An
electron transport chain couples a chemical reaction between an electron donor (such as
NADHNicotinamide adenine dinucleotide, abbreviated NAD
+, is a coenzyme found in all living cells. The compound is a dinucleotide, since it consists of two nucleotides joined through their phosphate groups, with one nucleotide containing an adenine base and the other containing...
) and an electron acceptor (such as
O2Oxygen Oxygen Oxygen (acid, literally "sharp", from the taste of acids) and -γενής (-genēs) (producer, literally begetter) is the element with atomic number 8 and represented by the symbol O...
) to the transfer of
H+ ionsThe proton is a subatomic particle with an electric charge of +1 elementary charge. It is found in the nucleus of each atom but is also stable by itself and has a second identity as the hydrogen ion, H
+...
across a
membraneThe cell membrane is the biological membrane separating the interior of a cell from the outside environment....
, through a set of mediating biochemical reactions. These H
+ ions are used to produce
adenosine triphosphateAdenosine-5'-triphosphate is a multifunctional nucleotide that plays an important role in cell biology as a coenzyme, that is, the "molecular unit of currency" of intracellular energy transfer. ATP transports chemical energy within cells for metabolism...
(
ATP), the main energy intermediate in living organisms, as they move back across the membrane. Electron transport chains are used for extracting energy from sunlight (
photosynthesisPhotosynthesis is a process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of Bacteria, but not in Archaea...
) and from
redoxRedox describes all chemical reactions in which atoms have their oxidation number changed....
reactions such as the oxidation of sugars (
respirationCellular respiration is one of the key ways a cell gains useful energy. It is the set of the metabolic reactions and processes that take place in organisms' cells to convert biochemical energy from nutrients into adenosine triphosphate , and then release waste products...
).
In
chloroplastChloroplasts are organelles found in plant cells and other eukaryotic organisms that conduct photosynthesis. Chloroplasts capture light energy to conserve free energy in the form of ATP and reduce NADP to NADPH through a complex set of processes called photosynthesis.The word chloroplast is...
s, light drives the conversion of water to oxygen and
NADP+Nicotinamide adenine dinucleotide phosphate is used in anabolic reactions, such as lipid and nucleic acid synthesis, which require NADPH as a reducing agent....
to NADPH and a transfer of H
+ ions. NADPH is used as an electron donor for
carbon fixationCarbon fixation refers to any process through which gaseous carbon dioxide is converted into a solid compound. It mostly refers to the processes found in autotrophs , usually driven by photosynthesis, whereby carbon dioxide is changed into sugars...
. In
mitochondriaIn cell biology, a mitochondrion is a membrane-enclosed organelle found in most eukaryotic cells. These organelles range from 0.5–10 micrometers in diameter...
, it is the conversion of oxygen to water, NADH to NAD
+ and succinate to fumarate that drives the transfer of H
+ ions. While some bacteria have electron transport chains similar to those in chloroplasts or mitochondria, other bacteria use different electron donors and acceptors. Both the respiratory and photosynthetic electron transport chains are major sites of premature electron leakage to
oxygenOxygen Oxygen Oxygen (acid, literally "sharp", from the taste of acids) and -γενής (-genēs) (producer, literally begetter) is the element with atomic number 8 and represented by the symbol O...
, thus being major sites of
superoxideSuperoxide is an anion with the chemical formula O
2−. It is important as the product of the one-electron reduction of dioxygen O
2, which occurs widely in nature...
production and drivers of
oxidative stressOxidative stress is caused by an imbalance between the production of reactive oxygen and a biological system's ability to readily detoxify the reactive intermediates or easily repair the resulting damage. All forms of life maintain a reducing environment within their cells...
.
Background
The electron transport chain is also called the
ETC.
An
enzymeEnzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process are called substrates, and the enzyme converts them into different molecules, called the products. Almost all processes in a biological cell need enzymes to occur at...
called
ATP synthaseAn ATP synthase is a general term for an enzyme that can synthesize adenosine triphosphate from adenosine diphosphate and inorganic phosphate by using some form of energy...
catalyzes a reaction to generate ATP. The structure of this enzyme and its underlying
genetic codeThe genetic code is the set of rules by which information encoded in genetic material is translated into proteins by living cells. A more precise term for the concept might be "genetic cipher". The code defines a mapping between tri-nucleotide sequences, called codons, and amino acids...
is remarkably
conservedIn biology, conserved sequences are similar or identical sequences that occur within nucleic acid sequences , protein sequences, protein structures or polymeric carbohydrates across species or within different molecules produced by the same organism...
in all known forms of life.
