Tyrosine D oxidation at cryogenic temperature in photosystem II

Photosystem II (PSII) contains two redox-active tyrosines (TyrZ and TyrD) possessing very different properties. TyrD is stable in its oxidized form, while TyrZ is kinetically competent in the water oxidizing reaction. The temperature dependence of the formation of light-induced tyrosyl radical was studied as a function of pH in Mn-depleted PS II from cyanobacteria and plants. Tyrosyl radical formation was observed in the majority of the centers at pH 8.5 and 15 K. The use of two Synechocystis mutants where either TyrZ or TyrD was replaced by a phenylalanine residue allowed a clear demonstration that only TyrD and not TyrZ was oxidized under these conditions. By lowering the pH, the fraction of centers undergoing TyrD oxidation greatly diminished. This pH dependence could be fitted by assuming a single protonable group with a pK(a) of approximately 7.6. This pH dependence correlates with the unexpectedly rapid oxidation of TyrD at room temperature [Faller, P., Debus, R. J., Brettel, K., Sugiura, M., Rutherford, A. W., and Boussac, A. (2001) Proc. Nat. Acad. Sci. U.S.A. 98, 14368]. The equally unexpectedly ability to undergo oxidation at cryogenic temperatures reported here, puts limitations on the nature of the protonation reactions associated with electron transfer in this system, raising the possibility of proton tunneling along the hydrogen bond or alternatively the presence of deprotonated TyrD (tyrosinate) prior to oxidation.