Porines

Porines are a term used in some literature to denote porins, a broad family of channel-forming proteins located in the outer membranes of Gram-negative bacteria and in the membranes of certain organelles such as mitochondria and chloroplasts. Porins form transmembrane pores that enable passive diffusion of small hydrophilic molecules, typically down their concentration gradients.

Most porins assemble as beta-barrel channels and may function as homooligomers; many monomers provide a pore of 16 to 18 antiparallel beta-strands, creating a curved barrel. A constriction region, often called the eyelet, narrows the pore and contributes to substrate selectivity and permeability. Porins are classified as general porins, allowing passage of a broad range of small solutes, or specific porins, which favor particular substrates. Classic examples in Escherichia coli include general porins such as OmpF and OmpC, and a maltose-specific porin LamB.

In organelles, porin homologues such as the mitochondrial voltage-dependent anion channels (VDACs) share the beta-barrel architecture and mediate metabolite exchange between cytosol and organelle.

In biology and medicine, porins influence nutrient uptake and the permeability of the outer membrane to many hydrophilic antibiotics. Changes in porin expression or structure can alter permeability and contribute to antibiotic resistance. The form “porine” is uncommon in modern English; most literature uses “porin,” with “porines” as an occasional plural.