ATP synthase is powered by a transmembrane electrochemical
potential gradientA potential gradient is the local space rate of change of the potential with respect to displacement.In electrostatics then, it is the local space rate of change of the electric potential:Units are volts per meter...
usually in the form of a proton gradient. The function of the electron transport chain is to produce this gradient. In all living organisms, a series of redox reactions is used to produce a transmembrane electrochemical potential gradient.
RedoxRedox describes all chemical reactions in which atoms have their oxidation number changed....
reactions are chemical reactions in which electrons are transferred from a donor molecule to an acceptor molecule. The underlying force driving these reactions is the
Gibbs free energyIn thermodynamics, the Gibbs free energy is a thermodynamic potential that measures the "useful" or process-initiating work obtainable from an isothermal, isobaric thermodynamic system...
of the reactants and products. The Gibbs free energy is the energy available ("free") to do work. Any reaction that decreases the overall Gibbs free energy of a system will proceed spontaneously.
The transfer of electrons from a high-energy molecule (the donor) to a lower-energy molecule (the acceptor) can be
spatially separated into a series of intermediate redox reactions. This is an electron transport chain.
The fact that a reaction is
thermodynamicallyIn physics, thermodynamics is the study of the conversion of energy into work and heat and its relation to macroscopic variables such as temperature, volume and pressure...
possible does not mean that it will actually occur; for example, a mixture of hydrogen gas and oxygen gas does not spontaneously ignite. It is necessary either to supply an
activation energyIn chemistry, activation energy is a term introduced in 1889 by the Swedish scientist Svante Arrhenius, that is defined as the energy that must be overcome in order for a chemical reaction to occur. Arrhenius' research was a follow up of the theories of reaction rate by Serbian physicist Nebojsa...
or to lower the intrinsic activation energy of the system, in order to make most biochemical reactions proceed at a useful rate. Living systems use complex
macromolecularA macromolecule is a very large molecule most often created by some form of polymerization. In the context of biochemistry, the term may be applied to the four conventional biopolymers , as well as non-polymeric molecules with large molecular mass such as macrocycles...
structures (enzymes) to lower the activation energies of biochemical reactions.
It is possible to couple a thermodynamically favorable reaction (a transition from a high-energy state to a lower-energy state) to a thermodynamically unfavorable reaction (such as a separation of charges, or the creation of an
osmoticOsmosis is the diffusion of water through a semi-permeable membrane. More specifically, it is the movement of water across a semi-permeable membrane from an area of high water potential to an area of low water potential...
gradient), in such a way that the overall free energy of the system decreases (making it thermodynamically possible), while useful
workIn thermodynamics, work performed by a system is the quantity of energy transferred by the system to another due to changes in the external parameters of the system. If these changes happen in a reversible way, then the performed work does not lead to a change of the entropy. It is a...
is done at the same time. Biological macromolecules that catalyze a thermodynamically unfavorable reaction
if and only if a thermodynamically favorable reaction occurs simultaneously underlie all known forms of life.
Electron transport chains capture energy in the form of a transmembrane electrochemical potential gradient. This energy can then be harnessed to do useful work. The gradient can be used to transport molecules across membranes. It can be used to do mechanical work, such as rotating bacterial flagella, and also to produce
ATPAdenosine-5'-triphosphate is a multifunctional nucleotide that plays an important role in cell biology as a coenzyme, that is, the "molecular unit of currency" of intracellular energy transfer. ATP transports chemical energy within cells for metabolism...
, a high-energy molecule which can go on to power other cellular reactions.
A small amount of ATP is available from
substrate-level phosphorylationSubstrate-level phosphorylation is a type of chemical reaction that results in the formation and creation of adenosine triphosphate by the direct transfer and donation of a phosphoryl group to adenosine diphosphate from a reactive intermediate...
(for example, in
glycolysisGlycolysis is the metabolic pathway that converts glucose, C
6H
12O
6, into pyruvate, C
3H
6O
3-...
). Some organisms can obtain ATP exclusively by
fermentationFermentation is the process of deriving energy from the oxidation of organic compounds, such as carbohydrates, using an endogenous electron acceptor, which is usually an organic compound. This is in contrast to cellular respiration, where electrons are donated to an exogenous electron acceptor,...
. In most organisms, however, the majority of ATP is generated by electron transport chains.
