Homotropic and heterotropic cooperativity in the tetrahaem cytochrome c3 from Desulfovibrio vulgaris.

The thermodynamic parameters which govern the homotropic (e-/e-) and heterotropic (e-/H+) cooperativity in the tetrahaem cytochrome c3 isolated from Desulfovibrio vulgaris (Hildenborough) were determined, using the paramagnetic shifts of haem methyl groups in the NMR spectra of intermediate oxidized states at different pH levels. A model is put forward to explain how the network of positive and negative cooperativities between the four haems and acid/base group(s) enables the protein to achieve a proton-assisted 2e- step.