Lipids constitute a broad group of naturally occurring
moleculeA molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...
s that include
fatFats consist of a wide group of compounds that are generally soluble in organic solvents and generally insoluble in water. Chemically, fats are triglycerides, triesters of glycerol and any of several fatty acids. Fats may be either solid or liquid at room temperature, depending on their structure...
s,
waxthumb|right|[[Cetyl palmitate]], a typical wax ester.Wax refers to a class of chemical compounds that are plastic near ambient temperatures. Characteristically, they melt above 45 °C to give a low viscosity liquid. Waxes are insoluble in water but soluble in organic, nonpolar solvents...
es,
sterolSterols, also known as steroid alcohols, are a subgroup of the steroids and an important class of organic molecules. They occur naturally in plants, animals, and fungi, with the most familiar type of animal sterol being cholesterol...
s, fat-soluble
vitaminA vitamin is an organic compound required as a nutrient in tiny amounts by an organism. In other words, an organic chemical compound is called a vitamin when it cannot be synthesized in sufficient quantities by an organism, and must be obtained from the diet. Thus, the term is conditional both on...
s (such as vitamins A, D, E, and K), monoglycerides, diglycerides,
triglycerideA triglyceride is an ester derived from glycerol and three fatty acids. There are many triglycerides, depending on the oil source, some are highly unsaturated, some less so....
s, phospholipids, and others. The main biological functions of lipids include energy storage, as structural components of
cell membraneThe cell membrane or plasma membrane is a biological membrane that separates the interior of all cells from the outside environment. The cell membrane is selectively permeable to ions and organic molecules and controls the movement of substances in and out of cells. It basically protects the cell...
s, and as important
signaling moleculesLipid signaling, broadly defined, refers to any biological signaling event involving a lipid messenger that binds a protein target, such as a receptor, kinase or phosphatase, which in turn mediate the effects of these lipids on specific cellular responses...
.
Lipids may be broadly defined as
hydrophobicIn chemistry, hydrophobicity is the physical property of a molecule that is repelled from a mass of water....
or amphiphilic small molecules; the amphiphilic nature of some lipids allows them to form structures such as
vesiclesA vesicle is a bubble of liquid within another liquid, a supramolecular assembly made up of many different molecules. More technically, a vesicle is a small membrane-enclosed sack that can store or transport substances. Vesicles can form naturally because of the properties of lipid membranes , or...
,
liposomeLiposomes are artificially prepared vesicles made of lipid bilayer. Liposomes can be filled with drugs, and used to deliver drugs for cancer and other diseases. Liposomes are composite structures made of phospholipids and may contain small amounts of other molecules...
s, or membranes in an aqueous environment. Biological lipids originate entirely or in part from two distinct types of biochemical subunits or "building-blocks":
ketoacylIn organic chemistry, a ketone is an organic compound with the structure RCR', where R and R' can be a variety of atoms and groups of atoms. It features a carbonyl group bonded to two other carbon atoms. Many ketones are known and many are of great importance in industry and in biology...
and
isopreneIsoprene , or 2-methyl-1,3-butadiene, is a common organic compound with the formula CH2=CCH=CH2. Under standard conditions it is a colorless liquid...
groups. Using this approach, lipids may be divided into eight categories: fatty acyls, glycerolipids,
glycerophospholipidGlycerophospholipids or phosphoglycerides are glycerol-based phospholipids. They are the main component of biological membranes.-Structures:...
s,
sphingolipidSphingolipids are a class of lipids containing a backbone of sphingoid bases, a set of aliphatic amino alcohols that includes sphingosine. They were discovered in brain extracts in the 1870s and were named for the mythological Sphinx because of their enigmatic nature. These compounds play...
s,
saccharolipidSaccharolipids describe compounds in which fatty acids are linked directly to a sugar backbone, forming structures that are compatible with membrane bilayers. In the saccharolipids, a monosaccharide substitutes for the glycerol backbone present in glycerolipids and glycerophospholipids...
s, and
polyketidePolyketides are secondary metabolites from bacteria, fungi, plants, and animals. Polyketides are usually biosynthesized through the decarboxylative condensation of malonyl-CoA derived extender units in a similar process to fatty acid synthesis...
s (derived from condensation of ketoacyl subunits); and sterol lipids and prenol lipids (derived from condensation of isoprene subunits).
Although the term
lipid is sometimes used as a synonym for
fatFats consist of a wide group of compounds that are generally soluble in organic solvents and generally insoluble in water. Chemically, fats are triglycerides, triesters of glycerol and any of several fatty acids. Fats may be either solid or liquid at room temperature, depending on their structure...
s, fats are a subgroup of lipids called
triglycerideA triglyceride is an ester derived from glycerol and three fatty acids. There are many triglycerides, depending on the oil source, some are highly unsaturated, some less so....
s. Lipids also encompass molecules such as
fatty acidIn chemistry, especially biochemistry, a fatty acid is a carboxylic acid with a long unbranched aliphatic tail , which is either saturated or unsaturated. Most naturally occurring fatty acids have a chain of an even number of carbon atoms, from 4 to 28. Fatty acids are usually derived from...
s and their derivatives (including
tri-A triglyceride is an ester derived from glycerol and three fatty acids. There are many triglycerides, depending on the oil source, some are highly unsaturated, some less so....
,
di-A diglyceride, or a diacylglycerol , is a glyceride consisting of two fatty acid chains covalently bonded to a glycerol molecule through ester linkages....
,
monoglycerideA monoglyceride, more correctly known as a monoacylglycerol, is a glyceride consisting of one fatty acid chain covalently bonded to a glycerol molecule through an ester linkage....
s, and
phospholipidPhospholipids are a class of lipids that are a major component of all cell membranes as they can form lipid bilayers. Most phospholipids contain a diglyceride, a phosphate group, and a simple organic molecule such as choline; one exception to this rule is sphingomyelin, which is derived from...
s), as well as other
sterolSterols, also known as steroid alcohols, are a subgroup of the steroids and an important class of organic molecules. They occur naturally in plants, animals, and fungi, with the most familiar type of animal sterol being cholesterol...
-containing
metaboliteMetabolites are the intermediates and products of metabolism. The term metabolite is usually restricted to small molecules. A primary metabolite is directly involved in normal growth, development, and reproduction. Alcohol is an example of a primary metabolite produced in large-scale by industrial...
s such as
cholesterolCholesterol is a complex isoprenoid. Specifically, it is a waxy steroid of fat that is produced in the liver or intestines. It is used to produce hormones and cell membranes and is transported in the blood plasma of all mammals. It is an essential structural component of mammalian cell membranes...
. Although humans and other mammals use various
biosynthetic pathwayMetabolism is the set of chemical reactions that happen in the cells of living organisms to sustain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolism is usually divided into two categories...
s to both break down and synthesize lipids, some essential lipids cannot be made this way and must be obtained from the diet.
Fatty acyls
Fatty acyls, a generic term for describing fatty acids, their conjugates, and derivatives, are a diverse group of molecules synthesized by chain-elongation of an
acetyl-CoAAcetyl coenzyme A or acetyl-CoA is an important molecule in metabolism, used in many biochemical reactions. Its main function is to convey the carbon atoms within the acetyl group to the citric acid cycle to be oxidized for energy production. In chemical structure, acetyl-CoA is the thioester...
primer with
malonyl-CoAMalonyl-CoA is a coenzyme A derivative.-Functions:It plays a key role in chain elongation in fatty acid biosynthesis and polyketide biosynthesis....
or
methylmalonyl-CoAMethylmalonyl-CoA is the coenzyme A linked form of methylmalonic acid. Methylmalonyl-CoA is formed from propionyl-CoA by propionyl-CoA carboxylase by help of biotin . It is converted into succinyl-CoA by methylmalonyl-CoA mutase, in a reaction that requires vitamin B12 as a cofactor...
groups in a process called
fatty acid synthesisFatty acid synthesis is the creation of fatty acids from acetyl-CoA and malonyl-CoA precursors through action of enzymes called fatty acid synthases...
. They are made of a hydrocarbon chain that terminates with a
carboxylic acidCarboxylic acids are organic acids characterized by the presence of at least one carboxyl group. The general formula of a carboxylic acid is R-COOH, where R is some monovalent functional group...
group; this arrangement confers the molecule with a
polarIn chemistry, polarity refers to a separation of electric charge leading to a molecule or its chemical groups having an electric dipole or multipole moment. Polar molecules interact through dipole–dipole intermolecular forces and hydrogen bonds. Molecular polarity is dependent on the difference in...
