The Crystal Structure of the Hexadeca-Heme Cytochrome Hmc and a Structural Model of Its Complex with Cytochrome c3

Abstract Sulfate-reducing bacteria contain a variety of multi-heme c -type cytochromes. The cytochrome of highest molecular weight (Hmc) contains 16 heme groups and is part of a transmembrane complex involved in the sulfate respiration pathway. We present the 2.42 A resolution crystal structure of the Desulfovibrio vulgaris Hildenborough cytochrome Hmc and a structural model of the complex with its physiological electron transfer partner, cytochrome c 3 , obtained by NMR restrained soft-docking calculations. The Hmc is composed of three domains, which exist independently in different sulfate-reducing species, namely cytochrome c 3 , cytochrome c 7 , and Hcc. The complex involves the last heme at the C-terminal region of the V-shaped Hmc and heme 4 of cytochrome c 3 , and represents an example for specific cytochrome-cytochrome interaction.