Cooperativity between redox and protonation centres is known to be crucial for the function of complex proteins, but it is often difficult to describe in terms of thermodynamic parameters. Cytochrome c 3 is a good model for these studies since, while retaining the overall complexity of larger systems, it is suitable for detailed crystallographic and spectroscopic studies. Assignment of the haem substituent NMR resonances, together with NMR redox titrations of cytochrome c 3 from D. desulfuricans ATCC 27774, was used to correlate relative redox potentials to specific haems in the structure: haem 00627-8/asset/equation/feb20014579396006278-math-si1.gif?v=1&s=9f31f5c883affbc5731b49ed1fea374670504855)
. This order is different from that determined for the homologous proteins studied and in disagreement with that previously reported for this cytochrome (Morais, J., Palma, N., Frazão, C., Caldeira, J., LeGall, J., Moura, I., Moura, J.J.G. and Carrondo, M.A. (1995) Biochemistry 34, 12830–12841).
