Nine-haem cytochrome c from Desulfovibrio desulfuricans ATCC 27774: Primary sequence determination, crystallographic refinement at 1.8 and modelling studies of its interaction with the tetrahaem cytochrome c₃

A monomeric nine-haem cytochrome c (9Hcc) with 292 amino acid residues was isolated from cells of the sulfate- and nitrate-reducing bacterium Desulfovibrio desulfuricans ATCC 27774 grown under both nitrate- and sulfate- respiring conditions. The nucleotide sequence encoding the 292 residues was determined, allowing the correction of about 10% of the previous primary structure, determined from 1.8 Δ electron density maps. The refinement at 1.8 Δ resolution of the structural model was completed, giving an R-value of 16.5%. The nine haem groups are arranged into two tetrahaem clusters, located at both ends of the molecule, with Fe-Fe distances and local protein fold very similar to tetrahaem cytochromes c₃, and the extra haem is located asymmetrically between the two regions. The new primary sequence determination confirmed the 39% sequence homology found between this cytochrome and the C-terminal region (residues 229-514) of the high- molecular-weight cytochrome c (Hmc) from D. vulgaris Hildenborough, providing strong evidence of structural similarity between 9Hcc and the C-terminal region of Hmc. The interaction between 9Hcc and the tetrahaem cytochrome c₃ from the same organism was studied by modelling methods, and the results suggest that a specific interaction is possible between haem 4 of tetrahaem cytochrome c₃ and haem 1 or haem 2 of 9Hcc, in agreement with previous kinetic experiments which showed the catalytic effect of the tetrahaem cytochrome c₃ upon the reduction of 9Hcc by the [NiFe] hydrogenase from D. desulfuricans ATCC 27774. These studies suggest a role for 9Hcc as part of the assembly of redox proteins involved in recycling the molecular hydrogen released by the cell as a result of substrate oxidation.