Electron transport chains in mitochondria
The cells of almost all eukaryotes contain intracellular
organelleIn cell biology, an organelle is a specialized subunit within a cell that has a specific function, and is usually separately enclosed within its own lipid membrane....
s called mitochondria, which produce ATP. Energy sources such as glucose are initially metabolized in the
cytoplasmThe cytoplasm is the part of a cell that is enclosed within the cell membrane. In eukaryotic cells, the cytoplasm contains organelles, such as mitochondria, which are filled with liquid that is kept separate from the rest of the cytoplasm by biological membranes. The contents of the cell nucleus...
. The products are imported into mitochondria. Mitochondria continue the process of
catabolismCatabolism is the set of metabolic pathways that break down molecules into smaller units and release energy. In catabolism, large molecules such as polysaccharides, lipids, nucleic acids and proteins are broken down into smaller units such as monosaccharides, fatty acids, nucleotides and amino...
using
metabolicMetabolism is the set of chemical reactions that occur in living organisms to maintain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolism is usually divided into two categories. Catabolism breaks down organic matter,...
pathways including the Krebs cycle,
fatty acidIn chemistry, especially biochemistry, a fatty acid is a carboxylic acid often with a long unbranched aliphatic tail , which is either saturated or unsaturated...
oxidation, and
amino acidAmino acids are molecules containing an amine group, a carboxylic acid group and one of the twenty R-groups. These molecules are particularly important in biochemistry, where this term refers to alpha-amino acids with the general formula H
2NCHRCOOH, where R is an organic substituent...
oxidation.
The end result of these pathways is the production of two kinds of energy-rich electron donors, NADH and succinate. Electrons from these donors are passed through an electron transport chain to oxygen, which is reduced to water. This is a multi-step redox process that occurs on the mitochondrial inner membrane. The enzymes that catalyze these reactions have the ability to simultaneously create a proton gradient across the membrane, producing a thermodynamically unlikely high-energy state with the potential to do work. Although electron transport occurs with great efficiency, a small percentage of electrons are prematurely leaked to oxygen, resulting in the formation of the toxic free-radical
superoxideSuperoxide is an anion with the chemical formula O
2−. It is important as the product of the one-electron reduction of dioxygen O
2, which occurs widely in nature...
.
The similarity between intracellular mitochondria and free-living bacteria is striking. The known structural, functional, and
DNADeoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms and some viruses. The main role of DNA molecules is the long-term storage of information...
similarities between mitochondria and bacteria provide strong evidence that mitochondria evolved from intracellular bacterial symbionts (
see Endosymbiotic theoryThe endosymbiotic theory concerns the origins of mitochondria and plastids , which are organelles of eukaryotic cells. According to this theory, these organelles originated as separate prokaryotic organisms that were taken inside the cell as endosymbionts...
).
Mitochondrial redox carriers
Four membrane-bound complexes have been identified in mitochondria. Each is an extremely complex transmembrane structure that is embedded in the inner membrane. Three of them are
proton pumpA proton pump is an integral membrane protein that is capable of moving protons across the membrane of a cell, mitochondrion, or other subcellular compartment.-Function:...
s. The structures are electrically connected by lipid-soluble electron carriers and water-soluble electron carriers. The overall electron transport chain
NADH →
Complex I →
Q →
Complex III →
cytochrome c →
Complex IV →
O2
↑
Complex II
Complex I
Complex I (
NADH dehydrogenaseNADH dehydrogenase is an enzyme located in the inner mitochondrial membrane that catalyzes the transfer of electrons from NADH to coenzyme Q .It is also called the NADH:quinone oxidoreductase.-Reaction:...
, also called NADH:ubiquinone oxidoreductase; ) removes two electrons from NADH and transfers them to a lipid-soluble carrier,
ubiquinone (Q). The reduced product,
ubiquinolUbiquinol is a benzoquinol and is the reduced product of ubiquinone also called coenzyme Q10.The reduction of ubiquinone to ubiquinol occurs in Complexes I & II in the electron transfer chain. The Q cycle is a process that occurs in cytochrome b, a component of Complex III in the...
(QH
2) is free to diffuse within the membrane. At the same time,
Complex I moves four protons (H
+) across the membrane, producing a proton gradient. Complex I is one of the main sites at which premature electron leakage to oxygen occurs, thus being one of main sites of production of a harmful free radical called
superoxideSuperoxide is an anion with the chemical formula O
2−. It is important as the product of the one-electron reduction of dioxygen O
2, which occurs widely in nature...
