Metalloprotein Electron Transfer Mechanisms

The rates of oxidation and reduction of various metalloproteins (several cytochromes c azurin, plastocyanin, stellacyanin, and HiPIP) by inorganic reagents (including Fe(EDTA)2-, Co(phen)33+, Fe(CN)63-/4-, derivatives of the latter two reagents, and Ru(NH3)62+) have been analyzed within the framework of the relative Marcus theory of outer-sphere electron transfer. This approach has allowed contributions to the overall free energy of activation (the specific activation of the reagent, the thermodynamic driving force for the reaction, and the general coulombic interactions of the reactants) to be factored out, leaving a quantity which is characteristic of the activation process the protein must undergo in order to transfer an electron with each reagent. These analyses have led to the definition of the "kinetic accessibility" of the electron transfer sites of several proteins, the order being stellacyanin ≫ plastocyanin > horse heart cytochrome ~ > Pseudomonas cytochrome c551 > HiPIP ≅ Pseudomonas azurin. As the kinetic accessibility of the proteins decreases, the variety of apparent electron transfer mechanisms increases. It is concluded that the two most important factors in controlling the reactivity of redox proteins with inorganic reagents (after the three previously mentioned contributions are eliminated) are the availability of extended π orbital systems to facilitate orbital overlap and thus adiabaticity, and the capacity of the reagents to penetrate the hydrophobic residues surrounding the metal sites in many proteins. Protein-protein electron transfer reactions are also considered, as are ionic strength and pH influences on the rate constants.