Question

Where are porins present?

Answer 1

Hint: Porins are beta-barrel proteins that cross the cell membrane and act as pores through which molecules can spread. Porins are broad enough to permit passive diffusion, unlike other membrane transport proteins.

Complete Answer:
- Porins serve as channels unique to various types of molecules. They are found in the outer membrane of gram-negative bacteria and some gram-positive mycobacteria (mycolic acid-containing actinomycetes), the outer membrane of mitochondria and the outer membrane of chloroplast.
- Porins are made up of beta sheets (β sheets) made up of beta strands (β strands) which are connected together by beta turns on the cytoplasmic side and long loops of amino acids on the other.
- The β strands lie in antiparallel fashion, forming a cylindrical tube called a beta barrel (β barrel)
- Most porins form homodimers in the outer membrane, which means that three identical porin subunits combine to form a three-channel porin superstructure.
- Hydrogen bonding and dipole-dipole interactions between each monomer in the homotrimer ensure that they do not dissolve and stay in the outer membrane together.
Several parameters were used to define the structure of the porin protein. These include tilting angle (α) , shear number (S), strand number (n) and barrel radius (R).
- The tilting angle refers to the relative angle of the membrane. The shear number (S) is the number of amino acid residues present in each β-strand. Strand number (n) is the number of β strands in the pore, and the barrel radius (R) is the radius of the pore opening.
- Porins are water-filled pores and canals found in bacterial and eukaryotic membranes. Porins are mainly involved in the passive transport of hydrophilic molecules of varying sizes and charges through the membrane.
There are two types of porins to transport different materials – general and selective.

Note: Many porins are targets for host immune cells, resulting in pathways of signalling that lead to bacterial degradation. Therapeutic procedures, such as vaccines and antibiotics, are used to supplement this immune response. The discovery of porins was credited to Hiroshi Nikaido, nicknamed the porinologist.