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

Light-induced electrogenic events associated with proton uptake upon forming QB− in bacterial wild-type and mutant reaction centers

AbstractLight-induced voltage changes (electrogenic events) were measured in wild-type and site-directed mutants of reaction centers (RCs) from Rhodobacter sphaeroides oriented in a lipid monolayer adsorbed to a Teflon film. A rapid increase in voltage associated with charge separation was followed by a slower increase attributed to proton transfer from solution to protonatable amino-acid residues in the vicinity of the QB site. In native reaction centers the proton-transfer voltage had a pH-dependent amplitude with two peaks at pH 4.5 and pH 9.7, respectively. In the Glu-L212→Gln RCs the high-pH peak was absent, whereas in the Asp-L213→Asn RCs the low-pH peak was absent and the high-pH peak was shifted to lower pH by about 1.3 pH units. The amplitudes of the electrogenic phases as a function of pH follow approximately the measured proton uptake from solution (P.H. McPherson, M.Y. Okamura, G. Feher, Biochim. Biophys. Acta, vol. 934, 1988, pp. 348–368) and are ascribed to proton transfer to amino acid residues upon QB− formation. The peak around pH 9.7 is ascribed to proton uptake predominantly by Glu-L212 and the peak around pH 4.5 to proton uptake predominantly by Asp-L213 or a residue strongly interacting with Asp-L213

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