Structure of a novel c₇-type three-heme cytochrome domain from a multidomain cytochrome c polymer

The structure of a novel c₇-type cytochrome domain that has two bis-histidine coordinated hemes and one heme with histidine, methionine coordination (where the sixth ligand is a methionine residue) was determined at 1.7 Å resolution. This domain is a representative of domains that form three polymers encoded by the Geobacter sulfurreducens genome. Two of these polymers consist of four and one protein of nine c₇-type domains with a total of 12 and 27 hemes, respectively. Four individual domains (termed A, B, C, and D) from one such multiheme cytochrome c (ORF03300) were cloned and expressed in Escherichia coli. The domain C produced diffraction quality crystals from 2.4 M sodium malonate (pH 7). The structure was solved by MAD method and refined to an R-factor of 19.5% and R-free of 21.8%. Unlike the two c₇ molecules with known structures, one from G. sulfurreducens (PpcA) and one from Desulfuromonas acetoxidans where all three hemes are bis-histidine coordinated, this domain contains a heme which is coordinated by a methionine and a histidine residue. As a result, the corresponding heme could have a higher potential than the other two hemes. The apparent midpoint reduction potential, E_app, of domain C is -105 mV, 50 mV higher than that of PpcA.