, hydrophilic end, and a nonpolar, hydrophobic end that is
insolubleSolubility is the property of a solid, liquid, or gaseous chemical substance called solute to dissolve in a solid, liquid, or gaseous solvent to form a homogeneous solution of the solute in the solvent. The solubility of a substance fundamentally depends on the used solvent as well as on...
in water. The fatty acid structure is one of the most fundamental categories of biological lipids, and is commonly used as a building-block of more structurally complex lipids. The carbon chain, typically between four and 24 carbons long, may be saturated or
unsaturatedIn organic chemistry, a saturated compound is a chemical compound that has of a chain of carbon atoms linked together by single bonds and has hydrogen atoms filling all of the other bonding orbitals of the carbon atoms. Alkanes are an example of saturated compounds...
, and may be attached to
functional groupIn organic chemistry, functional groups are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules. The same functional group will undergo the same or similar chemical reaction regardless of the size of the molecule it is a part of...
s containing
oxygenOxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
, halogens,
nitrogenNitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...
, and
sulfurSulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...
. Where a double bond exists, there is the possibility of either a
cis or a
trans geometric isomerism, which significantly affects the molecule's
molecular configurationThe configuration of a molecule is the permanent geometry that results from the spatial arrangement of its bonds. The ability of the same set of atoms to form two or more molecules with different configurations is stereoisomerism...
.
Cis-double bonds cause the fatty acid chain to bend, an effect that is more pronounced the more double bonds there are in a chain. This in turn plays an important role in the structure and function of cell membranes. Most naturally occurring fatty acids are of the
cis configuration, although the
trans form does exist in some natural and partially hydrogenated fats and oils.
Examples of biologically important fatty acids are the
eicosanoidIn biochemistry, eicosanoids are signaling molecules made by oxidation of twenty-carbon essential fatty acids, ....
s, derived primarily from
arachidonic acidArachidonic acid is a polyunsaturated omega-6 fatty acid 20:4.It is the counterpart to the saturated arachidic acid found in peanut oil, Arachidonic acid (AA, sometimes ARA) is a polyunsaturated omega-6 fatty acid 20:4(ω-6).It is the counterpart to the saturated arachidic acid found in peanut oil,...
and
eicosapentaenoic acidEicosapentaenoic acid is an omega-3 fatty acid. In physiological literature, it is given the name 20:5. It also has the trivial name timnodonic acid...
, which include
prostaglandinA prostaglandin is any member of a group of lipid compounds that are derived enzymatically from fatty acids and have important functions in the animal body. Every prostaglandin contains 20 carbon atoms, including a 5-carbon ring....
s,
leukotrieneLeukotrienes are fatty signaling molecules. They were first found in leukocytes . One of their roles is to trigger contractions in the smooth muscles lining the trachea; their overproduction is a major cause of inflammation in asthma and allergic rhinitis...
s, and
thromboxaneThromboxane is a member of the family of lipids known as eicosanoids. The two major thromboxanes are thromboxane A2 and thromboxane B2. The distinguishing feature of thromboxanes is a 6-membered ether-containing ring....
s. Other major lipid classes in the fatty acid category are the fatty esters and fatty amides. Fatty esters include important biochemical intermediates such as
wax esterAn ester of a fatty acid and a long-chain alcohol. These oils have similar properties as triglycerides, but are indigestible. They are found in some fish such as orange roughy, oilfish, escolar, black oreo, smooth oreo and other deep water fish. They are also present in marine copepods. Wax...
s, fatty acid thioester
coenzyme ACoenzyme A is a coenzyme, notable for its role in the synthesis and oxidation of fatty acids, and the oxidation of pyruvate in the citric acid cycle. All sequenced genomes encode enzymes that use coenzyme A as a substrate, and around 4% of cellular enzymes use it as a substrate...
derivatives, fatty acid thioester
ACPThe acyl carrier protein is an important component in both fatty acid and polyketide biosynthesis with the growing chain bound during synthesis as a thiol ester at the distal thiol of a 4'-phosphopantethiene moiety...
derivatives and fatty acid carnitines. The fatty amides include
N-acyl ethanolaminesAn N-acylethanolamine is a type of compound formed when one of several types of acyl group is linked to the nitrogen atom of ethanolamine. These amides conceptually can be formed from a fatty acid and ethanolamine with the release of a molecule of water, but the known biological synthesis relies...
, such as the cannabinoid neurotransmitter
anandamideAnandamide, also known as N-arachidonoylethanolamide or AEA, is an endogenous cannabinoid neurotransmitter. The name is taken from the Sanskrit word ananda, which means "bliss, delight", and amide. It is synthesized from N-arachidonoyl phosphatidylethanolamine by multiple pathways...
.
Glycerolipids
Glycerolipids are composed mainly of mono-, di-, and tri-substituted
glycerolGlycerol is a simple polyol compound. It is a colorless, odorless, viscous liquid that is widely used in pharmaceutical formulations. Glycerol has three hydroxyl groups that are responsible for its solubility in water and its hygroscopic nature. The glycerol backbone is central to all lipids...
s, the most well-known being the fatty acid triesters of glycerol (triacylglycerols), also known as triglycerides. In these compounds, the three hydroxyl groups of glycerol are each esterified, usually by different fatty acids. Because they function as a food store, these lipids comprise the bulk of storage fat in animal tissues. The hydrolysis of the
esterEsters are chemical compounds derived by reacting an oxoacid with a hydroxyl compound such as an alcohol or phenol. Esters are usually derived from an inorganic acid or organic acid in which at least one -OH group is replaced by an -O-alkyl group, and most commonly from carboxylic acids and...
bonds of triacylglycerols and the release of glycerol and fatty acids from
adipose tissueIn histology, adipose tissue or body fat or fat depot or just fat is loose connective tissue composed of adipocytes. It is technically composed of roughly only 80% fat; fat in its solitary state exists in the liver and muscles. Adipose tissue is derived from lipoblasts...
is called fat mobilization.
Additional subclasses of glycerolipids are represented by glycosylglycerols, which are characterized by the presence of one or more
sugar residuesMonosaccharides are the most basic units of biologically important carbohydrates. They are the simplest form of sugar and are usually colorless, water-soluble, crystalline solids. Some monosaccharides have a sweet taste. Examples of monosaccharides include glucose , fructose , galactose, xylose...
attached to glycerol via a glycosidic linkage. Examples of structures in this category are the digalactosyldiacylglycerols found in plant membranes and seminolipid from mammalian
sperm cellsA spermatozoon is a motile sperm cell, or moving form of the haploid cell that is the male gamete. A spermatozoon joins an ovum to form a zygote...
.
Glycerophospholipids
Glycerophospholipids, also referred to as
phospholipidPhospholipids are a class of lipids that are a major component of all cell membranes as they can form lipid bilayers. Most phospholipids contain a diglyceride, a phosphate group, and a simple organic molecule such as choline; one exception to this rule is sphingomyelin, which is derived from...
s, are ubiquitous in nature and are key components of the
lipid bilayerThe lipid bilayer is a thin membrane made of two layers of lipid molecules. These membranes are flat sheets that form a continuous barrier around cells. The cell membrane of almost all living organisms and many viruses are made of a lipid bilayer, as are the membranes surrounding the cell nucleus...
of cells, as well as being involved in
metabolismMetabolism is the set of chemical reactions that happen in the cells of living organisms to sustain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolism is usually divided into two categories...
and
cell signalingCell signaling is part of a complex system of communication that governs basic cellular activities and coordinates cell actions. The ability of cells to perceive and correctly respond to their microenvironment is the basis of development, tissue repair, and immunity as well as normal tissue...
. Neural tissue (including the brain) contains relatively high amounts of glycerophospholipids, and alterations in their composition has been implicated in various neurological disorders. Glycerophospholipids may be subdivided into distinct classes, based on the nature of the polar headgroup at the
sn-3 position of the glycerol backbone in
eukaryoteA eukaryote is an organism whose cells contain complex structures enclosed within membranes. Eukaryotes may more formally be referred to as the taxon Eukarya or Eukaryota. The defining membrane-bound structure that sets eukaryotic cells apart from prokaryotic cells is the nucleus, or nuclear...
s and eubacteria, or the
sn-1 position in the case of archaebacteria.
Examples of glycerophospholipids found in
biological membraneA biological membrane or biomembrane is an enclosing or separatingmembrane that acts as a selective barrier, within or around a cell. It consists of a lipid bilayer with embedded proteins that may constitute close to 50% of membrane content...
s are
phosphatidylcholinePhosphatidylcholines are a class of phospholipids that incorporate choline as a headgroup.They are a major component of biological membranes and can be easily obtained from a variety of readily available sources such as egg yolk or soy beans from which they are mechanically extracted or chemically...