.
The pathway of electrons occurs as follows:
NADH is oxidized to NAD
+, reducing
Flavin mononucleotideFlavin mononucleotide , or riboflavin-5′-phosphate, is a biomolecule produced from riboflavin by the enzyme riboflavin kinase and functions as prosthetic group of various oxidoreductases including NADH dehydrogenase...
to FMNH
2 in one two-electron step. The next electron carrier is a
Fe-S clusterFor biological Fe-S clusters, see iron-sulfur proteins.Iron-sulfur clusters are ensembles of iron and sulfide centres. Fe-S clusters are most often discussed in the context of the biological role for iron-sulfur proteins. Many Fe-S clusters are known in the area of organometallic chemistry and as...
, which can only accept one electron at a time to reduce the
ferricFerric refers to iron-containing materials or compounds. In chemistry the term is reserved for iron with an oxidation number of +3, also denoted iron or Fe3+. On the other hand, ferrous refers to iron with oxidation number of +2, denoted iron or Fe2+...
ion into a
ferrousFerrous, in chemical science, indicates a bivalent iron compound , as opposed to ferric, which indicates a trivalent iron compound ....
ion. In a convenient manner, FMNH
2 can be oxidized in only two one-electron steps, through a semiquinone intermediate. The electron thus travels from the FMNH
2 to the Fe-S cluster, then from the Fe-S cluster to the oxidized Q to give the free-radical (semiquinone) form of Q. This happens again to reduce the semiquinone form to the ubiquinol form, QH
2. During this process, four protons are translocated across the inner mitochondrial membrane, from the matrix to the intermembrane space. This creates a proton gradient that will be later used to generate ATP through
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 carry out oxidative phosphorylation to produce ATP, the molecule that...
.
Complex II
Complex II (succinate dehydrogenase; ) is not a proton pump. It serves to funnel additional electrons into the quinone pool (Q) by removing electrons from succinate and transferring them (via
FADIn biochemistry, flavin adenine dinucleotide is a redox cofactor involved in several important reactions in metabolism. FAD can exist in two different redox states and its biochemical role usually involves changing between these two states...
) to Q. Complex II consists of four protein subunits:
SDHASDHA is an acronym for succinate dehydrogenase complex subunit A.The term SDHA can refer to;* The protein subunit itself.* The gene that codes for this protein....
,
SDHBSDHB is an acronym for succinate dehydrogenase complex subunit B.The term SDHB can refer to:* The protein subunit itself.* The gene that codes for this protein....
,SDHC, and
SDHDSDHD, which stands for succinate dehydrogenase complex subunit D, is one of the two transmembrane subunits of the four-subunit succinate dehydrogenase protein complex that resides in the inner mitochondrial membrane. It also refers to the gene that codes for this protein. The other transmembrane...
. Other electron donors (e.g., fatty acids and glycerol 3-phosphate) also funnel electrons into Q (via FAD), again without producing a proton gradient.
Complex III
Complex III (cytochrome
bc1 complex; ) removes in a stepwise fashion two electrons from QH
2 at the Q
O site and sequentially transfers them to two molecules of
cytochrome cCytochrome c, or cyt c is a small heme protein found loosely associated with the inner membrane of the mitochondrion. It belongs to the cytochrome c family of proteins...
, a water-soluble electron carrier located within the intermembrane space. The two other electrons are sequentially passed across the protein to the Q
i site where
quinoneQuinones are "compounds having a fully conjugated cyclic dione structure, such as that of benzoquinones, derived from aromatic compounds by conversion of an even number of –CH= groups into –C– groups with any necessary rearrangement of double bonds ."Benzoquinone, sometimes referred to simply as...
part of ubiquinone is reduced to quinol. A proton gradient is formed because it takes 2 quinol (4H+4e-) oxidations at the Q
o site to form one quinol (2H+2e-) at the Q
i site. (in total 6 protons: 2 protons reduce quinone to quinol and 4 protons are released from 2 ubiquinol). The bc1 complex does NOT 'pump' protons, it helps build the proton gradient by an asymmetric absorption/release of protons.
When electron transfer is hindered (by a high membrane potential, point mutations or respiratory inhibitors such as antimycin A), Complex III may leak electrons to oxygen resulting in the formation of
superoxideSuperoxide is an anion with the chemical formula O
2−. It is important as the product of the one-electron reduction of dioxygen O
2, which occurs widely in nature...
