Proton-coupled electron transfer at the Q o site: what type of mechanism can account for the high activation barrier?

Abstract

In Rhodobacter sphaeroides, transfer of the first electron in quinol oxidation by the bc 1 complex shows kinetic features (a slow rate (approx. 1.5×10 3/s), high activation energy (approx. 65 kJ/mol) and reorganization energy, λ (2.5 V)) that are unexpected from Marcus theory and the distances shown by the structures. Reduction of the oxidized iron-sulfur protein occurs after formation of the enzyme-substrate complex, and involves a H-transfer in which the electron transfer occurs through the approx. 7 Å of a bridging histidine forming a H-bond with quinol and a ligand to 2Fe-2S. The anomalous kinetic features can be explained by a mechanism in which the electron transfer is constrained by coupled transfer of the proton. We discuss this in the context of mutant strains with modified E m,7 and p K for the iron-sulfur protein, and Marcus theory for proton-coupled electron transfer. We suggest that transfer of the second proton and electron involve movement of semiquinone in the Q o site, and rotation of the Glu of the conserved -PEWY- sequence. Mutational studies show a key role for the domain proximal to heme b L. The effects of mutation at Tyr-302 (Tyr-279 in bovine sequence) point to a possible linkage between conformational changes in the proximal domain, and changes leading to closure of the iron-sulfur protein access channel at the distal domain.