NMR studies of cooperativity in the tetrahaem cytochrome c₃ from Desulfovibrio vulgaris

The thermodynamic properties of the Desulfovibrio vulgaris (Hildenborough) tetrahaem cytochrome c₃ (Dvc₃) are rationalised by a model which involves both homotropic (e^-/e^-) and heterotropic (e^-/H⁺) cooperativity. The paramagnetic shifts of a methyl group from each haem of the Dvc₃ have been determined in each stage of oxidation at several pH values by means of two-dimensional exchange NMR. The thermodynamic parameters are obtained by fitting the model to the NMR data and to redox titrations followed by visible spectroscopy. They show significant positive cooperativity between two of the haems whereas the remaining interactions appear to be largely electrostatic in origin. These parameters imply that the protein undergoes a proton-assisted two-electron transfer which can be used for energy transduction. Comparison with the crystal structure together with measurement of the kinetics of proton exchange suggest that the pH dependence is mediated by a charged residue(s) readily acessible to the solvent and close to haem I.