, a highly-toxic species, which is thought to contribute to the pathology of a number of diseases, including aging.
Complex IV
Complex IV (
cytochrome c oxidaseThe enzyme cytochrome c oxidase or Complex IV is a large transmembrane protein complex found in bacteria and the mitochondrion.It is the last enzyme in the respiratory electron transport chain of mitochondria located in the mitochondrial membrane...
; ) removes four electrons from four molecules of
cytochrome cCytochrome c, or cyt c is a small heme protein found loosely associated with the inner membrane of the mitochondrion. It belongs to the cytochrome c family of proteins...
and transfers them to molecular oxygen (O
2), producing two molecules of water (H
2O). At the same time, it moves four protons across the membrane, producing a proton gradient.
Coupling with oxidative phosphorylation
The
chemiosmotic coupling hypothesisChemiosmosis is the diffusion of ions across a selectively-permeable membrane. More specifically, it relates to the generation of ATP by the movement of hydrogen ions across a membrane during cellular respiration....
, as proposed by
Nobel Prize in ChemistryThe Nobel Prize in Chemistry is awarded annually by the Royal Swedish Academy of Sciences to scientists in the various fields of chemistry. It is one of the five Nobel Prizes established by the will of Alfred Nobel in 1895, awarded for outstanding contributions in chemistry, physics, literature,...
winner
Peter D. MitchellPeter Dennis Mitchell was a British biochemist who was awarded the 1978 Nobel Prize for Chemistry for his discovery of the chemiosmotic mechanism of ATP synthesis.Mitchell was born in Mitcham, Surrey, England....
, explains that the electron transport chain and
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 carry out oxidative phosphorylation to produce ATP, the molecule that...
are coupled by a proton gradient across the inner mitochondrial membrane. The efflux of protons creates both a
pHpH is a measure of the acidity or basicity of a solution. It is defined as the cologarithm of the activity of dissolved hydrogen ions . Hydrogen ion activity coefficients cannot be measured experimentally, so they are based on theoretical calculations...
gradient and an
electrochemical gradientAn electrochemical gradient is a spatial variation of both electrical potential and chemical concentration across a membrane. Both components are often due to ion gradients, particularly proton gradients, and the result can be a type of potential energy available for work in a cell...
. This proton gradient is used by the F
OF
1 ATP synthaseAn ATP synthase is a general term for an enzyme that can synthesize adenosine triphosphate from adenosine diphosphate and inorganic phosphate by using some form of energy...
complex to make ATP via
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 carry out oxidative phosphorylation to produce ATP, the molecule that...
. ATP synthase is sometimes regarded as
complex V of the electron transport chain. The F
O component of
ATP synthaseAn ATP synthase is a general term for an enzyme that can synthesize adenosine triphosphate from adenosine diphosphate and inorganic phosphate by using some form of energy...
acts as an
ion channelIon channels are pore-forming proteins that help establish and control the small voltage gradient across the plasma membrane of all living cells by allowing the flow of ions down their electrochemical gradient. They are present in the membranes that surround all biological cells...
for return of protons back to mitochondrial matrix. During their return, the free energy produced during the generation of the oxidized forms of the electron carriers (
NAD+Nicotinamide adenine dinucleotide, abbreviated NAD
+, is a coenzyme found in all living cells. The compound is a dinucleotide, since it consists of two nucleotides joined through their phosphate groups, with one nucleotide containing an adenine base and the other containing...
and
QQ is the seventeenth letter of the basic modern Latin alphabet. Its name in English is spelled cue.- History :The Semitic sound value of Qôp was , a sound common to Semitic languages, but not found in English or most Indo-European ones...
) is released. This energy is used to drive ATP synthesis, catalyzed by the F
1 component of the complex.
Coupling with oxidative phosphorylation is a key step for ATP production. However, in certain cases, uncoupling may be biologically useful. The inner mitochondrial membrane of
brown adipose tissueBrown adipose tissue or brown fat is one of two types of fat or adipose tissue found in mammals. It is especially abundant in newborns and in hibernating mammals. Its primary function is to generate body heat in animals or newborns that do not shiver...
contains a large amount of
thermogeninThermogenin is an uncoupling protein found in the mitochondria of brown adipose tissue . It is used to generate heat by non-shivering thermogenesis...
