Cooperativity between electrons and protons in a monomeric Cytochrome C-3:The importance of mechano-chemical coupling for energy transduction

To fully understand the structural bases for the mechanisms of biological e nergy transduction, it is essential to determine the microscopic thermodyna mic parameters which describe the properties of each centre involved in the reactions, as well as its interactions with the others. These interactions between centres can then be interpreted in the light of structural feature s of the proteins. Redox titrations of cytochrome c(3), from Desulfovibrio desulfuricans ATCC27774 followed by NMR and visible spectroscopy were analy sed by using an equilibrium thermodynamic model. The network of homotropic and heterotropic cooperativities results in the coupled transfer of electro ns and protons under physiological conditions. The microscopic characterisa tion allows the identification of several pairs of centres for which there are clear conformational (non-Coulombic) contributions to their coupling en ergies, thus establishing the existence of localised redox- and acid - base -linked structural modifications in the protein (mechano-chemical coupling) . The modulation of interactions between centres observed for this cytochro me may be an important general phenomenon and is discussed in the framework of its physiological function and of the current focus of energy, transduc tion research.