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. 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 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+/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 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 Y D.(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 P 680+-P680 difference spectrum. These comparisons show that the electrostatic contribution of YD .(H+) to the reduction potential of redox couple P 680+/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.
ASJC Scopus subject areas
- Physics and Astronomy(all)
- Physical and Theoretical Chemistry