10] Cytochrome c3 (Mr 26,000) isolated from sulfate-reducing bacteria and its relationships to other polyhemic cytochromes from Desulfovibrio

Abstract

This chapter describes polyhemic cytochrome c3 isolated from sulfate-reducing bacteria from Desulfovibrio. It also describes cytochrome c3 relationships to other polyhemic cytochromes. Cytochrome c3 is a tetrahemic protein in which each heme exhibits a distinct redox potential in the –200 to −400-mV range and all have the same His-His iron atom axial ligands. Cytochrome c3 has been purified and characterized only in Desulfovibrio gigas (D. gigas) and Desulfomicrobium baculatum (Dsm. Baculatum). In D. gigas, cytochrome c3 has been described as being involved in the reduction of thiosulfate in a crude system. The cytochrome c3 purified from Dsm. Baculatum comprises two identical subunits of 111 amino acids resulting in a molecular mass of 14,987 Da, including that of the four heme groups and 29,974 Da in the case of the dimeric form. 26 amino-acid residues cytochrome c3 are invariant and are mainly located in the cysteine–histidine clusters that bind each heme. The histidine forming the sixth ligand of each heme is conserved and has been attributed to each heme in the three-dimensional model.