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

In Rhodobacter sphaeroides, transfer of the first electron in quinol oxidation by the bc1 complex shows kinetic features (a slow rate (approx. 1.5U10 3 /s), high activation energy (approx. 65 kJ/mol) and reorganization energy, V (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 A O 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 Em;7 and pK 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 Qo site, and rotation of the Glu of the conserved -PEWY- sequence. Mutational studies show a key role for the domain proximal to heme bL. 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. fl 2000 Elsevier Science B.V. All rights reserved.