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 c3 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 c3 from D, desulfuricans ATCC 27774, was used to correlate relative redox potentials to specific haems in the structure: haem II ≃ haem I < haem IV < haem III. 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., Frazao, C., Caldeira, J., LeGall, J., Moura, I., Moura, J.J.G. and Carrondo. M.A.
Louro, R. O., Pacheco, I., Turner, D. L., LeGall, J., & Xavier, A. V. (1996). Structural and functional characterization of cytochrome c3 from D. desulfuricans ATCC 27774 by 1H-NMR. FEBS Letters, 390(1), 59–62. https://doi.org/10.1016/0014-5793(96)00627-8