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, Y-Z and Y-D, of Photosystem II are oxidized by P-680(+) 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 t(1/2) in the microsecond to tens of microsecond range (pH 4-8). Sjodin and coworkers (M. Sjodin, S. Styring, B. Angstromkemark, L. Sun and L. Hammarstrom, 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 DeltaGdegrees is reported here through modification of the reduction potential of P-680(+)/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 P-680(+)/P-680. The influence of Y-D(.) (H+) on the P-680(+)/P-680 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(+)-P-680 difference spectrum. These comparisons show that the electrostatic contribution of Y-D(.)(H+) to the reduction potential of redox couple P-680(+)/P-680 is very small (less than or equal to10 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.