Porin (protein)


Porin (protein)
A sucrose specific porin from Salmonella typhimurium, a gram-negative bacterium. PDB 1A0S
A single monomer of the same protein in side view, illustrating the antiparallel beta barrel structure.

Porins are beta barrel proteins that cross a cellular membrane and act as a pore through which molecules can diffuse.[1] Unlike other membrane transport proteins, porins are large enough to allow passive diffusion, i.e., they act as channels that are specific to different types of molecules. They are present in the outer membrane of Gram-negative bacteria and some Gram-positive bacteria of the group Mycolata (mycolic acid-containing actinomycetes), the mitochondria, and the chloroplast.

Contents

Structure

Porins are composed of β strands, which are, in general, linked together by beta turns on the cytoplasmic side and long loops of amino acids on the other. The β strands lie in an antiparallel fashion and form a cylindrical tube, called a β barrel.[2] 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 lipid membrane, whereas the polar residues face inwards into the center of the beta barrel to interact with the aqueous channel. The phospholipids that compose the outer membrane give it the same semi-permeable characteristics as the cytoplasmic membrane.[citation needed]

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 and seven aspartic acid residues (in contrast to one histidine, two lysine and three arginine 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.[3]

Cellular roles

To transport medium-sized or charged molecules across, the molecules move through a porin, a water-filled channel or pore.[citation needed]

Porins typically control the diffusion of small metabolites like sugars, ions, and amino acids.

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.

The term "nucleoporin" refers to porins facilitating transport through nuclear pores in the nuclear envelope. However, they are often considered distinct from other porins (they are not classified as porins in MeSH.)

Porins are chemically selective – transport only one group of molecules, or may be specific for one molecule[citation needed]. For antibiotics to be effective against a bacterium, it must pass through the outer membrane, using a porin[citation needed]. Bacteria can develop resistance to the antibiotic by mutating the gene that encodes the porin – the antibiotic is then excluded from passing through the outer membrane[citation needed].

Discoverer

The discovery of porins has been attributed to Hiroshi Nikaido, nicknamed "the porinologist."[4]

See also

References

  1. ^ MeSH Porins
  2. ^ Schirmer T (1998). "General and specific porins from bacterial outer membranes". J. Struct. Biol. 121 (2): 101–9. doi:10.1006/jsbi.1997.3946. PMID 9615433. 
  3. ^ Branden and Tooze, Introduction to Protein Structure, second edition
  4. ^ Klebba PE (December 2005). "The porinologist". J. Bacteriol. 187 (24): 8232–6. doi:10.1128/JB.187.24.8232-8236.2005. PMC 1317029. PMID 16321927. http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=1317029. 

External links


Wikimedia Foundation. 2010.

Look at other dictionaries:

  • Porin (Protein) — Porine Schematische Darstellung eines Sucrose spezifischen Porins aus Salmonellen …   Deutsch Wikipedia

  • Porin — can be:* Porin (protein), a transmembrane protein * Porin (music award), Croatian music award * Porin (opera), the first Croatian opera …   Wikipedia

  • Protein K (porin) — Protein K is a porin expressed in some pathogenic strains of E. coli bacteria. It has a molecular weight of about 40 kDa and is localized to the outer membrane, through which it allows both inorganic and organic ions to pass. The strains in which …   Wikipedia

  • Protein K — The name Protein K is used for two different proteins. * Protein K (porin), a porin expressed in pathogenic strains of E. coli * Protein K (gene expression), a DNA and RNA binding protein expressed in the nucleus of eukaryotes …   Wikipedia

  • Transmembrane protein — A transmembrane protein is a protein that spans the entire biological membrane. Transmembrane proteins aggregate and precipitate in water. They require detergents or nonpolar solvents for extraction, although some of them (beta barrels) can be… …   Wikipedia

  • MspA porin — MspA Identifiers Symbol MspA Pfam PF09203 …   Wikipedia

  • Membrane transport protein — A membrane transport protein (or simply transporter) is a membrane protein[1] involved in the movement of ions, small molecules, or macromolecules, such as another protein across a biological membrane. Transport proteins are integral membrane… …   Wikipedia

  • Gap junction protein, alpha 1 — Gap junction protein, alpha 1, 43kDa PDB rendering based on 1r5s …   Wikipedia

  • Outer membrane protein G — (OmpG) Identifiers Symbol Porin OmpG Pfam PF09381 …   Wikipedia

  • Nucleoside-specific porin — Nucleoside specific channel forming protein, Tsx Identifiers Symbol Channel Tsx Pfam …   Wikipedia