Structure and function of the porin channel

The outer membrane bilayer of gram-negative bacteria, which encapsulates completely the plasma membrane, seems to act as a protection system against noxious macromolecules like proteases and bile-salt micelles [1]. The external monolayer is composed of lipopolysaccharides with a variable carbohydrate composition. Long oligosaccharide chains protect the cells against recognition by the complement system [2, 3] and hydrophobic substances [4]. The other monolayer facing the periplasmic space is mainly composed of phospholipid.This protection system should nevertheless allow free access of small biomolecules like mono- and disaccharides, nucleosides, and amino acids to the cytoplasmic membrane. The proteins allowing for rapid diffusion of these molecules across the outer membrane are called porins [5]. The “general diffusion porins” form aqueous channels with an exclusion limit of typically 600 Da and extremes of 5000 Da. Due to their high copy number they form the major integral protein component of the outer membrane and turn it into a molecular sieve. A single cell of E. coli has been estimated to possess approximately 105 OmpF porin molecules [6]