PARENT SESSION
Posters P7A Mechanisms of water oxidation. Abstracts (347-381)

Tyrosine z redox chemistry in active Photosystem II at cryogenic temperature. Chunxi Zhang^*,1, Alfred Rutherford¹, ¹ Service de Bioénergétique, CNRS URA 2096, Département de Biologie Joliot-Curie, Paris, France

ABSTRACT- TyrZ is one of the crucial components for water oxidation in PSII. Due to its fast redox chemistry in active PSII at room temperature, most previous studies of TyrZ have been carried out in inhibited PSII samples. Recently, we investigate the redox chemistry of TyrZ in different S-states PSII from Spinach and Thermosynechococcus elongates, at liquid helium temperature. We find that the oxidation of TyrZ occurs in more than 50% and 40% of the reaction centers in the S0 and S1 states, respectively. The oxidation of TyrZ in these two states give rise to two "split" EPR signals, S0TyrZ^. and S1TyrZ^., respectively. Here we report the influence of pH on TyrZ oxidation and TyrZ^. reduction in active PSII in S0 and S1 states, through monitoring the S0TyrZ^. and S1TyrZ^. EPR signals induced by illumination at low temperature (5K). The yield of the formation of both EPR signals, corresponding to the oxidation of TyrZ, shows a bell-shape pH curve. The maximum yield was obtained between pH 5 to 7, and was pH independent in this pH range. At low or high pH, the yield decreased with pKa values of 4.7 ± 0.2, 7.7 ± 0.1, respectively. The decay kinetics of S0TyrZ^. and S1TyrZ^. EPR signals, which show the reduction of TyrZ^. by charge recombination between TyrZ^. and QA^- , were essentially the same at all the pH values from 4.0 to 8.5. These results suggest that both the oxidation of TyrZ and reduction of TyrZ^. in active PSII are pH independent, which is dramatically different from the behaviour of Tyr in solution and TyrZ in Mn-depleted PSII. This may be taken as an evidence that there is no direct interaction between TyrZ and water in active enzyme. These low temperature redox chemistry studies of TyrZ provide new insight on the mechanism of TyrZ in active PSII.

KEY WORDS: Tyrosine Z, pH effect, Photosystem II, Low temperature EPR