(also known as PC, GPCho or
lecithinLecithin is a generic term to designate any group of yellow-brownish fatty substances occurring in animal and plant tissues, and in egg yolk, composed of phosphoric acid, choline, fatty acids, glycerol, glycolipids, triglycerides, and phospholipids .The word lecithin was originally coined in 1847 by...
),
phosphatidylethanolaminePhosphatidylethanolamine is a lipid found in biological membranes. It is synthesized by the addition of CDP-ethanolamine to diglyceride, releasing CMP. S-adenosyl methionine can subsequently methylate the amine of phosphatidyl ethanolamine to yield phosphatidyl choline.Cephalin is a phospholipid,...
(PE or GPEtn) and
phosphatidylserinePhosphatidylserine is a phospholipid component, usually kept on the inner-leaflet of cell membranes by an enzyme called flippase...
(PS or GPSer). In addition to serving as a primary component of cellular membranes and binding sites for intra- and intercellular proteins, some glycerophospholipids in eukaryotic cells, such as
phosphatidylinositolPhosphatidylinositol is a negatively charged phospholipid and a minor component in the cytosolic side of eukaryotic cell membranes....
s and phosphatidic acids are either precursors of or, themselves, membrane-derived
second messengersSecond messengers are molecules that relay signals from receptors on the cell surface to target molecules inside the cell, in the cytoplasm or nucleus. They relay the signals of hormones like epinephrine , growth factors, and others, and cause some kind of change in the activity of the cell...
. Typically, one or both of these hydroxyl groups are acylated with long-chain fatty acids, but there are also alkyl-linked and 1Z-alkenyl-linked (
plasmalogenPlasmalogens are a type of ether phospholipid characterized by the presence of a vinyl ether linkage at the sn-1 position and an ester linkage at the sn-2 position . In mammals, the sn-1 position is typically derived from C16:0, C18:0, or C18:1 fatty alcohols while the sn-2 position is most...
) glycerophospholipids, as well as dialkylether variants in archaebacteria.
Sphingolipids
Sphingolipids are a complex family of compounds that share a common structural feature, a sphingoid base backbone that is synthesized
de novoDe novo synthesis refers to the synthesis of complex molecules from simple molecules such as sugars or amino acids, as opposed to their being recycled after partial degradation. For example, nucleotides are not needed in the diet as they can be constructed from small precursor molecules such as...
from the amino acid
serineSerine is an amino acid with the formula HO2CCHCH2OH. It is one of the proteinogenic amino acids. By virtue of the hydroxyl group, serine is classified as a polar amino acid.-Occurrence and biosynthesis:...
and a long-chain fatty acyl CoA, then converted into
ceramideCeramides are a family of lipid molecules. A ceramide is composed of sphingosine and a fatty acid. Ceramides are found in high concentrations within the cell membrane of cells. They are one of the component lipids that make up sphingomyelin, one of the major lipids in the lipid bilayer...
s, phosphosphingolipids, glycosphingolipids and other compounds. The major sphingoid base of mammals is commonly referred to as
sphingosineSphingosine is an 18-carbon amino alcohol with an unsaturated hydrocarbon chain, which forms a primary part of sphingolipids, a class of cell membrane lipids that include sphingomyelin, an important phospholipid.-Functions:...
. Ceramides (N-acyl-sphingoid bases) are a major subclass of sphingoid base derivatives with an
amideIn chemistry, an amide is an organic compound that contains the functional group consisting of a carbonyl group linked to a nitrogen atom . The term refers both to a class of compounds and a functional group within those compounds. The term amide also refers to deprotonated form of ammonia or an...
-linked fatty acid. The fatty acids are typically saturated or mono-unsaturated with chain lengths from 16 to 26 carbon atoms.
The major phosphosphingolipids of mammals are
sphingomyelinSphingomyelin is a type of sphingolipid found in animal cell membranes, especially in the membranous myelin sheath that surrounds some nerve cell axons. It usually consists of phosphorylcholine and ceramide...
s (ceramide phosphocholines), whereas insects contain mainly ceramide phosphoethanolamines and fungi have phytoceramide phosphoinositols and
mannoseMannose is a sugar monomer of the aldohexose series of carbohydrates. Mannose is a C-2 epimer of glucose. It is not part of human metabolism, but is a component of microbial cell walls, and is therefore a target of the immune system and also of antibiotics....
-containing headgroups. The glycosphingolipids are a diverse family of molecules composed of one or more sugar residues linked via a
glycosidic bondIn chemistry, a glycosidic bond is a type of covalent bond that joins a carbohydrate molecule to another group, which may or may not be another carbohydrate....
to the sphingoid base. Examples of these are the simple and complex glycosphingolipids such as
cerebrosideCerebrosides is the common name for a group of glycosphingolipids called monoglycosylceramides which are important components in animal muscle and nerve cell membranes.They consist of a ceramide with a single sugar residue at the 1-hydroxyl moiety...
s and
gangliosideGanglioside is a molecule composed of a glycosphingolipid with one or more sialic acids linked on the sugar chain. The 60+ known gangliosides differ mainly in the position and number of NANA residues.It is a component of the cell plasma membrane that modulates cell signal transduction events...
s.
Sterol lipids
Sterol lipids, such as
cholesterolCholesterol is a complex isoprenoid. Specifically, it is a waxy steroid of fat that is produced in the liver or intestines. It is used to produce hormones and cell membranes and is transported in the blood plasma of all mammals. It is an essential structural component of mammalian cell membranes...
and its derivatives, are an important component of membrane lipids, along with the glycerophospholipids and sphingomyelins. The
steroidA steroid is a type of organic compound that contains a characteristic arrangement of four cycloalkane rings that are joined to each other. Examples of steroids include the dietary fat cholesterol, the sex hormones estradiol and testosterone, and the anti-inflammatory drug dexamethasone.The core...
s, all derived from the same fused four-ring core structure, have different biological roles as hormones and signaling molecules. The eighteen-carbon (C18) steroids include the
estrogenEstrogens , oestrogens , or œstrogens, are a group of compounds named for their importance in the estrous cycle of humans and other animals. They are the primary female sex hormones. Natural estrogens are steroid hormones, while some synthetic ones are non-steroidal...
family whereas the C19 steroids comprise the
androgenAndrogen, also called androgenic hormone or testoid, is the generic term for any natural or synthetic compound, usually a steroid hormone, that stimulates or controls the development and maintenance of male characteristics in vertebrates by binding to androgen receptors...
s such as
testosteroneTestosterone is a steroid hormone from the androgen group and is found in mammals, reptiles, birds, and other vertebrates. In mammals, testosterone is primarily secreted in the testes of males and the ovaries of females, although small amounts are also secreted by the adrenal glands...
and
androsteroneAndrosterone is a steroid hormone with weak androgenic activity. It is made in the liver from the metabolism of testosterone. Its beta-isomer is Epiandrosterone.-History:...
. The C21 subclass includes the progestogens as well as the
glucocorticoidGlucocorticoids are a class of steroid hormones that bind to the glucocorticoid receptor , which is present in almost every vertebrate animal cell...
s and mineralocorticoids. The
secosteroidA secosteroid is a molecule similar to a steroid but with a "broken" ring. Etym: L. Seco, to cut, _ steroidSecosteroids are very similar in structure to steroids except that two of the B-ring carbon atoms of the typical four steroid rings are not joined, whereas in steroids they are.In humans, the...
s, comprising various forms of
vitamin DVitamin D is a group of fat-soluble secosteroids. In humans, vitamin D is unique both because it functions as a prohormone and because the body can synthesize it when sun exposure is adequate ....
, are characterized by cleavage of the B ring of the core structure. Other examples of
sterolSterols, also known as steroid alcohols, are a subgroup of the steroids and an important class of organic molecules. They occur naturally in plants, animals, and fungi, with the most familiar type of animal sterol being cholesterol...
s are the
bile acidBile acids are steroid acids found predominantly in the bile of mammals. Bile salts are bile acids compounded with a cation, usually sodium. In humans, the salts of taurocholic acid and glycocholic acid represent approximately eighty percent of all bile salts. The two major bile acids are cholic...
s and their conjugates, which in mammals are oxidized derivatives of cholesterol and are synthesized in the liver. The plant equivalents are the phytosterols, such as β-sitosterol,
stigmasterolStigmasterol is one of a group of plant sterols, or phytosterols, that include beta-sitosterol, campesterol, ergosterol , brassicasterol, delta-7-stigmasterol and delta-7-avenasterol, that are chemically similar to animal cholesterol...
