Coordination of proton and electron transfer from the redox-active tyrosine, Y_z, of Photosystem II and examination of the electrostatic influence of oxidized tyrosine, Y_D^.(H⁺)

The redox active tyrosines, Y_Z and Y_D, of Photosystem II are oxidized by P₆₈₀⁺ to the neutral radical. Such oxidation requires coupling of electron transfer to the transfer of the phenolic proton. Studies of the multiphasic kinetics of Y_Z oxidation in Mn-depleted PSII core complexes have shown that the relative amplitudes of the kinetic components are pH-dependent with one component showing a pH-dependent _t1/2 in the microsecond to tens of microsecond range (pH 4-8). Sjödin and coworkers (M. Sjödin, S. Styring, B. Akemark, L. Sun and L. Hammarström, Philos. Trans. R. Soc. London. Ser. B, 2002, 357, 1471-1479) have suggested that the increase in rate of this latter component with pH reflects an increase in the driving force of the reaction by lowering the reduction potential of Y_Z^./ Y_Z, consistent with concerted electron and proton transfer (CEP mechanism). A similar dependence of the rate of Y_Z oxidation on ΔG° is reported here through modification of the reduction potential of P₆₈₀⁺/P ₆₈₀, that is, without modifying either the proton acceptor or the pathway for proton transfer. The results reported here support a CEP mechanism, though formation of the tyrosinate followed by electron transfer cannot be completely ruled out. The presence of oxidized tyrosine Y_D ^.(H⁺) has been shown to accelerate the photoactivation of the oxygen evolving complex, possibly by an increase in the reduction potential of P₆₈₀⁺/ P₆₈₀. The influence of Y _D^.(H⁺) on the P₆₈₀ ⁺/P₆₈₀ reduction potential is examined here by measuring the rate of Y_Z oxidation in Mn-depleted core complexes from the WT strain and from a Y_D-less strain of Synechocystis 6803. Also examined is the influence of Y_D^.(H⁺) on the P ₆₈₀⁺-P₆₈₀ difference spectrum. These comparisons show that the electrostatic contribution of Y_D ^.(H⁺) to the reduction potential of redox couple P ₆₈₀⁺/P₆₈₀ is very small (≤ 10 mV), implying that the role of Y_D^.(H⁺) in photoactivation may have more to do with its providing an oxidizing equivalent during assembly of the manganese cluster.