Reduction potentials of Rieske clusters: importance of the coupling between oxidation state and histidine protonation state.

TitleReduction potentials of Rieske clusters: importance of the coupling between oxidation state and histidine protonation state.
Publication TypeJournal Article
Year of Publication2003
AuthorsZu, Y, Couture, MM-J, Kolling, DRJ, Crofts, AR, Eltis, LD, Fee, JA, Hirst, J
JournalBiochemistry
Volume42
Issue42
Pagination12400-8
Date Published2003 Oct 28
ISSN0006-2960
KeywordsElectron Transport Complex III, Histidine, Iron-Sulfur Proteins, Oxidation-Reduction, Protons, Thermodynamics
Abstract

Rieske [2Fe-2S] clusters can be classified into two groups, depending on their reduction potentials. Typical high-potential Rieske proteins have pH-dependent reduction potentials between +350 and +150 mV at pH 7, and low-potential Rieske proteins have pH-independent potentials of around -150 mV at pH 7. The pH dependence of the former group is attributed to coupled deprotonation of the two histidine ligands. Protein-film voltammetry has been used to compare three Rieske proteins: the high-potential Rieske proteins from Rhodobacter sphaeroides (RsRp) and Thermus thermophilus (TtRp) and the low-potential Rieske ferredoxin from Burkholderia sp. strain LB400 (BphF). RsRp and TtRp differ because there is a cluster to serine hydrogen bond in RsRp, which raises its potential by 140 mV. BphF lacks five hydrogen bonds to the cluster and an adjacent disulfide bond. Voltammetry measurements between pH 3 and 14 reveal that all the proteins, including BphF, have pH-dependent reduction potentials with remarkably similar overall profiles. Relative to RsRp and TtRp, the potential versus pH curve of BphF is shifted to lower potential and higher pH, and the pK(a) values of the histidine ligands of the oxidized and reduced cluster are closer together. Therefore, in addition to simple electrostatic effects on E and pK(a), the reduction potentials of Rieske clusters are determined by the degree of coupling between cluster oxidation state and histidine protonation state. Implications for the mechanism of quinol oxidation at the Q(O) site of the cytochrome bc(1) and b(6)f complexes are discussed.

DOI10.1021/bi0350957
Alternate JournalBiochemistry
Citation Key10.1021/bi0350957
PubMed ID14567701
Grant ListGM35342 / GM / NIGMS NIH HHS / United States
GM35438 / GM / NIGMS NIH HHS / United States