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Assignment 3: Porins

Original- "Porin (protein)"

Cellular roles
To transport medium-sized or charged molecules across, the molecules move through a porin, a water-filled channel or pore.

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.

Porins are chemically selective – transporting only one group of molecules – or may be specific for one molecule. Beta-lactam and fluoroquinolone antibiotics must pass through porins to reach their targets in gram negative bacteria. Bacteria can develop resistance to these antibiotics by mutating the gene that encodes the porin – the antibiotics are then excluded from passing through the outer membrane.

The term "nucleoporin" refers to completely unrelated proteins facilitating transport through nuclear pores in the nuclear envelope.

Cellular roles
Porins are water-filled pores and channels found in membranes of both bacteria and eukaryotes. They are involved in passively transporting hydrophilic molecules of various sizes and charges across the membranes. For survival, certain required nutrients and substrates, like sugars and amino acids, must be transported into the cells, while toxins and wastes must be transported out.

Two types of porins exist – general and selective. General porins have no substrate specificity, though some exhibit slight preferences for anions or cations. Selective porins are smaller than general porins, and have specificities for chemical species. This specificity is determined by the threshold sizes of the porins and the amino acid residues lining them.

In gram-negative bacteria, the inner membrane is the major permeability barrier, whereas the outer membrane is more permeable to hydrophilic substances. The threshold sizes of transportable molecules depend on the type of bacteria and porin. For example, Escherichia coli can generally transport substrates between 600-800 daltons, while Pseudomonas aeruginosa have an upper limit of 6000 daltons.

Porins were first found in gram negative bacteria, but gram-positive bacteria that contain both general and specific porins have been discovered. They exhibit similar transport functions, but their diversity of porins is much more limited compared to the ones found in gram-negative bacteria. Porins are also found in eukaryotes, specifically in the outer membranes of mitochondria and chloroplasts. They are structurally and functionally similar to bacterial porins, but exhibit the same limited diversity as gram-positive porins. These similarities have supported the Endosymbiotic theory, through which eukaryotic organelles arose from gram-negative bacteria.

Many porins are targets for host immune responses, involving proteases and antibodies, which allow for bacterial degradation. Certain antibiotics have been designed to bind to porins, or to go through porins to reach their target molecules. However, due to selective pressure, bacteria can develop resistance to these antibiotics by mutating the porin gene. The mutations may lead to a loss of porins, resulting in the antibiotics having a lower permeability or being excluded from transport completely. These alterations have contributed to the global emergence of antibiotic resistance and increased mortality rates from infections.

The term "nucleoporin" refers to completely unrelated proteins facilitating transport through nuclear pores in the nuclear envelope.

Kwei3 (talk) 06:33, 9 October 2017 (UTC)

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" refers to unrelated proteins that facilitate transport through nuclear pores in the nuclear envelope.

Porins are primarily involved in passively transporting hydrophilic 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, 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 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 and chloroplasts. The organelles contain general porins that are structurally and functionally similar to bacterial ones. These similarities have supported the 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 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, and an increase in mortality rates from infections.

Kwei3 (talk) 19:38, 19 November 2017 (UTC)