, and
brassicasterolBrassicasterol is a 28-carbon sterol synthesised by several unicellular algae and some terrestrial plants, e.g., oilseed rape...
; the latter compound is also used as a biomarker for
algalAlgae are a large and diverse group of simple, typically autotrophic organisms, ranging from unicellular to multicellular forms, such as the giant kelps that grow to 65 meters in length. They are photosynthetic like plants, and "simple" because their tissues are not organized into the many...
growth. The predominant sterol in fungal cell membranes is
ergosterolErgosterol is a sterol found in fungi, and named for ergot, a common name for the members of the fungal genus Claviceps from which ergosterol was first isolated. Ergosterol does not occur in plant or animal cells...
.
Prenol lipids
Prenol lipids are synthesized from the 5 carbon precursors isopentenyl diphosphate and dimethylallyl diphosphate that are produced mainly via the
mevalonic acidMevalonic acid is a key organic compound in biochemistry. The anion of mevalonic acid, the predominant form in biological media, is known as mevalonate. This compound is of major pharmaceutical importance...
(MVA) pathway. The simple isoprenoids (linear alcohols, diphosphates, etc.) are formed by the successive addition of C5 units, and are classified according to number of these
terpeneTerpenes are a large and diverse class of organic compounds, produced by a variety of plants, particularly conifers, though also by some insects such as termites or swallowtail butterflies, which emit terpenes from their osmeterium. They are often strong smelling and thus may have had a protective...
units. Structures containing greater than 40 carbons are known as polyterpenes.
CarotenoidCarotenoids are tetraterpenoid organic pigments that are naturally occurring in the chloroplasts and chromoplasts of plants and some other photosynthetic organisms like algae, some bacteria, and some types of fungus. Carotenoids can be synthesized fats and other basic organic metabolic building...
s are important simple isoprenoids that function as
antioxidantAn antioxidant is a molecule capable of inhibiting the oxidation of other molecules. Oxidation is a chemical reaction that transfers electrons or hydrogen from a substance to an oxidizing agent. Oxidation reactions can produce free radicals. In turn, these radicals can start chain reactions. When...
s and as precursors of
vitamin AVitamin A is a vitamin that is needed by the retina of the eye in the form of a specific metabolite, the light-absorbing molecule retinal, that is necessary for both low-light and color vision...
. Another biologically important class of molecules is exemplified by the
quinoneA quinone is a class of organic compounds that are formally "derived from aromatic compounds [such as benzene or naphthalene] by conversion of an even number of –CH= groups into –C– groups with any necessary rearrangement of double bonds," resulting in "a fully conjugated cyclic dione structure."...
s and
hydroquinoneHydroquinone, also benzene-1,4-diol or quinol, is an aromatic organic compound that is a type of phenol, having the chemical formula C6H42. Its chemical structure, shown in the table at right, has two hydroxyl groups bonded to a benzene ring in a para position. It is a white granular solid...
s, which contain an isoprenoid tail attached to a quinonoid core of non-isoprenoid origin.
Vitamin EVitamin E is used to refer to a group of fat-soluble compounds that include both tocopherols and tocotrienols. There are many different forms of vitamin E, of which γ-tocopherol is the most common in the North American diet. γ-Tocopherol can be found in corn oil, soybean oil, margarine and dressings...
and
vitamin KVitamin K is a group of structurally similar, fat soluble vitamins that are needed for the posttranslational modification of certain proteins required for blood coagulation and in metabolic pathways in bone and other tissue. They are 2-methyl-1,4-naphthoquinone derivatives...
, as well as the ubiquinones, are examples of this class. Prokaryotes synthesize polyprenols (called
bactoprenolBactoprenol is a lipid synthesized by lactobacilli.Bactoprenol phosphate transports NAM and NAG across the cell membrane in the synthesis of peptidoglycan. Bacitracin inhibits this process....
s) in which the terminal isoprenoid unit attached to oxygen remains unsaturated, whereas in animal polyprenols (
dolicholDolichol refers to any of a group of long-chain mostly unsaturated organic compounds that are made up of varying numbers of isoprene units terminating in an α-saturated isoprenoid group, containing an alcohol functional group.-Functions:...
s) the terminal isoprenoid is reduced.
Saccharolipids
Saccharolipids describe compounds in which fatty acids are linked directly to a sugar backbone, forming structures that are compatible with membrane bilayers. In the saccharolipids, a
monosaccharideMonosaccharides are the most basic units of biologically important carbohydrates. They are the simplest form of sugar and are usually colorless, water-soluble, crystalline solids. Some monosaccharides have a sweet taste. Examples of monosaccharides include glucose , fructose , galactose, xylose...
substitutes for the glycerol backbone present in glycerolipids and glycerophospholipids. The most familiar saccharolipids are the acylated
glucosamineGlucosamine is an amino sugar and a prominent precursor in the biochemical synthesis of glycosylated proteins and lipids. Glucosamine is part of the structure of the polysaccharides chitosan and chitin, which compose the exoskeletons of crustaceans and other arthropods, cell walls in fungi and...
precursors of the
Lipid ALipid A is a lipid component of an endotoxin held responsible for toxicity of Gram-negative bacteria. It is the innermost of the three regions of the lipopolysaccharide molecule, and its hydrophobic nature allows it to anchor the LPS to the outer membrane...
component of the
lipopolysaccharideLipopolysaccharides , also known as lipoglycans, are large molecules consisting of a lipid and a polysaccharide joined by a covalent bond; they are found in the outer membrane of Gram-negative bacteria, act as endotoxins and elicit strong immune responses in animals.-Functions:LPS is the major...
s in Gram-negative bacteria. Typical lipid A molecules are disaccharides of glucosamine, which are derivatized with as many as seven fatty-acyl chains. The minimal lipopolysaccharide required for growth in
E. coliEscherichia coli is a Gram-negative, rod-shaped bacterium that is commonly found in the lower intestine of warm-blooded organisms . Most E. coli strains are harmless, but some serotypes can cause serious food poisoning in humans, and are occasionally responsible for product recalls...
is Kdo
2-Lipid A, a hexa-acylated disaccharide of glucosamine that is glycosylated with two 3-deoxy-D-manno-octulosonic acid (Kdo) residues.
Polyketides
Polyketides are synthesized by polymerization of
acetylIn organic chemistry, acetyl is a functional group, the acyl with chemical formula COCH3. It is sometimes represented by the symbol Ac . The acetyl group contains a methyl group single-bonded to a carbonyl...
and
propionylPropionyl-CoA is a coenzyme A derivative of propionic acid.-Production:There are several different ways in which it is formed:* It is formed as a product of beta-oxidation of odd-chain fatty acids....
subunits by classic enzymes as well as iterative and multimodular enzymes that share mechanistic features with the
fatty acid synthaseFatty acid synthase is an enzyme that in humans is encoded by the FASN gene.Fatty acid synthase is a multi-enzyme protein that catalyzes fatty acid synthesis...
s. They comprise a large number of
secondary metaboliteSecondary metabolites are organic compounds that are not directly involved in the normal growth, development, or reproduction of an organism. Unlike primary metabolites, absence of secondary metabolities does not result in immediate death, but rather in long-term impairment of the organism's...
s and natural products from animal, plant, bacterial, fungal and marine sources, and have great structural diversity. Many
polyketidePolyketides are secondary metabolites from bacteria, fungi, plants, and animals. Polyketides are usually biosynthesized through the decarboxylative condensation of malonyl-CoA derived extender units in a similar process to fatty acid synthesis...
s are cyclic molecules whose backbones are often further modified by
glycosylationGlycosylation is the reaction in which a carbohydrate, i.e. a glycosyl donor, is attached to a hydroxyl or other functional group of another molecule . In biology glycosylation refers to the enzymatic process that attaches glycans to proteins, lipids, or other organic molecules...
,
methylationIn the chemical sciences, methylation denotes the addition of a methyl group to a substrate or the substitution of an atom or group by a methyl group. Methylation is a form of alkylation with, to be specific, a methyl group, rather than a larger carbon chain, replacing a hydrogen atom...
,
hydroxylationHydroxylation is a chemical process that introduces a hydroxyl group into an organic compound. In biochemistry, hydroxylation reactions are often facilitated by enzymes called hydroxylases. Hydroxylation is the first step in the oxidative degradation of organic compounds in air...
