Charge recombination and proton transfer in manganese-depleted photosystem II.

The proton transfer reactions induced by the oxidation and reduction of the secondary donor, tyrosine YZ, have been studied in photosystem II after inactivation (Mn-depletion) of the oxygen-evolving complex. The rate of the recombination reaction of YZox with the reduced primary acceptor QA- appears modulated by a protonatable group with pK approximately 6 in the presence of YZox. The finding of monophasic recombination kinetics requires that the proton equilibration of this group is faster than the recombination rate. The same group modulates the extent of proton release, from 0 below pH 5 to 1 per center above pH 7. The kinetics of proton appearance and disappearance in the bulk medium are markedly dependent on the material used. In PSII core particles, the release is observed in the 100 micros range and the uptake accompanies the recombination reaction. In PSII membranes, both of these reactions are markedly delayed, so that the uptake considerably lags behind the completion of the recombination reaction. An electrochromic shift of a chlorophyll is present during the whole lifetime of YZox, suggesting a charged character of this species. A fast decreasing phase of this signal was observed in particles in the same time range as proton release. These results are discussed in the framework of a model where the proton originating from the formation of the neutral oxidized tyrosine radical (YZ.) remains locally trapped. In turn, this proton shifts the pK of a nearby group from a value >/=9 to a value of 6.