Structural and Functional Consequences of a Glu L212 .fwdarw. Lys Mutation in the QB Binding Site of the Photosynthetic Reaction Center of Rhodopseudomonas viridis

Abstract

The properties of the quinone acceptor complex in the photosynthetic reaction center of the atrazine-resistant Rhodopseudomonas viridis mutant A2 (Glu L212-->Lys) were studied by EPR spectroscopy and by photoelectric measurements. The EPR signal attributed to the semiquinone-iron (QB-Fe2+) was significantly different from wild type and resembled that found in PS II. Essentially normal oscillations of QB-Fe2+ were observed upon flash illumination. The kinetics of the first and the second electron transfer from QA to QB were characterized by a photoelectric double-flash method. Compared to wild type, the rate of the first electron transfer in the large majority of reaction centers was decreased drastically from k1 = (18 microseconds)-1 in the wild type to (70 ms)-1 in the mutant, whereas the second electron transfer was only slightly slowed down with a rate of k2 = (260 microseconds)-1 compared to (65 microseconds)-1 in wild type (pH 7). When the pH was raised above 10, in a major fraction of the reaction centers a fast kinetics of the first electron transfer, like that in wild type, reappeared. The experimental results are interpreted as an effect of the positive charge on the lysine causing a significant structural change of the QB binding pocket and a strongly diminished affinity for ubiquinone. The slow QA(-)-->QB electron transfer kinetics are thus attributed to ubiquinone binding, which is rate limiting. The possible role of the residue Glu L212, which is conserved in all purple bacteria, in electron and proton transfer to QB is discussed.

Radicals and Radical Pairs in Photosynthesis

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Theoretical studies on the influence of molecular interactions on the mechanism of electron transfer in photosynthetic reaction center ofRps. viridis

“…fig. the charge-separated state, changes, the direction of the electron transfer does not [7] . The SCRF treatment of the protein surrounding affects the frontier orbital energy levels of the electron donor with a smaller degree than the axial coordination and hydrogen bonds of amino acid residues do.…”

“…Two molecules of quinones (Q A and Q B ), and one non-heme Fe (II) ion position between two quinines in the PRC ( fig. There are many disputes about it according to the reports that have been published [5][6][7][8] . All of these molecules are arranged by a quasi-C B 2 symmetry.…”