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

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Abstract

In Rhodobacter sphaeroides, transfer of the first electron in quinol oxidation by the bc1 complex shows kinetic features (a slow rate (approx. 1.5×103/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 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.

Keywords

Myxothiazol
Stigmatellin
Activation energy
Mutagenesis
Proton-coupled electron transfer

Abbreviations

cyt, cytochrome
cyt bH, the higher potential heme of cyt b
cyt bL, the lower potential heme of cyt b
ES, enzyme-substrate
ET, electron transfer
ISP, iron-sulfur protein
ISPox, H-ISPred, oxidized and reduced forms of ISP
MOA-, methoxyacrylate
-PEWY-, conserved span of cyt b, starting with Pro-271 in bovine sequence, with the sequence in single-letter code
PT, proton transfer
Q, quinone
QH2, hydroquinone or quinol
Qi site, quinone reducing site
Qo site, hydroquinone oxidizing site
SQ-ISP, putative complex between semiquinone and ISP

Cited by (0)

1

Present address: Service de Photosynthèse, Institut de Biologie Physico-Chimique, 13, rue Pierre et Marie Curie, 75005 Paris, France.

2

Present address: Department of Biochemistry and Biophysics, University of Pennsylvania, Philadelphia, PA 19104-6059, USA.

3

Present address: Department of Biological Chemistry, Johns Hopkins University, School of Medicine, 725 N. Wolfe Street, Baltimore, MD 21205, USA.