M. Czjzek et al., CRYSTAL-STRUCTURE OF A DIMERIC OCTAHEME CYTOCHROME C(3) (M(R) 26000) FROM DESULFOVIBRIO-DESULFURICANS NORWAY, Structure, 4(4), 1996, pp. 395-404
Background: The octaheme cytochrome c(3) (M(r) 26 000; cc(3)) from Des
ulfovibrio desulfuricans Norway is a dimeric cytochrome made up of two
identical subunits, each containing four heme groups. It is involved
in the redox transfer chain of sulfate-reducing bacteria, which links
the periplasmic oxidation of hydrogen to the cytoplasmic reduction of
sulfate. The amino-acid sequence of cc(3) shows similarities to that o
f the tetraheme cytochrome c(3) (M(r) 13 000; c(3)) from the same bact
eria. Structural analysis of cc(3) forms a basis for understanding the
precise roles of the multiheme-containing redox proteins and the reas
on for the presence of several different multiheme cytochromes in one
bacterial strain. Results: The crystal structure of cytochrome cc(3) h
as been determined at 2.16 Angstrom resolution, The subunits display t
he c(3) structural fold with significant amino-acid substitutions, rel
ative to the tetraheme cytochromes c(3), in the regions of the dimer i
nterface, The identical subunits are related by a crystallographic two
fold axis, with one heme of each subunit in close contact, The overall
structure and the environments of the different heme groups are compa
red with those of the tetraheme cytochromes c(3). Conclusions: A commo
n scheme for interactions between these types of cytochrome and their
redox partners involves the interaction of a heme crevice, surrounded
by positively charged lysine residues, with acidic residues surroundin
g the redox partner's functional group, Despite the relatively acidic
character of cytochrome cc(3), the crevice of one heme is surrounded b
y a high number of positively charged residues, in the same manner as
has been reported for cytochromes c(3), The environment of this heme i
s formed by four flexible surface loops which are variable in length a
nd orientation in the different c(3)-type cytochromes although the ove
rall structural folds are very similar, It has been proposed that this
region, adapted in topology and charge, is the interaction site for p
hysiological partners and is also most likely to be the interaction si
te in the dimeric cytochrome cc(3).