(an uncoupling protein), which acts as uncoupler by forming an alternative pathway for the flow of protons back to matrix. This results in consumption of energy in
thermogenesisThermogenesis is the process of heat production in organisms. It occurs mostly in warm-blooded animals, but a few species of thermogenic plants exist.-Types:...
rather than ATP production. This may be useful in cases when heat production is required, for example in colds or during arise of
hibernatingHibernation is a state of inactivity and metabolic depression in animals, characterized by lower body temperature, slower breathing, and lower metabolic rate. Hibernating animals conserve energy, especially during winter when food is short, tapping energy reserves, body fat, at a slow rate...
animals. Synthetic uncouplers (e.g.,
2,4-dinitrophenol2,4-Dinitrophenol , C6H4N2O5, is a cellular metabolic poison. It uncouples oxidative phosphorylation by carrying protons across the mitochondrial membrane, leading to a rapid consumption of energy without generation of ATP.Dinitrophenols as a class of...
) also exist, and, at high doses, are lethal.
Summary
The mitochondrial electron transport chain removes electrons from an electron donor (NADH or QH
2) and passes them to a terminal electron acceptor (O
2) via a series of redox reactions. These reactions are coupled to the creation of a proton gradient across the mitochondrial inner membrane. There are three proton pumps:
I,
III, and
IV. The resulting transmembrane proton gradient is used to make ATP via ATP synthase.
The reactions catalyzed by
Complex I and
Complex III exist roughly at equilibrium. This means that these reactions are readily reversible, simply by increasing the concentration of the products relative to the concentration of the reactants (for example, by increasing the proton gradient). ATP synthase is also readily reversible. Thus ATP can be used to make a proton gradient, which in turn can be used to make NADH. This process of
reverse electron transport is important in many prokaryotic electron transport chains.
Electron transport chains in bacteria
In eukaryotes, NADH is the most important electron donor. The associated electron transport chain is
NADH →
Complex I →
Q →
Complex III →
cytochrome c →
Complex IV →
O2
where
Complexes I, III and
IV are proton pumps, while Q and cytochrome
c are mobile electron carriers. The electron acceptor is molecular oxygen.
In prokaryotes (
bacteriaThe bacteria are a large group of unicellular microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
and
archaeaThe Archaea are a group of single-celled microorganisms. A single individual or species from this domain is called an archaeon . They have no cell nucleus or any other organelles within their cells...
) the situation is more complicated, because there is a number of different electron donors and a number of different electron acceptors. The generalized electron transport chain in bacteria is:
Donor Donor Donor
↓ ↓ ↓
dehydrogenase →
quinone →
bc1 →
cytochrome
↓ ↓
oxidase(reductase) oxidase(reductase)
↓ ↓
Acceptor Acceptor
Note that electrons can enter the chain at three levels: at the level of a
dehydrogenaseA dehydrogenase is an enzyme that oxidizes a substrate by transferring one or more hydrides to an acceptor, usually NAD+/NADP+ or a flavin coenzyme such as FAD or FMN.-Examples:...
, at the level of the quinone pool, or at the level of a mobile
cytochromeCytochromes are, in general, membrane-bound hemoproteins that contain heme groups and carry out electron transport.They are found either as monomeric proteins or as subunits of bigger enzymatic complexes that catalyze redox reactions...
electron carrier. These levels correspond to successively more positive redox potentials, or to successively decreased potential differences relative to the terminal electron acceptor. In other words, they correspond to successively smaller Gibbs free energy changes for the overall redox reaction
Donor → Acceptor.
Individual bacteria use multiple electron transport chains, often simultaneously. Bacteria can use a number of different electron donors, a number of different dehydrogenases, a number of different oxidases and reductases, and a number of different electron acceptors. For example,
E. coli (when growing aerobically using glucose as an energy source) uses two different NADH dehydrogenases and two different quinol oxidases, for a total of four different electron transport chains operating simultaneously.
A common feature of all electron transport chains is the presence of a proton pump to create a transmembrane proton gradient. Bacterial electron transport chains may contain as many as three proton pumps, like mitochondria, or they may contain only one or two. They always contain at least one proton pump.
Electron donors
In the present day biosphere, the most common electron donors are organic molecules. Organisms that use organic molecules as an energy source are called
organotrophs. Organotrophs (animals, fungi, protists) and
phototrophs (plants and algae) constitute the vast majority of all familiar life forms.
