Sulfate respiration in Desulfovibrio vulgaris Hildenborough: Structure of the 16-heme cytochrome c HmcA at 2.5-Å resolution and a view of its role in transmembrane electron transfer

The crystal structure of the high molecular mass cytochrome c HmcA from Desulfovibrio vulgaris Hildenborough is described. HmcA contains the unprecedented number of sixteen hemes c attached to a single polypeptide chain, is associated with a membranebound redox complex, and is involved in electron transfer from the periplasmic oxidation of hydrogen to the cytoplasmic reduction of sulfate. The structure of HmcA is organized into four tetraheme cytochrome c₃-like domains, of which the first is incomplete and contains only three hemes, and the final two show great similarity to the nine-heme cytochrome c from Desulfovibrio desulfuricans. An isoleucine residue fills the vacant coordination space above the iron atom in the five-coordinated high-spin Heme 15. The characteristics of each of the tetraheme domains of HmcA, as well as its surface charge distribution, indicate this cytochrome has several similarities with the nine-heme cytochrome c and the Type II cytochrome c₃ molecules, in agreement with their similar genetic organization and mode of reactivity and further support an analogous physiological function for the three cytochromes. Based on the present structure, the possible electron transfer sites between HmcA and its redox partners (namely Type I cytochrome c₃ and other proteins of the Hmc complex), as well as its physiological role, are discussed.