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Porins are
beta barrel In protein structures, a beta barrel is a beta-sheet The beta sheet, (β-sheet) (also β-pleated sheet) is a common structural motif, motif of the regular protein secondary structure. Beta sheets consist of beta strands (β-strands) connected late ...
proteins Proteins are large biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and Sir John Cowdery Kendrew in 1958, for which they received a No ...

proteins
that cross a
cellular membrane cell membrane vs. Prokaryotes The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates the interior of all cells from the out ...

cellular membrane
and act as a pore, through which
molecules A molecule is an electrically Electricity is the set of physical phenomena associated with the presence and motion Image:Leaving Yongsan Station.jpg, 300px, Motion involves a change in position In physics, motion is the phenomenon ...

molecules
can
diffuse 250px, Diffusion from a microscopic and macroscopic point of view. Initially, there are solution, solute molecules on the left side of a barrier (purple line) and none on the right. The barrier is removed, and the solute diffuses to fill the wh ...

diffuse
. Unlike other
membrane transport protein A membrane transport protein (or simply transporter) is a membrane protein Membrane proteins are common proteins that are part of, or interact with, biological membranes. Membrane proteins fall into several broad categories depending on their l ...
s, porins are large enough to allow passive diffusion, i.e., they act as
channels Channel, channels, channeling, etc., may refer to: Geography * Channel (geography), in physical geography, a landform consisting of the outline (banks) of the path of a narrow body of water. Australia * Channel Country, region of outback Austr ...

channels
that are specific to different types of molecules. They are present in the outer membrane of
gram-negative bacteria Gram-negative bacteria are bacteria Bacteria (; common noun bacteria, singular bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), do ...
and some
gram-positive In bacteriology, gram-positive bacteria are bacteria that give a positive result in the Gram stain test, which is traditionally used to quickly classify bacteria into two broad categories according to their type of cell wall. Gram-positive bacte ...
mycobacteria ''Mycobacterium'' is a genus Genus /ˈdʒiː.nəs/ (plural genera /ˈdʒen.ər.ə/) is a taxonomic rank In biological classification In biology, taxonomy () is the scientific study of naming, defining (Circumscription (taxonomy), ci ...
(mycolic acid-containing
actinomycetes The Actinomycetales are an order of Actinobacteria. A member of the order is often called an actinomycete. The actinomycetes are diverse and contain a variety of subdivisions, as well as yet-unclassified isolates, mainly because some genera ar ...

actinomycetes
), the outer membrane of
mitochondria A mitochondrion (; ) is a double-membrane A membrane is a selective barrier; it allows some things to pass through but stops others. Such things may be molecules, ions, or other small particles. Biological membranes include cell membranes ...

mitochondria
, and the outer chloroplast membrane.


Structure

Porins are composed of
beta sheet The beta sheet, (β-sheet) (also β-pleated sheet) is a common motif Motif may refer to: General concepts * Motif (chess composition), an element of a move in the consideration of its purpose * Motif (folkloristics), a recurring element that cre ...
s (β sheets) made up of
beta strand Beta (, ; uppercase , lowercase , or cursive Cursive (also known as script, among other names) is any style of penmanship Penmanship is the technique of writing Writing is a medium of human communication that involves the represe ...

beta strand
s (β strands) which are linked together by beta turns on the
cytoplasm In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes ...
ic side and long loops of
amino acid Amino acids are organic compound In , organic compounds are generally any s that contain - . Due to carbon's ability to (form chains with other carbon s), millions of organic compounds are known. The study of the properties, reactions, a ...

amino acid
s on the other. The β strands lie in an antiparallel fashion and form a cylindrical tube, called a
beta barrel In protein structures, a beta barrel is a beta-sheet The beta sheet, (β-sheet) (also β-pleated sheet) is a common structural motif, motif of the regular protein secondary structure. Beta sheets consist of beta strands (β-strands) connected late ...
(β barrel). The amino acid composition of the porin β strands are 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 In chemistry Chemistry is the scientific Science () is a systematic enterprise that builds and organizes knowledge Knowledge is a familiarity or awareness, of someone or something, such as facts A fact is an occurrence in ...
lipid In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanis ...
s of outer membrane, whereas the polar residues face inwards into the center of the beta barrel to create the
aqueous An aqueous solution is a solution Solution may refer to: * Solution (chemistry) Image:SaltInWaterSolutionLiquid.jpg, upMaking a saline water solution by dissolving Salt, table salt (sodium chloride, NaCl) in water. The salt is the solute an ...
channel. The specific amino acids in the channel determine the specificity of the porin to different molecules. The β barrels that make up a porin are composed of as few as eight β strands to as many as twenty-two β strands. The individual strands are joined together by loops and turns. The majority of porins are
monomer In chemistry Chemistry is the scientific Science () is a systematic enterprise that builds and organizes knowledge Knowledge is a familiarity or awareness, of someone or something, such as facts A fact is an occurrence in th ...