Some prokaryotes can use inorganic matter as an energy source. Such organisms are called
lithotrophs ("rock-eaters"). Inorganic electron donors include hydrogen, carbon monoxide, ammonia, nitrite, sulfur, sulfide, and ferrous iron. Lithotrophs have been found growing in rock formations thousands of meters below the surface of Earth. Because of their volume of distribution, lithotrophs may actually outnumber organotrophs and phototrophs in our biosphere.
The use of inorganic electron donors as an energy source is of particular interest in the study of evolution. This type of metabolism must logically have preceded the use of organic molecules as an energy source.
Dehydrogenases
Bacteria can use a number of different electron donors. When organic matter is the energy source, the donor may be NADH or succinate, in which case electrons enter the electron transport chain via NADH dehydrogenase (similar to
Complex I in mitochondria) or succinate dehydrogenase (similar to
Complex II). Other dehydrogenases may be used to process different energy sources: formate dehydrogenase, lactate dehydrogenase, glyceraldehyde-3-phosphate dehydrogenase, H
2 dehydrogenase (
hydrogenaseA hydrogenase is an enzyme that catalyses the reversible oxidation of molecular hydrogen . Hydrogenases play a vital role in anaerobic metabolism....
), etc. Some dehydrogenases are also proton pumps; others simply funnel electrons into the quinone pool.
Most of dehydrogenases are synthesized only when needed. Depending on the environment in which they find themselves, bacteria select different enzymes from their DNA library and synthesize only those that are needed for growth.Enzymes that are synthesized only when needed are said to be 'inducible'.
Quinone carriers
Quinones are mobile, lipid-soluble carriers that shuttle electrons (and protons) between large, relatively immobile macromolecular complexes embedded in the membrane. Bacteria use
ubiquinone (the same quinone that mitochondria use) and related quinones such as
menaquinone.
Proton pumps
A
proton pump is any process that creates a proton gradient across a membrane. Protons can be physically moved across a membrane; this is seen in mitochondrial
Complexes I and
IV. The same effect can be produced by moving electrons in the opposite direction. The result is the disappearance of a proton from the cytoplasm and the appearance of a proton in the periplasm. Mitochondrial
Complex III uses this second type of proton pump, which is mediated by a quinone (the
Q cycle- History :The Q cycle describes a series of reactions first proposed by Peter Mitchell that describe how the sequential oxidation and reduction of the lipophilic electron carrier, ubiquinol-ubiquinone , can result in the net pumping of protons across a lipid bilayer...
).
Some dehydrogenases are proton pumps; others are not. Most oxidases and reductases are proton pumps, but some are not. Cytochrome
bc1 is a proton pump found in many, but not all, bacteria (it is not found in
E. coli). As the name implies, bacterial
bc1 is similar to mitochondrial
bc1 (
Complex III).
Proton pumps are the heart of the electron transport process. They produce the transmembrane electrochemical gradient that supplies energy to the cell.
Cytochrome electron carriers
Cytochromes are pigments that contain iron. They are found in two very different environments.
Some cytochromes are water-soluble carriers that shuttle electrons to and from large, immobile macromolecular structures imbedded in the membrane. The mobile cytochrome electron carrier in mitochondria is cytochrome
c. Bacteria use a number of different mobile cytochrome electron carriers.
Other cytochromes are found within macromolecules such as
Complex III and
Complex IV. They also function as electron carriers, but in a very different, intramolecular, solid-state environment.
Electrons may enter an electron transport chain at the level of a mobile cytochrome or quinone carrier. For example, electrons from inorganic electron donors (nitrite, ferrous iron, etc.) enter the electron transport chain at the cytochrome level. When electrons enter at a redox level greater than NADH, the electron transport chain must operate in reverse to produce this necessary, higher-energy molecule.
Terminal oxidases and reductases
When bacteria grow in aerobic environments, the terminal electron acceptor (O
2) is reduced to water by an enzyme called an
oxidase. When bacteria grow in
anaerobicHypoxia or oxygen depletion is a phenomenon that occurs in aquatic environments as dissolved oxygen becomes reduced in concentration to a point detrimental to aquatic organisms living in the system...
environments, the terminal electron acceptor is reduced by an enzyme called a
reductase.
In mitochondria the terminal membrane complex (
Complex IV) is cytochrome oxidase. Aerobic bacteria use a number of different terminal oxidases. For example,
E. coli does not have a cytochrome oxidase or a
bc1 complex. Under aerobic conditions, it uses two different terminal quinol oxidases (both proton pumps) to reduce oxygen to water.