, oxidation, and/or other processes. Many commonly used anti-microbial,
anti-parasiticAntiparasitics are a class of medications which are indicated for the treatment of parasitic diseases such as nematodes, cestodes, trematodes, infectious protozoa, and amoebas.-Antinematodes:* Mebendazole...
, and anti-cancer agents are polyketides or polyketide derivatives, such as
erythromycinErythromycin is a macrolide antibiotic that has an antimicrobial spectrum similar to or slightly wider than that of penicillin, and is often used for people who have an allergy to penicillins. For respiratory tract infections, it has better coverage of atypical organisms, including mycoplasma and...
s,
tetracyclinesTetracyclines are a group of broad-spectrum antibiotics whose general usefulness has been reduced with the onset of bacterial resistance. Despite this, they remain the treatment of choice for some specific indications....
,
avermectinThe avermectins are a series 16-membered macrocyclic lactone derivatives with potent anthelmintic and insecticidal properties. These naturally occurring compounds are generated as fermentation products by Streptomyces avermitilis, a soil actinomycete...
s, and antitumor
epothiloneThe epothilones are a new class of cancer drugs. Like taxanes, they prevent cancer cells from dividing by interfering with tubulin, but in early trials epithilones have better efficacy and milder adverse effects than taxanes....
s.
Membranes
Eukaryotic cells are compartmentalized into membrane-bound organelles that carry out different biological functions. The glycerophospholipids are the main structural component of biological membranes, such as the cellular plasma membrane and the intracellular membranes of organelles; in animal cells the plasma membrane physically separates the
intracellularNot to be confused with intercellular, meaning "between cells".In cell biology, molecular biology and related fields, the word intracellular means "inside the cell".It is used in contrast to extracellular...
components from the
extracellularIn cell biology, molecular biology and related fields, the word extracellular means "outside the cell". This space is usually taken to be outside the plasma membranes, and occupied by fluid...
environment. The glycerophospholipids are amphipathic molecules (containing both hydrophobic and hydrophilic regions) that contain a glycerol core linked to two fatty acid-derived "tails" by
esterEsters are chemical compounds derived by reacting an oxoacid with a hydroxyl compound such as an alcohol or phenol. Esters are usually derived from an inorganic acid or organic acid in which at least one -OH group is replaced by an -O-alkyl group, and most commonly from carboxylic acids and...
linkages and to one "head" group by a
phosphateA phosphate, an inorganic chemical, is a salt of phosphoric acid. In organic chemistry, a phosphate, or organophosphate, is an ester of phosphoric acid. Organic phosphates are important in biochemistry and biogeochemistry or ecology. Inorganic phosphates are mined to obtain phosphorus for use in...
ester linkage. While glycerophospholipids are the major component of biological membranes, other non-glyceride lipid components such as
sphingomyelinSphingomyelin is a type of sphingolipid found in animal cell membranes, especially in the membranous myelin sheath that surrounds some nerve cell axons. It usually consists of phosphorylcholine and ceramide...
and
sterolSterols, also known as steroid alcohols, are a subgroup of the steroids and an important class of organic molecules. They occur naturally in plants, animals, and fungi, with the most familiar type of animal sterol being cholesterol...
s (mainly
cholesterolCholesterol is a complex isoprenoid. Specifically, it is a waxy steroid of fat that is produced in the liver or intestines. It is used to produce hormones and cell membranes and is transported in the blood plasma of all mammals. It is an essential structural component of mammalian cell membranes...
in animal cell membranes) are also found in biological membranes. In plants and algae, the galactosyldiacylglycerols, and sulfoquinovosyldiacylglycerol, which lack a phosphate group, are important components of membranes of chloroplasts and related organelles and are the most abundant lipids in photosynthetic tissues, including those of higher plants, algae and certain bacteria.
Bilayers have been found to exhibit high levels of
birefringenceBirefringence, or double refraction, is the decomposition of a ray of light into two rays when it passes through certain anisotropic materials, such as crystals of calcite or boron nitride. The effect was first described by the Danish scientist Rasmus Bartholin in 1669, who saw it in calcite...
, which can be used to probe the degree of order (or disruption) within the bilayer using techniques such as dual polarization interferometry
A biological membrane is a form of
lipid bilayerThe lipid bilayer is a thin membrane made of two layers of lipid molecules. These membranes are flat sheets that form a continuous barrier around cells. The cell membrane of almost all living organisms and many viruses are made of a lipid bilayer, as are the membranes surrounding the cell nucleus...
. The formation of lipid bilayers is an energetically preferred process when the glycerophospholipids described above are in an aqueous environment. In an aqueous system, the polar heads of lipids align towards the polar, aqueous environment, while the hydrophobic tails minimize their contact with water and tend to cluster together, forming a
vesicleA vesicle is a bubble of liquid within another liquid, a supramolecular assembly made up of many different molecules. More technically, a vesicle is a small membrane-enclosed sack that can store or transport substances. Vesicles can form naturally because of the properties of lipid membranes , or...
; depending on the
concentrationIn colloidal and surface chemistry, the critical micelle concentration is defined as the concentration of surfactants above which micelles form and almost all additional surfactants added to the system go to micelles....
of the lipid, this biophysical interaction may result in the formation of
micelleA micelle is an aggregate of surfactant molecules dispersed in a liquid colloid. A typical micelle in aqueous solution forms an aggregate with the hydrophilic "head" regions in contact with surrounding solvent, sequestering the hydrophobic single tail regions in the micelle centre. This phase is...
s, liposomes, or
lipid bilayerThe lipid bilayer is a thin membrane made of two layers of lipid molecules. These membranes are flat sheets that form a continuous barrier around cells. The cell membrane of almost all living organisms and many viruses are made of a lipid bilayer, as are the membranes surrounding the cell nucleus...
s. Other aggregations are also observed and form part of the polymorphism of amphiphile (lipid) behavior. Phase behavior is an area of study within
biophysicsBiophysics is an interdisciplinary science that uses the methods of physical science to study biological systems. Studies included under the branches of biophysics span all levels of biological organization, from the molecular scale to whole organisms and ecosystems...
and is the subject of current academic research. Micelles and bilayers form in the polar medium by a process known as the
hydrophobic effectThe hydrophobic effect is the observed tendency of nonpolar substances to aggregate in aqueous solution and exclude water molecules. The name, literally meaning "water-fearing," describes the segregation and apparent repulsion between water and nonpolar substances...
. When dissolving a lipophilic or amphiphilic substance in a polar environment, the polar molecules (i.e., water in an aqueous solution) become more ordered around the dissolved lipophilic substance, since the polar molecules cannot form
hydrogen bondA hydrogen bond is the attractive interaction of a hydrogen atom with an electronegative atom, such as nitrogen, oxygen or fluorine, that comes from another molecule or chemical group. The hydrogen must be covalently bonded to another electronegative atom to create the bond...
s to the lipophilic areas of the amphiphile. So in an aqueous environment, the water molecules form an ordered "clathrate" cage around the dissolved lipophilic molecule.
Energy storage
Triacylglycerols, stored in adipose tissue, are a major form of energy storage in animals. The adipocyte, or fat cell, is designed for continuous synthesis and breakdown of triacylglycerols, with breakdown controlled mainly by the activation of hormone-sensitive enzyme
lipaseA lipase is an enzyme that catalyzes the formation or cleavage of fats . Lipases are a subclass of the esterases.Lipases perform essential roles in the digestion, transport and processing of dietary lipids in most, if not all, living organisms...
. The complete oxidation of fatty acids provides high caloric content, about 9 kcal/g, compared with 4 kcal/g for the breakdown of
carbohydrateA carbohydrate is an organic compound with the empirical formula ; that is, consists only of carbon, hydrogen, and oxygen, with a hydrogen:oxygen atom ratio of 2:1 . However, there are exceptions to this. One common example would be deoxyribose, a component of DNA, which has the empirical...
s and
proteinProteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
s. Migratory birds that must fly long distances without eating use stored energy of triacylglycerols to fuel their flights.
Signaling
In recent years, evidence has emerged showing that
lipid signalingLipid signaling, broadly defined, refers to any biological signaling event involving a lipid messenger that binds a protein target, such as a receptor, kinase or phosphatase, which in turn mediate the effects of these lipids on specific cellular responses...
is a vital part of the
cell signalingCell signaling is part of a complex system of communication that governs basic cellular activities and coordinates cell actions. The ability of cells to perceive and correctly respond to their microenvironment is the basis of development, tissue repair, and immunity as well as normal tissue...