monomer
s; however, some dimeric porins have been discovered, as well as an octameric porin. Depending on the size of the porin, the interior of the protein may either be filled with water, have up to two β strands folded back into the interior, or contain a "stopper" segment composed of β strands. All porins form homotrimers in the outer membrane, meaning that three identical porin subunits associate together to form a porin super-structure with three channels.
Hydrogen bond A hydrogen bond (or H-bond) is a primarily electrostatic Electrostatics is a branch of physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department ...

Hydrogen bond
ing and
dipole-dipole interactions
dipole-dipole interactions
between each monomer in the homotrimer ensure that they do not dissociate, and remain together in the outer membrane. Several parameters have been used to describe the structure of a porin protein. They include the tilting angle (α), shear number (S), strand number (n), and barrel radius (R). The tilting angle refers to the angle relative to the membrane. The shear number (S) is the number of amino acid residues found in each β strands. Strand number (n) is the amount of β strands in the porin, and barrel radius (R) refers to the radius of the opening of the porin. These parameters are related via the following formulas: : 2 \pi R = \frac and, :\tan(\alpha) = \frac Using these formulas, the structure of a porin can be determined by knowing only a few of the available parameters. While the structure of many porins have been determined using
X-ray crystallography X-ray crystallography is the experimental science determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to Diffraction, diffract into many specific directions. By measurin ...

X-ray crystallography
, the alternative method of sequencing
protein primary structure Protein primary structure is the linear sequence of amino acid Amino acids are organic compound In , organic compounds are generally any s that contain - . Due to carbon's ability to (form chains with other carbon s), millions of organ ...

protein primary structure
may also be used instead.


Cellular roles

Porins are water-filled pores and channels found in the membranes of bacteria and eukaryotes. Porin-like channels have also been discovered in archaea. Note that the term "
nucleoporin Nucleoporins are a family of proteins which are the constituent building blocks of the nuclear pore A nuclear pore is a part of a large complex of proteins, known as a nuclear pore complex that spans the nuclear envelope, which is the double ...
" refers to unrelated proteins that facilitate transport through nuclear pores in the
nuclear envelope The nuclear envelope, also known as the nuclear membrane, is made up of two lipid bilayer The lipid bilayer (or phospholipid bilayer) is a thin polar membrane A polarized membrane is a lipid bilayer, lipid membrane that has a positive elect ...

nuclear envelope
. Porins are primarily involved in passively transporting
hydrophilic A hydrophile is a molecule A molecule is an electrically Electricity is the set of physical phenomena associated with the presence and motion Image:Leaving Yongsan Station.jpg, 300px, Motion involves a change in position In phys ...
molecules of various sizes and charges across the membrane. For survival, certain required nutrients and substrates must be transported into the cells. Likewise, toxins and wastes must be transported out to avoid toxic accumulation. Additionally, porins can regulate permeability and prevent lysis by limiting the entry of detergents into the cell. Two types of porins exist to transport different materials– general and selective. General porins have no substrate specificities, though some exhibit slight preferences for anions or cations. Selective porins are smaller than general porins, and have specificities for chemical species. These specificities are determined by the threshold sizes of the porins, and the amino acid residues lining them. In
gram-negative bacteria Gram-negative bacteria are bacteria Bacteria (; common noun bacteria, singular bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), do ...
, the inner membrane is the major permeability barrier. The outer membrane is more permeable to hydrophilic substances, due to the presence of porins. Porins have threshold sizes of transportable molecules that depend on the type of bacteria and porin. Generally, only substances less than 600 Daltons in size can diffuse through.


Diversity

Porins were first discovered in gram-negative bacteria, but
gram-positive bacteria In bacteriology, gram-positive bacteria are bacteria that give a positive result in the Gram stain test, which is traditionally used to quickly classify bacteria into two broad categories according to their type of cell wall. Gram-positive bacte ...
with both types of porins have been found. They exhibit similar transport functions but have a more limited variety of porins, compared to the distribution found in gram-negative bacteria. Gram-positive bacteria lack outer membranes, so these porin channels are instead bound to specific lipids within the cell walls. Porins are also found in eukaryotes, specifically in the outer membranes of
mitochondria A mitochondrion (; ) is a double-membrane A membrane is a selective barrier; it allows some things to pass through but stops others. Such things may be molecules, ions, or other small particles. Biological membranes include cell membranes ...

mitochondria
and
chloroplasts A chloroplast is a type of membrane-bound organelle In cell biology, an organelle is a specialized subunit, usually within a cell (biology), cell, that has a specific function. The name ''organelle'' comes from the idea that these structure ...

chloroplasts
. The organelles contain general porins that are structurally and functionally similar to bacterial ones. These similarities have supported the
Endosymbiotic theory Symbiogenesis, endosymbiotic theory, or serial endosymbiotic theory, is the leading evolutionary theory of the origin of eukaryotic Eukaryotes () are organism In biology, an organism () is any organic, life, living system that fun ...

