Correlation between rate constant for reduction and redox potential as a basis for systematic investigation of reaction mechanisms of electron transfer proteins.

Rate constants for the reduction of approximately 40 electron transfer proteins by photoreduced flavins have been determined by laser flash photolysis techniques. The data for a series of 12 homologous cytochromes and 10 homologous high redox potential ferredoxins (HiPIPs) are in excellent agreement with semi-empirical equations relating rate constant and thermodynamic redox potential that have proven applicable to nonbiological electron transfer systems. These correlations allow the establishment of relative reactivities within structurally homologous classes of biological oxidation-reduction proteins, including cytochromes and HiPIPs, and a variety of nonhomologous heme-, iron-sulfur-, copper-, and flavin-containing proteins. A qualitative correspondence is shown to exist between such relative reactivity and the extent of solvent exposure of the redox centers in a particular structural class. The implications of these results are considered, and it is concluded that free energy relationships provide a sound basis for systematic analysis of reaction mechanisms of electron transfer proteins.