Recent models for water oxidation in photosystem II propose that His190 of the D1 polypeptide facilitates electron transfer from tyrosine Y(Z) to P680+ by accepting the hydroxyl proton from Y(Z). To test these models, and to further define the role of D1-His190 in the proton-coupled electron transfer reactions of PSII, the rates of P680+ reduction, Y(Z) oxidation, Q(A)- oxidation, and Y(Z)· reduction were measured in PSII particles isolated from several D1-His190 mutants constructed in the cyanobacterium Synechocystis sp. PCC 6803. These measurements were conducted in the absence and presence of imidazole and other small organic bases. In all mutants examined, the rates of P680+ reduction, Y(Z) oxidation, and Y(Z)· reduction after a single flash were slowed dramatically and the rate of Q(A)- oxidation was accelerated to values consistent with the reduction of P680+ by Q(A)- rather than by Y(Z). There appeared to be little correlation between these rates and the nature of the residue substituted for D1-His190. However, in nearly all mutants examined, the rates of P680+ reduction, Y(Z) oxidation, and Y(Z)· reduction were accelerated dramatically in the presence of imidazole and other small organic bases (e.g., methyl- substituted imidazoles, histidine, methylamine, ethanolamine, and TRIS). In addition, the rate of Q(A)- oxidation was decelerated substantially. For example, in the presence of 100 mM imidazole, the rate of electron transfer from Y(Z) to P680+ in most D1-His190 mutants increased 26-87-fold. Furthermore, in the presence of 5 mM imidazole, the rate of Y(Z)· reduction in the D1-His190 mutants increased to values comparable to that of Mn- depleted wild-type PSII particles in the absence of imidazole. On the basis of these results, we conclude that D1-His190 is the immediate proton acceptor for Y(Z) and that the hydroxyl proton of Y(Z) remains bound to D1-His190 during the lifetime of Y(Z)·, thereby facilitating the reduction of Y(Z)·.
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