. Lipid signaling may occur via activation of
G protein-coupledG protein-coupled receptors , also known as seven-transmembrane domain receptors, 7TM receptors, heptahelical receptors, serpentine receptor, and G protein-linked receptors , comprise a large protein family of transmembrane receptors that sense molecules outside the cell and activate inside signal...
or
nuclear receptorIn the field of molecular biology, nuclear receptors are a class of proteins found within cells that are responsible for sensing steroid and thyroid hormones and certain other molecules...
s, and members of several different lipid categories have been identified as signaling molecules and
cellular messengersSecond messengers are molecules that relay signals from receptors on the cell surface to target molecules inside the cell, in the cytoplasm or nucleus. They relay the signals of hormones like epinephrine , growth factors, and others, and cause some kind of change in the activity of the cell...
. These include
sphingosine-1-phosphateSphingosine-1-phosphate is a signaling sphingolipid. It is also referred to as a bioactive lipid mediator. Sphingolipids at large form a class of lipids characterized by a particular aliphatic aminoalcohol, which is sphingosine.-Production:...
, a sphingolipid derived from ceramide that is a potent messenger molecule involved in regulating calcium mobilization, cell growth, and apoptosis; diacylglycerol (DAG) and the
phosphatidylinositolPhosphatidylinositol is a negatively charged phospholipid and a minor component in the cytosolic side of eukaryotic cell membranes....
phosphates (PIPs), involved in calcium-mediated activation of
protein kinase CProtein kinase C also known as PKC is a family of enzymes that are involved in controlling the function of other proteins through the phosphorylation of hydroxyl groups of serine and threonine amino acid residues on these proteins. PKC enzymes in turn are activated by signals such as increases in...
; the prostaglandins, which are one type of fatty-acid derived eicosanoid involved in
inflammationInflammation is part of the complex biological response of vascular tissues to harmful stimuli, such as pathogens, damaged cells, or irritants. Inflammation is a protective attempt by the organism to remove the injurious stimuli and to initiate the healing process...
and
immunityImmunity is a biological term that describes a state of having sufficient biological defenses to avoid infection, disease, or other unwanted biological invasion. Immunity involves both specific and non-specific components. The non-specific components act either as barriers or as eliminators of wide...
; the steroid hormones such as
estrogenEstrogens , oestrogens , or œstrogens, are a group of compounds named for their importance in the estrous cycle of humans and other animals. They are the primary female sex hormones. Natural estrogens are steroid hormones, while some synthetic ones are non-steroidal...
,
testosteroneTestosterone is a steroid hormone from the androgen group and is found in mammals, reptiles, birds, and other vertebrates. In mammals, testosterone is primarily secreted in the testes of males and the ovaries of females, although small amounts are also secreted by the adrenal glands...
and
cortisolCortisol is a steroid hormone, more specifically a glucocorticoid, produced by the adrenal gland. It is released in response to stress and a low level of blood glucocorticoids. Its primary functions are to increase blood sugar through gluconeogenesis; suppress the immune system; and aid in fat,...
, which modulate a host of functions such as reproduction, metabolism and blood pressure; and the
oxysterolOxysterols are oxidized derivatives of cholesterol, which are important in many biological processes, including cholesterol homeostasis, sphingolipid metabolism, platelet aggregation, apoptosis, and protein prenylation.Some examples of oxysterols include:*...
s such as 25-hydroxy-cholesterol that are
liver X receptorThe liver X receptor is a member of the nuclear receptor family of transcription factors and is closely related to nuclear receptors such as the PPARs, FXR and RXR. Liver X receptors are important regulators of cholesterol, fatty acid, and glucose homeostasis...
agonistAn agonist is a chemical that binds to a receptor of a cell and triggers a response by that cell. Agonists often mimic the action of a naturally occurring substance...
s.
Other functions
The "fat-soluble" vitamins (
ARetinol is one of the animal forms of vitamin A. It is a diterpenoid and an alcohol. It is convertible to other forms of vitamin A, and the retinyl ester derivative of the alcohol serves as the storage form of the vitamin in animals....
, D,
ETocopherols are a class of chemical compounds of which many have vitamin E activity. It is a series of organic compounds consisting of various methylated phenols...
and
KPhylloquinone is a polycyclic aromatic ketone, based on 2-methyl-1,4-naphthoquinone, with a 3-phytyl substituent.It is a fat-soluble vitamin that is stable to air and moisture but decomposes in sunlight. It is found naturally in a wide variety of green plants.-Terminology:It is often called...
) – which are isoprene-based lipids – are essential nutrients stored in the liver and fatty tissues, with a diverse range of functions. Acyl-carnitines are involved in the transport and metabolism of fatty acids in and out of mitochondria, where they undergo
beta oxidationBeta oxidation is the process by which fatty acids, in the form of Acyl-CoA molecules, are broken down in mitochondria and/or in peroxisomes to generate Acetyl-CoA, the entry molecule for the Citric Acid cycle....
. Polyprenols and their phosphorylated derivatives also play important transport roles, in this case the transport of
oligosaccharideAn oligosaccharide is a saccharide polymer containing a small number of component sugars, also known as simple sugars...
s across membranes. Polyprenol phosphate sugars and polyprenol diphosphate sugars function in extra-cytoplasmic glycosylation reactions, in extracellular polysaccharide biosynthesis (for instance,
peptidoglycanPeptidoglycan, also known as murein, is a polymer consisting of sugars and amino acids that forms a mesh-like layer outside the plasma membrane of bacteria , forming the cell wall. The sugar component consists of alternating residues of β- linked N-acetylglucosamine and N-acetylmuramic acid...
polymerization in bacteria), and in eukaryotic protein N-
glycosylationGlycosylation is the reaction in which a carbohydrate, i.e. a glycosyl donor, is attached to a hydroxyl or other functional group of another molecule . In biology glycosylation refers to the enzymatic process that attaches glycans to proteins, lipids, or other organic molecules...
.
CardiolipinCardiolipin is an important component of the inner mitochondrial membrane, where it constitutes about 20% of the total lipid composition. The only other place that cardiolipin can be found is in the membranes of most bacteria. The name ‘cardiolipin’ is derived from the fact that it was first...
s are a subclass of glycerophospholipids containing four acyl chains and three glycerol groups that are particularly abundant in the inner mitochondrial membrane. They are believed to activate enzymes involved with
oxidative phosphorylationOxidative phosphorylation is a metabolic pathway that uses energy released by the oxidation of nutrients to produce adenosine triphosphate . Although the many forms of life on earth use a range of different nutrients, almost all aerobic organisms carry out oxidative phosphorylation to produce ATP,...
. Lipids also form the basis of steroid hormones.
Metabolism
The major dietary lipids for humans and other animals are animal and plant triglycerides, sterols, and membrane phospholipids. The process of lipid metabolism synthesizes and degrades the lipid stores and produces the structural and functional lipids characteristic of individual tissues.
Biosynthesis
In animals, when there is an oversupply of dietary carbohydrate, the excess carbohydrate is converted to triacylglycerol. This involves the synthesis of fatty acids from acetyl-CoA and the esterification of fatty acids in the production of triacylglycerol, a process called
lipogenesisLipogenesis is the process by which acetyl-CoA is converted to fats. The former is an intermediate stage in metabolism of simple sugars, such as glucose, a source of energy of living organisms. Through lipogenesis, the energy can be efficiently stored in the form of fats...
. Fatty acids are made by
fatty acid synthaseFatty acid synthase is an enzyme that in humans is encoded by the FASN gene.Fatty acid synthase is a multi-enzyme protein that catalyzes fatty acid synthesis...
s that polymerize and then reduce acetyl-CoA units. The acyl chains in the fatty acids are extended by a cycle of reactions that add the acetyl group, reduce it to an alcohol,
dehydrateIn chemistry and the biological sciences, a dehydration reaction is usually defined as a chemical reaction that involves the loss of water from the reacting molecule. Dehydration reactions are a subset of elimination reactions...
it to an
alkeneIn organic chemistry, an alkene, olefin, or olefine is an unsaturated chemical compound containing at least one carbon-to-carbon double bond...
group and then reduce it again to an
alkaneAlkanes are chemical compounds that consist only of hydrogen and carbon atoms and are bonded exclusively by single bonds without any cycles...
group. The enzymes of fatty acid biosynthesis are divided into two groups, in animals and fungi all these fatty acid synthase reactions are carried out by a single multifunctional protein, while in plant
plastidPlastids are major organelles found in the cells of plants and algae. Plastids are the site of manufacture and storage of important chemical compounds used by the cell...
s and bacteria separate enzymes perform each step in the pathway. The fatty acids may be subsequently converted to triacylglycerols that are packaged in lipoproteins and secreted from the liver.
