The Sanderson Group Webpages
Department of Chemistry
Durham University, Durham, UK
OmpF - A Bacterial Porin
| Display Options | |
| Structural Features |
The Porins
The porins are a family of proteins
which are found in the outer membrane of Gram-negative bacteria.
Their role is to provide a pathway for the diffusion of small hydrophilic
compounds into and out of the cell. General properties of porins
include a high thermal stability and resistance to protease and detergent
degradation, essential for the survival of Gram-negative bacteria in harsh
environments such as the intestinal tract (Schirmer, T. and Rosenbusch,
J. P., Curr. Opinion Struct. Biol., 1, 539-545 (1991)).
Most porin channels show a weak ion selectivity and moderate voltage gating
behaviour. Underlying the high stability of these proteins is their
unusual structure, consisting of a 16-stranded antiparallel β-barrel
(Cowan, S. W. et al, Nature, 358, 727-733 (1992)) with a
shear number of 20. Extra stability is derived from the formation
of a salt bridge between the N- and C-termini to produce an essentially
closed structure, with the tilt of the strands at 35-50 deg. relative to
the barrel axis. The exits of the channel are bordered by β-turns,
although there is a marked difference between the two ends. The extracellular
end or rough end is marked by the presence of extended loop structures,
some of which protrude into the channel pore and provide a means of regulating
channel selectivity. Short β-turns are
found at the intracellular end or smooth end of the porin channel
(see the display opposite, for example). The pore itself is irregular
in shape due to the extended loops of the rough end, but is predominantly
populated with polar residues and is 22 Å in diameter at its widest
point.