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Mutants Affecting the Quinol Oxidation Site of Ubiquinolcyt c 2 -Oxidoreductase

Ubiquinol-cyt c 2 -oxidoreductase (cyt bc1.) is part of the cyclic electron transfer system of the purple, non-sulphur bacteria Rhodobacter capsulatus and Rhodobacter sphaeroides. The enzyme, like the cyt b 6 f complex of plant chloroplasts and the cyt bc 1 of mitochondria, oxidizes quinol (QH2) from a membranebound pool (Qpool), reduces a c-type cytochrome, and, through a process of reaox-linked charge separation, generates an electrochemical gradient of protons (ΔμH+;see 1,2 for reviews). A preponderance of evidence supports a Q-cycle-type mechanism (3,4) for the the formation of Δψ and ΔpH. In any Q-cycle variant (5) two quinone catalytic sites are postulated, one of them involved in net quinol oxidation (Qo or Qz) and the other in net quinone reduction (Qi or Qc). The sites are placed at opposite ends of a charge separating cyt b H reduction and oxidation. In other words, one electron from QH2 at Qz passes to a high potential sequence including the FeS center and cyt c 1 while the other reduces first, cyt b L and then cyt b H. Cyt b H is, in turn oxidized by Q at site Qc.