Endosymbiotic theory
, through which eukaryotic organelles arose from gram-negative bacteria. However, eukaryotic porins exhibit the same limited diversity as gram-positive porins, and also display a greater voltage-dependent role during metabolism. Archaea also contain ion channels that have originated from general porins. The channels are found in the cell envelope and help facilitate solute transfer. They have similar characteristics as bacterial and mitochondrial porins, indicating physiological overlaps over all three domains of life.


Antibiotic resistance

Many porins are targets for host immune cells, resulting in
signaling pathways Signal transduction is the process by which a chemical or physical signal is transmitted through a cell as a series of molecular events, most commonly protein phosphorylation residue Protein phosphorylation is a reversible post-translational ...
that lead to bacterial degradation. Therapeutic treatments, like vaccinations and antibiotics, are used to supplement this immune response. Specific antibiotics have been designed to travel through porins in order to inhibit cellular processes. However, due to selective pressure, bacteria can develop resistance through mutations in the porin gene. The mutations may lead to a loss of porins, resulting in the antibiotics having a lower permeability or being completely excluded from transport. These changes have contributed to the global emergence of
antibiotic resistance Antimicrobial resistance (AMR or AR) occurs when microbe A microorganism, or microbe,, ''mikros'', "small") and ''organism In biology Biology is the natural science that studies life and living organisms, including their a ...

antibiotic resistance
, and an increase in mortality rates from infections.


Discovery

The discovery of porins has been attributed to Hiroshi Nikaido, nicknamed "the porinologist."


Classification

According to
TCDB The Transporter Classification Database (or TCDB) is an International Union of Biochemistry and Molecular Biology (IUBMB)-approved classification system for membrane transport proteins, including ion channels. Classification The upper level of cla ...
, there are five evolutionarily independent superfamilies of porins. Porin superfamily I includes 47 families of porins with a range of numbers of trans-membrane β-strands (β-TMS). These include the GBP, SP and RPP porin families. While PSF I includes 47 families, PSF II-V each contain only 2 families. While PSF I derives members from
gram-negative bacteria Gram-negative bacteria are bacteria Bacteria (; common noun bacteria, singular bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), do ...
primarily one family of eukaryotic mitochondrial porins, PSF II and V porins are derived from
Actinobacteria The Actinobacteria are a phylum of mostly Gram-positive bacteria In bacteriology, gram-positive bacteria are bacteria that give a positive result in the Gram stain test, which is traditionally used to quickly classify bacteria into two broad ...
. PSF III and V are derived from eukaryotic organelle.


Porin Superfamily I

1.B.1 - The General bacterial porin family
1.B.2 - The Chlamydial Porin (CP) Family
1.B.3 - The Sugar porin (SP) Family
1.B.4 - The '' Brucella-Rhizobium porin'' (BRP) Family
1.B.5 - The ''Pseudomonas'' OprP Porin (POP) Family
1.B.6 - OmpA-OmpF porin (OOP) family
1.B.7 ''Rhodobacter'' PorCa porin (RPP) family
1.B.8 Mitochondrial and plastid porin (MPP) family
1.B.9 FadL outer membrane protein (FadL) family
1.B.10 Nucleoside-specific channel-forming outer membrane porin (Tsx) family
1.B.11 Outer membrane fimbrial usher porin (FUP) family
1.B.12 Autotransporter-1 (AT-1) family
1.B.13 Alginate export porin (AEP) family
1.B.14 Outer membrane receptor (OMR) family
1.B.15 Raffinose porin (RafY) family
1.B.16 Short chain amide and urea porin (SAP) family
1.B.17 Outer membrane factor (OMF) family
1.B.18 Outer membrane auxiliary (OMA) protein family
1.B.19 Glucose-selective OprB porin (OprB) family
1.B.20 Two-partner secretion (TPS) family
1.B.21 OmpG porin (OmpG) family
1.B.22 Outer bacterial membrane secretin (secretin) family
1.B.23 Cyanobacterial porin (CBP) family
1.B.24 Mycobacterial porin
1.B.25 Outer membrane porin (Opr) family
1.B.26 Cyclodextrin porin (CDP) family
1.B.31 ''
Campylobacter jejuni ''Campylobacter jejuni'' () is one of the most common causes of food poisoning in Europe and in the United States. The vast majority of cases occur as isolated events, not as part of recognized outbreaks. Active surveillance through the Foodborne ...

