CYTOCHROME C(3) FROM DESULFOVIBRIO-GIGAS - CRYSTAL-STRUCTURE AT 1.8-ANGSTROM RESOLUTION AND EVIDENCE FOR A SPECIFIC CALCIUM-BINDING SITE

Crystals of the tetraheme cytochrome c(3) from sulfate-reducing bacter ia Desulfovibrio gigas (Dg) (MW 13 kDa, 111 residues, four heme groups ) were obtained and X-ray diffraction data collected to 1.8 Angstrom r esolution. The structure was solved by the method of molecular replace ment and the resulting model refined to a conventional R-factor of 14. 9%. The three-dimensional structure shows many similarities to other k nown crystal structures of tetraheme c(3) cytochromes, but it also sho ws some remarkable differences. In particular, the location of the aro matic residues around the heme groups, which may play a fundamental ro le in the electron transfer processes of the molecule, are well conser ved in the cases of hemes I, III, and IV. However, heme II has an arom atic environment that is completely different to that found in other r elated cytochromes c(3). Another unusual feature is the presence of a Ca2+ ion coordinated by oxygen atoms supplied by the protein within a loop near the N-terminus. It is speculated that this loop may be stabi lized by the presence of this Ca2+ ion, may contribute to heme-redox p erturbation, and might even be involved in the specificity of recognit ion with its redox partner.