Tyrozine D oxidation and redox equilibrium in photosystem II

Tyrosine D (TyrD) is an auxiliary redox active tyrosine residue in photosystem II (PSII). The mechanism of TyrD oxidation was investigated by EPR spectroscopy, flash-induced fluorescence decay and thermoluminescence measurements in PSII enriched membranes from spinach. PSII membranes were chemically treated with 3 mM ascorbate and 1 mM diaminodurene and subsequent washing, leading to the complete reduction of TyrD. TyrD oxidation kinetics and competing recombination reactions were measured after a single saturating flash in the absence and presence of DCMU (inhibitor of the QB-site) in the pH range of 4.7-8.5. Two kinetic phases of TyrD oxidation were observed by the time resolved EPR spectroscopy - the fast phase (msec-sec time range) and the pH dependent slow phase (tens of seconds time range). In the presence of DCMU, TyrD oxidation kinetics was monophasic in the entire pH range, i.e. only the fast kinetics was observed. The results obtained from the fluorescence and thermoluminescence analysis show that when forward electron transport is blocked in the presence of DCMU, the S2QA− recombination outcompetes the slow phase of TyrD oxidation by the S2 state. Modelling of the whole complex of these electron transfer events associated with TyrD oxidation fitted very well with our experimental data. Based on these data, structural information and theoretical considerations we confirm our assignment of the fast and slow oxidation kinetics to two populations of PSII centers with different water positions (proximal and distal) in the TyrD vicinity.