The synthesis of unsaturated fatty acids involves a
desaturationA fatty acid desaturase is an enzyme that removes two hydrogen atoms from a fatty acid, creating a carbon/carbon double bond. These desaturases are classified as...
reaction, whereby a double bond is introduced into the fatty acyl chain. For example, in humans, the desaturation of
stearic acidStearic acid is the saturated fatty acid with an 18 carbon chain and has the IUPAC name octadecanoic acid. It is a waxy solid, and its chemical formula is CH316CO2H. Its name comes from the Greek word στέαρ "stéatos", which means tallow. The salts and esters of stearic acid are called stearates...
by
stearoyl-CoA desaturase-1Stearoyl-CoA desaturase is a protein that in humans is encoded by the SCD gene.Stearoyl-CoA desaturase-1is a key enzyme in fatty acid metabolism. It is responsible for forming a double bond in Stearoyl-CoA...
produces
oleic acidOleic acid is a monounsaturated omega-9 fatty acid found in various animal and vegetable fats. It has the formula CH37CH=CH7COOH. It is an odorless, colourless oil, although commercial samples may be yellowish. The trans isomer of oleic acid is called elaidic acid...
. The doubly unsaturated fatty acid
linoleic acidLinoleic acid is an unsaturated n-6 fatty acid. It is a colorless liquid at room temperature. In physiological literature, it has a lipid number of 18:2...
as well as the triply unsaturated
α-linolenic acidα-Linolenic acid is an organic compound found in many common vegetable oils. In terms of its structure, it is named all-cis-9,12,15-octadecatrienoic acid. In physiological literature, it is given the name 18:3 ....
cannot be synthesized in mammalian tissues, and are therefore
essential fatty acidEssential fatty acids, or EFAs, are fatty acids that humans and other animals must ingest because the body requires them for good health but cannot synthesize them...
s and must be obtained from the diet.
Triacylglycerol synthesis takes place in the
endoplasmic reticulumThe endoplasmic reticulum is an organelle of cells in eukaryotic organisms that forms an interconnected network of tubules, vesicles, and cisternae...
by metabolic pathways in which acyl groups in fatty acyl-CoAs are transferred to the hydroxyl groups of glycerol-3-phosphate and diacylglycerol.
TerpeneTerpenes are a large and diverse class of organic compounds, produced by a variety of plants, particularly conifers, though also by some insects such as termites or swallowtail butterflies, which emit terpenes from their osmeterium. They are often strong smelling and thus may have had a protective...
s and
isoprenoidsThe terpenoids , sometimes called isoprenoids, are a large and diverse class of naturally occurring organic chemicals similar to terpenes, derived from five-carbon isoprene units assembled and modified in thousands of ways. Most are multicyclic structures that differ from one another not only in...
, including the
carotenoidCarotenoids are tetraterpenoid organic pigments that are naturally occurring in the chloroplasts and chromoplasts of plants and some other photosynthetic organisms like algae, some bacteria, and some types of fungus. Carotenoids can be synthesized fats and other basic organic metabolic building...
s, are made by the assembly and modification of
isopreneIsoprene , or 2-methyl-1,3-butadiene, is a common organic compound with the formula CH2=CCH=CH2. Under standard conditions it is a colorless liquid...
units donated from the reactive precursors
isopentenyl pyrophosphateIsopentenyl pyrophosphate is an intermediate in the classical, HMG-CoA reductase pathway used by organisms in the biosynthesis of terpenes and terpenoids. IPP is formed from acetyl-CoA via mevalonic acid...
and
dimethylallyl pyrophosphateDimethylallyl pyrophosphate is an intermediate product of both mevalonic acid pathway and DOXP/MEP pathway. It is an isomer of isopentenyl pyrophosphate and exists in virtually all life forms...
. These precursors can be made in different ways. In animals and
archaeaThe Archaea are a group of single-celled microorganisms. A single individual or species from this domain is called an archaeon...
, the mevalonate pathway produces these compounds from acetyl-CoA, while in plants and bacteria the
non-mevalonate pathwayThe non-mevalonate pathway or 2-C-methyl-D-erythritol 4-phosphate/1-deoxy-D-xylulose 5-phosphate pathway of isoprenoid biosynthesis is an alternative metabolic pathway leading to the formation of isopentenyl pyrophosphate and dimethylallyl pyrophosphate that has been elucidated only...
uses pyruvate and
glyceraldehyde 3-phosphateGlyceraldehyde 3-phosphate, also known as triose phosphate or 3-phosphoglyceraldehyde and abbreviated as G3P, GADP, GAP, TP, GALP or PGAL, is a chemical compound that occurs as an intermediate in several central metabolic pathways of all organisms...
as substrates. One important reaction that uses these activated isoprene donors is steroid biosynthesis. Here, the isoprene units are joined together to make
squaleneSqualene is a natural organic compound originally obtained for commercial purposes primarily from shark liver oil, though plant sources are used as well, including amaranth seed, rice bran, wheat germ, and olives. All plants and animals produce squalene, including humans...
and then folded up and formed into a set of rings to make
lanosterolLanosterol is a tetracyclic triterpenoid, which is the compound from which all steroids are derived.-Role in creation of steroids:Elaboration of lanosterol under enzyme catalysis leads to the core structure of steroids. 14-Demethylation of lanosterol by CYP51 eventually yields...
. Lanosterol can then be converted into other steroids such as
cholesterolCholesterol is a complex isoprenoid. Specifically, it is a waxy steroid of fat that is produced in the liver or intestines. It is used to produce hormones and cell membranes and is transported in the blood plasma of all mammals. It is an essential structural component of mammalian cell membranes...
and
ergosterolErgosterol is a sterol found in fungi, and named for ergot, a common name for the members of the fungal genus Claviceps from which ergosterol was first isolated. Ergosterol does not occur in plant or animal cells...
.
Degradation
Beta oxidationBeta oxidation is the process by which fatty acids, in the form of Acyl-CoA molecules, are broken down in mitochondria and/or in peroxisomes to generate Acetyl-CoA, the entry molecule for the Citric Acid cycle....
is the metabolic process by which fatty acids are broken down in the mitochondria and/or in peroxisomes to generate
acetyl-CoAAcetyl coenzyme A or acetyl-CoA is an important molecule in metabolism, used in many biochemical reactions. Its main function is to convey the carbon atoms within the acetyl group to the citric acid cycle to be oxidized for energy production. In chemical structure, acetyl-CoA is the thioester...
. For the most part, fatty acids are oxidized by a mechanism that is similar to, but not identical with, a reversal of the process of fatty acid synthesis. That is, two-carbon fragments are removed sequentially from the carboxyl end of the acid after steps of
dehydrogenationDehydrogenation is a chemical reaction that involves the elimination of hydrogen . It is the reverse process of hydrogenation. Dehydrogenation reactions may be either large scale industrial processes or smaller scale laboratory procedures....
,
hydrationIn organic chemistry, a hydration reaction is a chemical reaction in which a hydroxyl group and a hydrogen cation are added to the two carbon atoms bonded together in the carbon-carbon double bond which makes up an alkene functional group. The reaction usually runs in a strong acidic, aqueous...
, and oxidation to form a
beta-keto acidKeto acids are organic compounds that contain a carboxylic acid group and a ketone group. The alpha-keto acids are especially important in biology as they are involved in the Krebs citric acid cycle and in glycolysis...
, which is split by
thiolysisThiolysis is a reaction with a thiol that cleaves one compound into two. This reaction is similar to hydrolysis, which involves water instead of a thiol. This reaction is seen in β-oxidation of fatty acids. The depolymerisation of condensed tannins with the use of benzyl mercaptan as...
. The acetyl-CoA is then ultimately converted into
ATPAdenosine-5'-triphosphate is a multifunctional nucleoside triphosphate used in cells as a coenzyme. It is often called the "molecular unit of currency" of intracellular energy transfer. ATP transports chemical energy within cells for metabolism...
, CO
2, and H
2O using the
citric acid cycleThe citric acid cycle — also known as the tricarboxylic acid cycle , the Krebs cycle, or the Szent-Györgyi-Krebs cycle — is a series of chemical reactions which is used by all aerobic living organisms to generate energy through the oxidization of acetate derived from carbohydrates, fats and...
and the
electron transport chainAn electron transport chain couples electron transfer between an electron donor and an electron acceptor with the transfer of H+ ions across a membrane. The resulting electrochemical proton gradient is used to generate chemical energy in the form of adenosine triphosphate...
.
Hence the Krebs Cycle can start at acetyl-CoA when fat is being broken down for energy if there is little or no glucose available.
