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Porin (protein)

 

 
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Porin (protein)
 
 
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Porins are beta barrel
Beta barrel

A beta barrel is a large beta-sheet that twists and coils to form a closed structure in which the first strand is hydrogen bonded to the last....
 proteins that cross a cellular membrane
Cell membrane

The cell membrane is the interface between the cellular machinery inside the cell and the fluid outside.It is a semipermeable lipid bilayer found in all cell ....
 and act as a pore through which molecules can diffuse
Diffusion

Molecular diffusion, often called simply diffusion, is a net transport of molecules from a region of higher concentration to one of lower concentration by random molecular motion....
. Unlike other membrane transport
Membrane transport

Membrane transport is the Motion of biochemistrys and other atomic or molecule substances across biological membranes.Usually, two types are distinguished: Active transport requires chemical energy, while passive transport does not....
 proteins, porins are large enough to allow passive diffusion
Diffusion

Molecular diffusion, often called simply diffusion, is a net transport of molecules from a region of higher concentration to one of lower concentration by random molecular motion....
, i.e., they act as channels
Ion channel

Ion channels are pore-forming proteins that help establish and control the small voltage gradient across the plasma membrane of all living cell s by allowing the flow of ions down their electrochemical gradient....
 that are specific to different types of molecules. They are present in the outer membrane
Outer membrane

The bacterial outer membrane is found in Gram-negative bacteria. Its composition is distinct from that of the cytoplasmic membrane - among other things, the outer leaflet of the membrane include a complex lipopolysaccharide whose lipid portion acts as an endotoxin - and it is linked to the cell's peptidoglycan by Braun's lipoprotein....
 of Gram-negative bacteria and some Gram-positive bacteria of the group Mycolata, the mitochondria, and the chloroplast
Chloroplast

Chloroplasts are organelles found in plant cells and other eukaryote organisms that conduct photosynthesis. Chloroplasts capture light energy to conserve Thermodynamic free energy in the form of Adenosine triphosphate and reduce NADP to NADPH through a complex set of processes called photosynthesis....
.

ns are composed of beta sheet
Beta sheet

The ? sheet is the second form of regular secondary structure in proteins consisting of beta strands connected laterally by three or more hydrogen bonds, forming a generally twisted, pleated sheet ....
s, which are, in general, linked together by beta turns on the cytoplasm
Cytoplasm

The cytoplasm is the part of a Cell that is enclosed within the plasma membrane. In eukaryote cells the cytoplasm contains organelles, such as mitochondrion, that are filled with liquid kept separate from the rest of the cytoplasm by biological membranes....
ic side and long loops of amino acid
Amino acid

In chemistry, an amino acid is a molecule containing both amine and carboxyl functional groups. These molecules are particularly important in biochemistry, where this term refers to alpha-amino acids with the general formula H2NCHRCOOH, where R is an organic substituent....
s on the other.






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Porins are beta barrel
Beta barrel

A beta barrel is a large beta-sheet that twists and coils to form a closed structure in which the first strand is hydrogen bonded to the last....
 proteins that cross a cellular membrane
Cell membrane

The cell membrane is the interface between the cellular machinery inside the cell and the fluid outside.It is a semipermeable lipid bilayer found in all cell ....
 and act as a pore through which molecules can diffuse
Diffusion

Molecular diffusion, often called simply diffusion, is a net transport of molecules from a region of higher concentration to one of lower concentration by random molecular motion....
. Unlike other membrane transport
Membrane transport

Membrane transport is the Motion of biochemistrys and other atomic or molecule substances across biological membranes.Usually, two types are distinguished: Active transport requires chemical energy, while passive transport does not....
 proteins, porins are large enough to allow passive diffusion
Diffusion

Molecular diffusion, often called simply diffusion, is a net transport of molecules from a region of higher concentration to one of lower concentration by random molecular motion....
, i.e., they act as channels
Ion channel

Ion channels are pore-forming proteins that help establish and control the small voltage gradient across the plasma membrane of all living cell s by allowing the flow of ions down their electrochemical gradient....
 that are specific to different types of molecules. They are present in the outer membrane
Outer membrane

The bacterial outer membrane is found in Gram-negative bacteria. Its composition is distinct from that of the cytoplasmic membrane - among other things, the outer leaflet of the membrane include a complex lipopolysaccharide whose lipid portion acts as an endotoxin - and it is linked to the cell's peptidoglycan by Braun's lipoprotein....
 of Gram-negative bacteria and some Gram-positive bacteria of the group Mycolata, the mitochondria, and the chloroplast
Chloroplast

Chloroplasts are organelles found in plant cells and other eukaryote organisms that conduct photosynthesis. Chloroplasts capture light energy to conserve Thermodynamic free energy in the form of Adenosine triphosphate and reduce NADP to NADPH through a complex set of processes called photosynthesis....
.

