Membrane-Anchored Cytochrome cy Mediated Microsecond Time Range Electron Transfer from the Cytochrome bc1 Complex to the Reaction Center in Rhodobacter capsulatus

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Abstract
In Rhodobacter capsulatus, the soluble cytochrome (cyt) c2 and membrane-associated cyt cy are the only electron carriers which operate between the photochemical reaction center (RC) and the cyt bc1 complex. In this work, cyt cy mediated microsecond time range electron transfer kinetics were studied by light-activated time-resolved absorption spectroscopy using a mutant strain lacking cyt c2. In intact cells and in isolated chromatophores of this mutant, only ∼30% of the RCs had their photooxidized primary donor rapidly rereduced by cyt cy. Of these 30%, about half were reduced with a half-time of ∼5 μs attributed to preformed complexes, and the other half with a half-time of ∼40 μs attributed to cyt cy having to move from another site. This slower phase was affected by addition of glycerol, indicating its dependence on the viscosity of the medium. Cyt cy , despite its rereduction by ubihydroquinone oxidation in the millisecond time range, remained virtually unable to deliver electrons to other RCs which stayed photooxidized for several seconds. Furthermore, using two flashes separated by a variable time interval, it was shown that the fast electron donating complex was reformed in about 60 μs, a time span probably reflecting electron transfer from cyt c1 to cyt cy. In the absence of the cyt bc1 complex, the steady-state level of cyt cy in the chromatophore membranes obtained using cells grown in minimal medium was decreased to ∼50%. The remaining cyt cy , however, was able to form the fast electron donating complex with the RC (half-time of ∼5 μs), whereas the slower phase with a half-time of ∼40 μs was strongly decelerated. This finding suggests a role for the cyt bc1 complex in stabilizing cyt cy and providing its “other” site, possibly via a close association between these components. Taken together, it is concluded that although cyt cy is present in substoichiometric amount compared to the RCs, it supports efficiently photosynthetic growth of R. capsulatus in the absence of cyt c2 because it can mediate fast electron transfer from the cyt bc1 complex to the RC during multiple turnovers of the cyclic electron flow.

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