The crystal structure of the hexadeca-heme cytochrome Hmc and a structural model of its complex with cytochrome c3

Sulfate-reducing bacteria contain a variety of multiheme 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 Angstrom 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.