Structure

Porins are composed of beta sheet
Beta sheet

The ? sheet is the second form of regular secondary structure in proteins consisting of beta strands connected laterally by three or more hydrogen bonds, forming a generally twisted, pleated sheet ....
s, which are, in general, linked together by beta turns on the cytoplasm
Cytoplasm

The cytoplasm is the part of a Cell that is enclosed within the plasma membrane. In eukaryote cells the cytoplasm contains organelles, such as mitochondrion, that are filled with liquid kept separate from the rest of the cytoplasm by biological membranes....
ic side and long loops of amino acid
Amino acid

In chemistry, an amino acid is a molecule containing both amine and carboxyl functional groups. These molecules are particularly important in biochemistry, where this term refers to alpha-amino acids with the general formula H2NCHRCOOH, where R is an organic substituent....
s on the other. The beta sheet
Beta sheet

The ? sheet is the second form of regular secondary structure in proteins consisting of beta strands connected laterally by three or more hydrogen bonds, forming a generally twisted, pleated sheet ....
s lie in an antiparallel fashion and form a cylindrical tube, called a beta barrel
Beta barrel

A beta barrel is a large beta-sheet that twists and coils to form a closed structure in which the first strand is hydrogen bonded to the last....
. The amino acid composition of the porin beta sheets is unique in that polar and nonpolar residues alternate along them. This means that the nonpolar residues face outward so as to interact with the nonpolar lipid
Lipid

Lipids are broadly defined as any fat-soluble , naturally-occurring molecule, such as fats, oils, waxes, cholesterol, sterols, fat-soluble vitamins , monoglycerides, diglycerides, phospholipids, and others....
 membrane, whereas the polar residues face inwards into the center of the beta barrel to interact with the aqueous channel.

The porin channel is partially blocked by a loop, called the eyelet, which projects into the cavity. In general, it is found between strands 5 and 6 of each barrel, and it defines the size of solute that can traverse the channel. It is lined almost exclusively with charged amino acids arranged on opposite sides of the channel, creating a transversal electric field across the pore. The eyelet has a local surplus of negative charges from four glutamic acid
Glutamic acid

Glutamic acid is one of the 20 proteinogenic amino acids and its codons are GAA and GAG. It is a non-essential amino acid. The carboxylate anions and salt of glutamic acid are known as glutamates....
 and seven aspartic acid
Aspartic acid

Aspartic acid is an a-amino acid with the chemical formula HO2CCHCH2CO2H. The carboxylate anion of aspartic acid is known as aspartate....
 residues (in contrast to one histidine
Histidine

Histidine is one of the 20 standard amino acids present in proteins. In the nutritional sense, in humans, histidine is considered an essential amino acid, but only in children....
, two lysine
Lysine

Lysine is an a-amino acid with the chemical formula HO2CCH4NH2. This amino acid is an essential amino acid, which means that humans cannot synthesize it....
 and three arginine
Arginine

Arginine is an a-amino acid. The Optical isomerism is one of the 20 most common natural amino acids. Its codons are CGU, CGC, CGA, CGG, AGA, and AGG....
  residues) is partially compensated for by two bound calcium atoms, and this asymmetric arrangement of molecules is thought to have an influence in the selection of molecules that can pass through the channel.

Cellular roles

Porins typically control the diffusion of small metabolites like sugars, ion
Ion

An ion is an atom or molecule which has lost or gained one or more electrons, giving it a positive or negative electrical charge. According to the Bohr_model this will be from or in the outer shield 'n'....
s, and amino acid
Amino acid

In chemistry, an amino acid is a molecule containing both amine and carboxyl functional groups. These molecules are particularly important in biochemistry, where this term refers to alpha-amino acids with the general formula H2NCHRCOOH, where R is an organic substituent....
s.

In gram-negative bacteria, the inner membrane is the major permeability barrier, whereas the outer membrane contains porins, which render it largely permeable to molecules less than about 1500 daltons
Atomic mass unit

The unified atomic mass unit , or dalton or, sometimes, universal mass unit, is a Units of measurement of mass used to express atomic weight and molecular masses....
.

The term "nucleoporin
Nucleoporin

The nucleoporins are a family of proteins which are the constituent building blocks of the nuclear pore complex. Nuclear pores in turn allow the transport of water-soluble molecules across the nuclear envelope....
" refers to porins facilitating transport through nuclear pore
Nuclear pore

Nuclear pores are large protein complexes that cross the nuclear envelope, which is the double Endomembrane system surrounding the eukaryote cell cell nucleus....
s in the nuclear envelope
Nuclear envelope

The nuclear envelope is a double lipid bilayer that encloses the genetic material in eukaryote cell . The nuclear envelope also serves as the physical barrier, separating the contents of the nucleus from the cytosol ....
. However, they are often considered distinct from other porins (they are not classified as porins in MeSH
Mesh

Mesh consists of semi-permeable barrier made of connected strands of metal, fiber, or other flexible/ductile material. Mesh is similar to spider web or Net in that it has many attached or woven strands....
.)

External links

  • at University of Hamburg
    University of Hamburg

    The University of Hamburg is a university in Hamburg, Germany. It was founded on 1 April 1919 by William Stern and others. It grew out of the previous Allgemeines Vorlesungswesen and the Kolonialinstitut as well as the Akademisches Gymnasium....
- OmpG porin - Trimeric porins - Sugar porins