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Figure 30.
Bacteriorhodopsin, 1ap9, [98]. CATH:
1.20.1070
.10. (Alpha, up-down bundle, rhopdopsin 7-
helix transmembrane proteins) SCOP: membrane and
cell surface proteins and peptides, family A G protein-
coupled receptor-like protein.
Beta-structures can also span the membrane if all the main chain polar groups are
engaged in inter-strand hydrogen bonding, a criterion satisfied by the closed topology of a
β
-barrel. Porins are trimeric proteins in which each sub-unit is made up of between 12 and
18
-barrel spanning the outer membrane of gram negative
bacteria (figure 31). The radius of each barrel is large enough for the interior to form a pore
lined with hydrophilic residues. Within the porin trimer there is a hydrophobic core around
the symmetry axis, which usually lends the structure a degree of stability, however
dissociation of the porin timer is accompanied by denaturation of the subunits.
β
-strands that form an up-down
β
a) b)
Figure 31. Matrix OmpF porin, 1bt9, [99].
CATH
: 2.40.160.10 (Beta, barrel, porin)
SCOP
: membrane and cell surface proteins and peptides, transmembrane beta-
barrels. a) Side view; b) Looking down the barrel.