The energy yield of the complete oxidation of the fatty acid palmitate is 106 ATP. Unsaturated and odd-chain fatty acids require additional enzymatic steps for degradation.
Nutrition and health
Most of the lipid found in food is in the form of triacylglycerols, cholesterol, and phospholipids. A minimum amount of dietary fat is necessary to facilitate absorption of fat-soluble vitamins (
ARetinol is one of the animal forms of vitamin A. It is a diterpenoid and an alcohol. It is convertible to other forms of vitamin A, and the retinyl ester derivative of the alcohol serves as the storage form of the vitamin in animals....
, D,
ETocopherols are a class of chemical compounds of which many have vitamin E activity. It is a series of organic compounds consisting of various methylated phenols...
, and
KPhylloquinone is a polycyclic aromatic ketone, based on 2-methyl-1,4-naphthoquinone, with a 3-phytyl substituent.It is a fat-soluble vitamin that is stable to air and moisture but decomposes in sunlight. It is found naturally in a wide variety of green plants.-Terminology:It is often called...
) and carotenoids. Humans and other mammals have a dietary requirement for certain essential fatty acids, such as
linoleic acidLinoleic acid is an unsaturated n-6 fatty acid. It is a colorless liquid at room temperature. In physiological literature, it has a lipid number of 18:2...
(an
omega-6 fatty acidn−6 fatty acids are a family of unsaturated fatty acids that have in common a final carbon–carbon double bond in the n−6 position, that is, the sixth bond, counting from the methyl end.The biological effects of the n−6 fatty acids are largely mediated by their conversion to n-6 eicosanoids...
) and
alpha-linolenic acidα-Linolenic acid is an organic compound found in many common vegetable oils. In terms of its structure, it is named all-cis-9,12,15-octadecatrienoic acid. In physiological literature, it is given the name 18:3 ....
(an omega-3 fatty acid) because they cannot be synthesized from simple precursors in the diet. Both of these fatty acids are 18-carbon
polyunsaturated fatty acidsIn nutrition, polyunsaturated fat, or polyunsaturated fatty acid, are fatty acids in which more than one carbon–carbon double bond exists within the representative molecule. That is, the molecule has two or more points on its structure capable of supporting hydrogen atoms not currently part of the...
differing in the number and position of the double bonds. Most
vegetable oilsVegetable fats and oils are lipid materials derived from plants. Physically, oils are liquid at room temperature, and fats are solid. Chemically, both fats and oils are composed of triglycerides, as contrasted with waxes which lack glycerin in their structure...
are rich in linoleic acid (
safflowerSafflower is a highly branched, herbaceous, thistle-like annual. It is commercially cultivated for vegetable oil extracted from the seeds. Plants are 30 to 150 cm tall with globular flower heads having yellow, orange or red flowers. Each branch will usually have from one to five flower heads...
,
sunflowerSunflower oil is the non-volatile oil expressed from sunflower seeds. Sunflower oil is commonly used in food as a frying oil, and in cosmetic formulations as an emollient. Sunflower oil was first industrially produced in 1835 in the Russian Empire.- Composition :Sunflower oil is mainly a...
, and
corn oilCorn oil is oil extracted from the germ of corn . Its main use is in cooking, where its high smoke point makes refined corn oil a valuable frying oil. It is also a key ingredient in some margarines. Corn oil is generally less expensive than most other types of vegetable oils. One bushel of corn...
s). Alpha-linolenic acid is found in the green leaves of plants, and in selected seeds, nuts, and legumes (in particular
flaxLinseed oil, also known as flaxseed oil, is a clear to yellowish oil obtained from the dried ripe seeds of the flax plant . The oil is obtained by cold pressing, sometimes followed by solvent extraction...
,
rapeseedRapeseed , also known as rape, oilseed rape, rapa, rappi, rapaseed is a bright yellow flowering member of the family Brassicaceae...
,
walnutJuglans is a plant genus of the family Juglandaceae, the seeds of which are known as walnuts. They are deciduous trees, 10–40 meters tall , with pinnate leaves 200–900 millimetres long , with 5–25 leaflets; the shoots have chambered pith, a character shared with the wingnuts , but not the hickories...
, and soy).
Fish oilFish oil is oil derived from the tissues of oily fish. Fish oils contain the omega-3 fatty acids eicosapentaenoic acid , and docosahexaenoic acid , precursors of certain eicosanoids that are known to reduce inflammation throughout the body, and are thought to have many health benefits.Fish do not...
s are particularly rich in the longer-chain omega-3 fatty acids
eicosapentaenoic acidEicosapentaenoic acid is an omega-3 fatty acid. In physiological literature, it is given the name 20:5. It also has the trivial name timnodonic acid...
(EPA) and
docosahexaenoic acidDocosahexaenoic acid is an omega-3 fatty acid that is a primary structural component of the human brain and retina. In chemical structure, DHA is a carboxylic acid with a 22-carbon chain and six cis double bonds; the first double bond is located at the third carbon from the omega end...
(DHA). A large number of studies have shown positive health benefits associated with consumption of omega-3 fatty acids on infant development, cancer, cardiovascular diseases, and various mental illnesses, such as depression, attention-deficit hyperactivity disorder, and dementia. In contrast, it is now well-established that consumption of
trans fatTrans fat is the common name for unsaturated fat with trans-isomer fatty acid. Because the term refers to the configuration of a double carbon-carbon bond, trans fats are sometimes monounsaturated or polyunsaturated, but never saturated....
s, such as those present in partially hydrogenated vegetable oils, are a risk factor for
cardiovascular diseaseHeart disease or cardiovascular disease are the class of diseases that involve the heart or blood vessels . While the term technically refers to any disease that affects the cardiovascular system , it is usually used to refer to those related to atherosclerosis...
.
A few studies have suggested that total dietary fat intake is linked to an increased risk of obesity and diabetes. However, a number of very large studies, including the Women's Health Initiative Dietary Modification Trial, an eight year study of 49,000 women, the Nurses' Health Study and the Health Professionals Follow-up Study, revealed no such links. None of these studies suggested any connection between percentage of calories from fat and risk of cancer, heart disease, or weight gain. The Nutrition Source, a website maintained by the Department of Nutrition at the
Harvard School of Public HealthThe Harvard School of Public Health is one of the professional graduate schools of Harvard University, located in the Longwood Area of the Boston, Massachusetts neighborhood of Mission Hill, which is next to Harvard Medical School. HSPH is considered a significant school focusing on health in the...
, summarizes the current evidence on the impact of dietary fat: "Detailed research—much of it done at Harvard—shows that the total amount of fat in the diet isn't really linked with weight or disease."
See also
- Lipidomics
Lipidomics may be defined as the large-scale study of pathways and networks of cellular lipids in biological systems The word "lipidome" is used to describe the complete lipid profile within a cell, tissue or organism and is a subset of the "metabolome" which also includes the three other major...
- Essential fatty acid
Essential fatty acids, or EFAs, are fatty acids that humans and other animals must ingest because the body requires them for good health but cannot synthesize them...
- Lipid microdomain
Lipid microdomains are formed when lipids undergo lateral phase separations yielding stable coexisting lamellar domains. These phase separations can be induced by changes in temperature, pressure, ionic strength or by the addition of divalent cations or proteins...
- Lipid signaling
Lipid signaling, broadly defined, refers to any biological signaling event involving a lipid messenger that binds a protein target, such as a receptor, kinase or phosphatase, which in turn mediate the effects of these lipids on specific cellular responses...
- Protein-lipid interaction
Protein–lipid interaction is the influence of membrane proteins on the lipid physical state or vice versa.The questions which are relevant to understanding of the structure and function of the membrane are: 1) Do intrinsic membrane proteins bind tightly to lipids, and what is the nature of the...
- Lipoprotein
A lipoprotein is a biochemical assembly that contains both proteins and lipids water-bound to the proteins. Many enzymes, transporters, structural proteins, antigens, adhesins, and toxins are lipoproteins...
External links
Introductory
Nomenclature
Databases
- LIPID MAPS - Comprehensive lipid and lipid-associated gene/protein databases.
- LipidBank - Japanese database of lipids and related properties, spectral data and references.
- LIPIDAT - Database composed mainly of phospholipids and associated thermodynamic data.
General
- ApolloLipids - Provides dyslipidemia and cardiovascular disease prevention and treatment information as well as continuing medical education programs
- National Lipid Association - Professional medical education organization for health care professionals who seek to prevent morbidity and mortality stemming from dyslipidemias and other cholesterol-related disorders.