We have examined the temperature dependence of the intramolecular electron transfer (ET) between heme b and heme o3 in CO-mixed valence cytochrome bo3 (Cbo) from Escherichia coli. Upon photolysis of CO-mixed valence Cbo rapid ET occurs between heme o3 and heme b with a rate constant of 2.2 × 105 s-1 at room temperature. The corresponding rate of CO recombination is found to be 86 s-1. From Eyring plots the activation energies for these two processes are found to be 3.4 kcal/mol and 6.7 kcal/mol for the ligand binding and ET reactions, respectively. Using variants of the Marcus equation the reorganization energy (λ), electronic coupling factor (HAB), and the ET distance were found to be 1.4 ± 0.2 eV, (2 ± 1) × 10-3 eV, and 9 ± 1 Å, respectively. These values are quite distinct from the analogous values previously obtained for bovine heart cytochrome c oxidase (CcO) (0.76 eV, 9.9 × 10-5 eV, 13.2 Å). The differences in mechanisms/pathways for heme b/heme o3 and heme a/heme a3 ET suggested by the Marcus parameters can be attributed to structural changes at the CuB site upon change in oxidation state as well as differences in electronic coupling pathways between Heme b and heme o3.
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