ROLE OF D1-HIS190 IN PROTON-COUPLED ELECTRON-TRANSFER REACTIONS IN PHOTOSYSTEM-II - A CHEMICAL COMPLEMENTATION STUDY

Recent models for water oxidation in photosystem II propose that His19 0 of the D1 polypeptide facilitates electron transfer from tyrosine Y- Z to P-680(+) by accepting the hydroxyl proton from Y-Z. To test these models, and to further define the role of D1-His190 in the proton-cou pled electron transfer reactions of PSII, the rates of P-680(+) reduct ion, Y-Z oxidation, Q(A)(-) oxidation, and Y-Z(.) reduction were measu red in PSII particles isolated from several D1-His190 mutants construc ted in the cyanobacterium Synechocystis sp. PCC 6803. These measuremen ts were conducted in the absence and presence of imidazole and other s mall organic bases. In all mutants examined, the rates of P-680(+) red uction, 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 P-680(+) by Q(A)(-) rather than by Y-Z. There appeared to be little correlation between these rat es and the nature of the residue substituted for D1-His190, However, i n nearly all mutants examined, the rates of P-680(+) reduction, Y-Z ox idation, and Y-Z(.) reduction were accelerated dramatically in the pre sence of imidazole and other small organic bases (e.g., methyl-substit uted imidazoles, histidine, methylamine, ethanolamine, and TRIS), In a ddition, 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 P-680(+) 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 comparabl e to that of Mn-depleted wild-type PSII particles in the absence of im idazole. 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 fa cilitating the reduction of Y-Z(.).