Campylobacter jejuni
'' major outer membrane porin (MomP) family
1.B.32 Fusobacterial outer membrane porin (FomP) family
1.B.33 Outer membrane protein insertion porin (Bam complex) (OmpIP) family
1.B.34 Corynebacterial porins
1.B.35 Oligogalacturonate-specific porin (KdgM) family
1.B.39 Bacterial porin, OmpW (OmpW) family
1.B.42 - The Outer Membrane Lipopolysaccharide Export Porin (LPS-EP) Family
1.B.43 - The ''Coxiella'' Porin P1 (CPP1) Family
1.B.44 - The Probable Protein Translocating ''Porphyromonas gingivalis'' Porin (PorT) Family
1.B.49 - The ''Anaplasma'' P44 (A-P44) Porin Family
1.B.54 -
Intimin Intimin is a virulence factor Virulence factors (preferably known as pathogenicity factors or effectors in plant science) are cellular structures, molecules and regulatory systems that enable microbial pathogens (bacteria, viruses, fungi, and pro ...
/Invasin (Int/Inv) or Autotransporter-3 family
1.B.55 - The Poly Acetyl Glucosamine Porin (PgaA) Family
1.B.57 - The Legionella Major-Outer Membrane Protein (LM-OMP) Family
1.B.60 - The Omp50 Porin (Omp50 Porin) Family
1.B.61 - The Delta-Proteobacterial Porin (Delta-Porin) Family
1.B.62 - The Putative Bacterial Porin (PBP) Family
1.B.66 - The Putative Beta-Barrel Porin-2 (BBP2) Family
1.B.67 - The Putative Beta Barrel Porin-4 (BBP4) Family
1.B.68 - The Putative Beta Barrel Porin-5 (BBP5) Superfamily
1.B.70 - The Outer Membrane Channel (OMC) Family
1.B.71 - The Proteobacterial/Verrucomicrobial Porin (PVP) Family
1.B.72 - The Protochlamydial Outer Membrane Porin (PomS/T) Family
1.B.73 - The Capsule Biogenesis/Assembly (CBA) Family
1.B.78 - The DUF3374 Electron Transport-associated Porin (ETPorin) Family


Porin Superfamily II (MspA Superfamily)

1.B.24 - Mycobacterial porin
1.B.58 - Nocardial Hetero-oligomeric Cell Wall Channel (NfpA/B) Family


Porin Superfamily III

1.B.28 - The Plastid Outer Envelope Porin of 24 kDa (OEP24) Family
1.B.47 - The Plastid Outer Envelope Porin of 37 kDa (OEP37) Family


Porin Superfamily IV (Tim17/OEP16/PxMPL (TOP) Superfamily)

This superfamily includes protein that comprise pores in multicomponent protein translocases as follows: 3.A.8 - im17 (P39515) Tim22 (Q12328) Tim23 (P32897) 1.B.69 - XMP4 (Q9Y6I8) PMP24 (A2R8R0) 3.D.9 - DH 21.3 kDa component (P25710) 1.B.30 - The Plastid Outer Envelope Porin of 16 kDa (OEP16) Family
1.B.69 - The Peroxysomal Membrane Porin 4 (PxMP4) Family
3.A.8 - The Mitochondrial Protein Translocase (MPT) Family


Porin Superfamily V (Corynebacterial PorA/PorH Superfamily)

1.B.34 - The Corynebacterial Porin A (PorA) Family 1.B.59 - The Outer Membrane Porin, PorH (PorH) Family


See also

*
Voltage-dependent anion channel Voltage-dependent anion channels, or mitochondrial porins, are a class of porin (protein), porin ion channel located on the outer mitochondrial membrane. There is debate as to whether or not this channel is expressed in the cell surface membrane ...
*
Aquaporin Aquaporins, also called water channels, are channel protein Channel, channels, channeling, etc., may refer to: Geography * Channel (geography), in physical geography, a landform consisting of the outline (banks) of the path of a narrow body ...

Aquaporin


References


External links


Overview
at
University of Hamburg The University of Hamburg (german: Universität Hamburg, also referred to as UHH) is a university A university () is an educational institution, institution of higher education, higher (or Tertiary education, tertiary) education and researc ...
* - OmpG porin * - Trimeric porins * - Sugar porins {{DEFAULTSORT:Porin (Protein) Transmembrane proteins Outer membrane proteins