Coordination of proton and electron transfer from the redox-active tyrosine, YZ, of Photosystem II and examination of the electrostatic influence of oxidized tyrosine, YD˙(H+)
The redox active tyrosines, YZ and YD, 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 YZ 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. Åkemark, 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 YZ˙/ YZ, consistent with concerted electron and proton transfer (CEP mechanism). A similar dependence of the rate of YZ oxidation on ΔG° is reported here through modification of the reduction potential of P680+/P680, 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 YD˙(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 YD˙(H+) on the P680+/P680 reduction potential is examined here by measuring the rate of YZ oxidation in Mn-depleted core complexes from the WT strain and from a YD-less strain of Synechocystis 6803. Also examined is the influence of YD˙(H+) on the P680+–P680 difference spectrum. These comparisons show that the electrostatic contribution of YD˙(H+) to the reduction potential of redox couple P680+/P680 is very small (≤10 mV), implying that the role of YD˙(H+) in photoactivation may have more to do with its providing an oxidizing equivalent during assembly of the manganese cluster.