PROTEIN-PROTEIN ELECTRON-TRANSFER - MARCUS THEORY ANALYSIS OF REACTIONS BETWEEN C-TYPE CYTOCHROMES AND BLUE COPPER PROTEINS

The electron-transfer rate and equilibrium constants and enthalpies of reaction for seven protein-protein reactions in 0.05 M phosphate buffer at pH 7 and 25 °C have been determined by the chemical relaxation temperature-jump method. The rate constants for the reduction by cytochrome C553 (Bumilleriopsis filiformis) of the following proteins, Pseudomonas aeruginosa azurin, Alcaligenes faecalis azurin, Alcaligenes spp. azurin, Phaseolus vulgaris plastocyanin, Scenedesmus acutus plastocyanin, and horse heart cytochrome c, and the reduction by Pseudomonas aeruginosa cytochrome C551 of Alcaligenes spp. azurin are 1.0 X 107, 5.2 X 105, 2.1 X 106, 4.0 X 105, 7.4 X 105, 4.0 X 104, and 3.0 X 106 M-1 s-1, respectively. For the reaction of cytochrome C553 with Ps. azurin there is a second relaxation mode attributed to an isomerization of reduced azurin to an inactive form. The rate constant for the formation of the inactive form was 5 s-1 and the equilibrium constant 0.1. These data and others from the literature are analyzed with the Marcus theory formalism. For 13 protein-protein electron-transfer reactions, the data can be excellently fit by 6 apparent electron self-exchange rate constants; k11 (M-1 s-1) is 2.8 X 108 for cytochrome C553,4.6 X 107 for cytochrome c551, 9.9 X 105 for Ps. azurin, 2.6 X 105 for the other azurins, 6.6 X 102 for the plastocyanins, and 1.5 X 102 for cytochrome c. The constancy of the values for each protein is taken as evidence that there is no kinetic selectivity in these reactions. Implications of this conclusion for the pathway of electron transport in photosynthesis and bacterial respiration are discussed. The k11 values calculated are compared to those previously calculated for some reactions of these proteins with inorganic transition metal complexes. © 1978, American Chemical Society. All rights reserved.