Tyrozine D oxidation and redox equilibrium in photosystem II

Tyrosine ID (Tyr(D)) is an auxiliary redox active tyrosine residue in photosystem II (PSII). The mechanism of Tyr(D) 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 Tyr(D). Tyr(D) oxidation kinetics and competing recombination reactions were measured after a single saturating flash in the absence and presence of DCMU (inhibitor of the Q(B)-site) in the pH range of 4.7-8.5. Two kinetic phases of Tyro 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, Tyr(D) 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 S(2)Q((S) over bar) recombination outcompetes the slow phase of Tyr(D) oxidation by the S-2 state. Modelling of the whole complex of these electron transfer events associated with Tyr(D) 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 Tyr(D) vicinity. (C) 2017 Elsevier B.V. All rights reserved.