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

Abstract

Light-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 Q B 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 Q B − 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.