CRYSTAL-STRUCTURE OF A DIMERIC OCTAHEME CYTOCHROME C(3) (M(R) 26000) FROM DESULFOVIBRIO-DESULFURICANS NORWAY

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).