AnaerobicAn anaerobic organism or anaerobe is any organism that does not require oxygen for growth and may even die in its presence. There are three types: obligate anaerobes, which cannot use oxygen for growth and are even harmed by it; aerotolerant organisms, which cannot use oxygen for growth, but...
bacteria, which do not use oxygen as a terminal electron acceptor, have terminal reductases individualized to their terminal acceptor. For example,
E. coli can use fumarate reductase, nitrate reductase, nitrite reductase, DMSO reductase, or trimethylamine-N-oxide reductase, depending on the availability of these acceptors in the environment.
Most terminal oxidases and reductases are
inducible. They are synthesized by the organism as needed, in response to specific environmental conditions.
Electron acceptors
Just as there are a number of different electron donors (organic matter in organotrophs, inorganic matter in lithotrophs), there are a number of different electron acceptors, both organic and inorganic. If oxygen is available, it is invariably used as the terminal electron acceptor, because it generates the greatest Gibbs free energy change and produces the most energy.
In anaerobic environments, different electron acceptors are used, including nitrate, nitrite, ferric iron, sulfate, carbon dioxide, and small organic molecules such as fumarate.
Since electron transport chains are redox processes, they can be described as the sum of two redox pairs. For example, the mitochondrial electron transport chain can be described as the sum of the NAD
+/NADH redox pair and the O
2/H
2O redox pair. NADH is the electron donor and O
2 is the electron acceptor.
Not every donor-acceptor combination is thermodynamically possible. The redox potential of the acceptor must be more positive than the redox potential of the donor. Furthermore, actual environmental conditions may be far different from
standard conditions (1 molar concentrations, 1 atm partial pressures, pH = 7), which apply to
standard redox potentials. For example, hydrogen-evolving bacteria grow at an ambient partial pressure of hydrogen gas of 10
-4 atm. The associated redox reaction, which is thermodynamically favorable in nature, is thermodynamically impossible under “standard” conditions.
Summary
Bacterial electron transport pathways are, in general, inducible. Depending on their environment, bacteria can synthesize different transmembrane complexes and produce different electron transport chains in their cell membranes. Bacteria select their electron transport chains from a DNA library containing multiple possible dehydrogenases, terminal oxidases and terminal reductases. The situation is often summarized by saying that electron transport chains in bacteria are
branched,
modular, and
inducible.
Photosynthetic electron transport chains
In
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 carry out oxidative phosphorylation to produce ATP, the molecule that...
, electrons are transferred from a high-energy electron donor (e.g., NADH) to an electron acceptor (e.g., O
2) through an electron transport chain. In
photophosphorylationThe production of ATP using the energy of sunlight is called photophosphorylation. Only two sources of energy are available to living organisms: sunlight and oxidation-reduction reactions...
, the energy of sunlight is used to
create a high-energy electron donor and an electron acceptor. Electrons are then transferred from the donor to the acceptor through another electron transport chain.
Photosynthetic electron transport chains have many similarities to the oxidative chains discussed above. They use mobile, lipid-soluble carriers (quinones) and mobile, water-soluble carriers (cytochromes, etc.). They also contain a proton pump. It is remarkable that the proton pump in
all photosynthetic chains resembles mitochondrial
Complex III.
Photosynthetic electron transport chains are discussed in greater detail in the articles
PhotophosphorylationThe production of ATP using the energy of sunlight is called photophosphorylation. Only two sources of energy are available to living organisms: sunlight and oxidation-reduction reactions...
,
PhotosynthesisPhotosynthesis is a process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of Bacteria, but not in Archaea...
, Photosynthetic reaction center and Light-dependent reaction.
Summary
Electron transport chains are redox reactions that transfer electrons from an electron donor to an electron acceptor. The transfer of electrons is coupled to the translocation of protons across a membrane, producing a proton gradient. The proton gradient is used to produce useful work.
The coupling of thermodynamically favorable to thermodynamically unfavorable biochemical reactions by biological macromolecules is an example of an
emergent property – a property that could not have been predicted, even given full knowledge of the primitive geochemical systems from which these macromolecules evolved. It is an open question whether such emergent properties evolve only by chance, or whether they
necessarily evolve in any large biogeochemical system, given the underlying laws of physics.
External links
- Complexes with cytochrome b-like domains - Bacterial and mitochondrial cytochrome c oxidases - Photosynthetic reaction centers and photosystems - Cytochrome c family - Cupredoxins - Adrenodoxin reductase - Electron transfer flavoproteins