Proton transfer pathways and mechanism in bacterial reaction centers

AbstractThe focus of this minireview is to discuss the state of knowledge of the pathways and rates of proton transfer in the bacterial reaction center (RC) from Rhodobacter sphaeroides. Protons involved in the light driven catalytic reduction of a quinone molecule QB to quinol QBH2 travel from the aqueous solution through well defined proton transfer pathways to the oxygen atoms of the quinone. Three main topics are discussed: (1) the pathways for proton transfer involving the residues: His-H126, His-H128, Asp-L210, Asp-M17, Asp-L213, Ser-L223 and Glu-L212, which were determined by a variety of methods including the use of proton uptake inhibiting metal ions (e.g. Zn2+ and Cd2+); (2) the rate constants for proton transfer, obtained from a ‘chemical rescue’ study was determined to be 2×105 s−1 and 2×104 s−1 for the proton uptake to Glu-L212 and QB−, respectively; (3) structural studies of altered proton transfer pathways in revertant RCs that lack the key amino acid Asp-L213 show a series of structural changes that propagate toward L213 potentially allowing Glu-H173 to participate in the proton transfer processes

Proton and electron transfer in bacterial reaction centers

AbstractThe bacterial reaction center couples light-induced electron transfer to proton pumping across the membrane by reactions of a quinone molecule QB that binds two electrons and two protons at the active site. This article reviews recent experimental work on the mechanism of the proton-coupled electron transfer and the pathways for proton transfer to the QB site. The mechanism of the first electron transfer, k(1)AB, Q−AQB→QAQ−B, was shown to be rate limited by conformational gating. The mechanism of the second electron transfer, k(2)AB, was shown to involve rapid reversible proton transfer to the semiquinone followed by rate-limiting electron transfer, H++Q−AQ−B⇔Q−AQBH→QA(QBH)−. The pathways for transfer of the first and second protons were elucidated by high-resolution X-ray crystallography as well as kinetic studies showing changes in the rate of proton transfer due to site directed mutations and metal ion binding