W0364

Structure of a Novel c₇-type Three-heme Cytochrome Domain from a Multi-heme Cytochrome-c. P.R. Pokkuluri, Y.Y. Londer, N. Duke, J. Erickson and M. Schiffer, Biosciences Div., Argonne National Laboratory, Argonne, IL 60439.

Structure of a c₇-type cytochrome domain that has one high potential and two low potential hemes was determined at 1.7 Å resolution. This domain is characteristic of three multi-heme cytochromes c encoded in Geobacter sulfurreducens genome. Two of these proteins consist of four and one protein of nine c₇-type domains with 12 and 27 hemes, respectively. The function(s) of these proteins are unknown. Four individual domains (termed A,B,C,D) from one such multi-heme cytochrome c (ORF03300) were cloned into Escherichia coli. The domain C produced diffraction quality crystals from 2.4 M sodium malonate, pH 7. The structure was solved by MAD method using data collected at the Fe K-absoption edge 19BM (APS). The structure refinement is in progress, the R-factor is 25%, with data collected to 1.7 Å resolution at 19ID (APS). Unlike the two c₇ molecules with known structures, one from G. sulfurreducens (PpcA) and one from Desulfuromonas acetoxidans, this protein contains a heme which is coordinated by a methionine and a histidine residue. As a result, the corresponding heme is high potential compared to the other two hemes which are coordinated by two histidines each and are low potential. Although domain C also contains three hemes and similar number of amino acid residues, its structure differs significantly from that of the PpcA determined earlier in our laboratory. 20 out of 82 residues are identical between the two proteins. The three hemes are closer together in domain C than in PpcA, however, some of the secondary structure elements are maintained in the two structures.

This work was supported by the U.S. Department of Energy’s Natural and Accelerated Bioremdiation Research (NABIR) program, under contract No. W-31-109-ENG-38.