PORINS: CHARACTERISTICS, FUNCTION AND ASSEMBLY - BIOLOGY - 2025

The porins are a type of integral membrane proteins that permit the diffusion of various types of medium - sized molecules. They are found on the outer membranes of gram-negative bacteria (as in E. coli) and of mitochondria and chloroplasts. These transmembrane proteins completely penetrate membranes and their structure is made up of β sheets.

Unlike transporter proteins, porins are channel proteins, that is, they form open channels or pores that cross the membrane where it is located, allowing the free diffusion of molecules with an appropriate size and charge.

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characteristics

Porins are proteins that create transmembrane channels and, unlike most proteins that cross a membrane, the portion that crosses the membrane is made up of β sheets instead of α helices.

These integral membrane proteins may lack selectivity with respect to the molecules that will pass through them, or they may exhibit certain degrees of size and charge selectivity. They can also allow the passage of molecules smaller than a certain size.

The polar region of the porin chain is found on the inside covering the aqueous channel, while the apolar portion projects to the outside of the barrel, interacting with the hydrophobic region of the lipid bilayer of the membrane.

Diffusion through the channels formed by these proteins is due to a concentration gradient of the molecule or substance that crosses the membrane and is a passive diffusion, that is, it does not require metabolic energy for the movement of the substance.

This diffusion is controlled by specific mechanisms that can open or close the channel and can be inhibited by various compounds.

Structure

At the end of the 1980s, by means of X-ray crystallography, the atomic structure of a porin was determined for the first time, corresponding to the cell membrane of Rhodobacter capsulatus, which is a photosynthetic bacterium.

Said porin consists of a trimer, where each monomer forms a β barrel with 16 β sheets that roll together, forming a cylindrical structure capable of passing through the membrane and containing a pore filled with water inside.

Various types of porins have since been described, both from prokaryotic and eukaryotic cells. All of them consist of joined β sheets that form a β barrel-like structure and a water-filled channel that can measure between 0.6 and 3nm in diameter.

In mitochondria, porins consist of 19 β sheets that when folded together form the β barrel.

In many bacteria, porins are composed of 16 to 18 anti-parallel β-sheets that form a β-barrel, exhibiting hydrogen bonds between nearby molecules along the chain.

Function

The porins of bacteria and the membranes of mitochondria and chloroplasts are functionally similar, operating in a similar way, due to their similarity in terms of pore dimensions, atomic structure, and passive diffusion properties.

The width of the porin channel is defined by the alignment of polypeptides on the inner wall of the structure, allowing the size of the molecules that pass through them to be restricted.

The similarity of these structures reinforces the endosymbiotic theory, according to which the mitochondria of eukaryotic cells come from a prokaryotic organism that was phagocytosed by a eukaryotic cell precursor.

In eukaryotes

In eukaryotic cells, porins are found in the outer membranes of mitochondria and plastids. The porins found in plastids have been very little studied.

In the case of mitochondria, they are known as mitochondrial porins or voltage-gated anion channels (VDAC). They are wide channels with an approximate diameter of 3nm that make up the largest protein constituent in the outer membranes. They represent about 30% of the total proteins in this membrane.

They generate permeability to uncharged molecules of up to 5000 Da. Mitochondrial porins allow passage into the intermembrane space of small molecules, ions, and metabolites.

The molecules and ions that cross the intermembrane space do not pass through the inner mitochondrial membrane, since it has greater impermeability. Therefore, the space between the two membranes is loaded with small molecules and ions, being similar to the cytoplasm.

In prokaryotes

In gram-negative bacteria, an outer membrane allows them to isolate themselves from the environment as a protective measure. This membrane contains porins that are permeable to the nutrients required by bacteria.

About 100,000 porins can be found in the membrane of a prokaryotic cell, representing about 70% of the total proteins in that structure.

In intestinal bacteria, the outer membrane represents protection against external damaging agents such as antibiotics, bile salts and proteases.

Porins ensure the uptake and elimination of small hydrophilic molecules, thus allowing the cell to obtain nutrients necessary for its proper functioning and to be able to free itself of waste products. In E. coli, porins allow passage of disaccharides, phosphates, and other small molecules.

Assembly of porins in eukaryotes and prokaryotes

Mitochondrial porins are imported into the mitochondria by a protein complex called TOM (translocase of the outer mitochondrial membrane) and are inserted by the SAM complex (protein classification and assembly machinery).

Many studies have described how some bacterial outer membrane proteins are imported into the mitochondria of eukaryotic cells by a mechanism managed by the TOM and SAM complexes, indicating that this insertion process has been conserved between the two systems.

In bacteria, porins are inserted by a complex possessed by the β-barrel assembly machinery called BAM. This complex consists of five proteins, four of which are lipoproteins.

Although the insertion process of porins and their structures are similar between eukaryotic and prokaryotic cells, a notable difference is that, in prokaryotes, the presence of lipoproteins is required for the insertion of these structures.

On the other hand, its assembly in the mitochondrial outer membranes depends on the presence of two accessory proteins of the SAM complex: the proteins Sam35 and Sam36.

References

1. Alberts, B., Bray, D., Watson, JD, Lewis, J., Roberts, K. & Raff, M. (2002). Molecular Biology of the cell. Fourth edition. Ed. Garland Science.
2. Devlin, MT (1993). Texbook of Biochemistry With Chemical Correlations. Ed. John Wiley & Sons, Inc.
3. Lodish, H. (2005). Cellular and molecular biology. Panamerican Medical Ed.
4. Schirmer, T., & Rosenbusch, JP (1991). Prokaryotic and eukaryotic porins. Current Opinion in Structural Biology, 1 (4), 539-545.
5. Schulz, GE (1993). Bacterial porins: structure and function. Current Opinion in Structural Biology, 5 (4), 701-707.
6. Voet, D., & Voet, JG (2006). Biochemistry. Panamerican Medical Ed.
7. Zeth, K., & Thein, M. (2010). Porins in prokaryotes and eukaryotes: common themes and variations. Biochemical Journal, 431 (1), 13-22.