Ab initio structure solution of a dimeric cytochrome c(3) from Desulfovibrio gigas containing disulfide bridges

The 1.2 Angstrom resolution crystal structure of the 29 kDa di-tetrahaem cy tochrome c(3) from the sulfate reducing bacterium Desulfovibrio gigas was s olved by ab initio methods, making this the largest molecule to be solved b y this procedure. The actual refined model of the cysteine-linked dimeric m olecule reveals that this molecule is very similar to the non-covalently li nked symmetrical dimer of the di-tetrahaem cytochrome c(3) from Desulfomicr obium norvegicum. Each monomer has the typical polypeptide fold, haem arran gement and iron coordination found for the tetrahaem cytochrome c(3) molecu les. The interface between the covalently linked monomers in the asymmetric unit of the crystal shows a pseudo two-fold arrangement, disturbed from sy mmetry by crystal packing forces. The fact that D. gigas contains a dimeric tetrahaem cytochrome with solvent accessible disulfide bridges and that th is cytochrome specifically couples hydrogen oxidation to thiosulfate reduct ion in bacterial extracts provides an interesting aspect related to disulfi de exchange reactions in this microorganism.