Temperature dependence of the oxidation kinetics of TyrZ and TyrD in oxygen-evolving photosystem II complexes throughout the range from 320K to 5K.

ABSTRACT

The photo-induced oxidation of TyrZ and TyrD by P680(•+), that involves both electron and proton transfer (PCET), has been studied in oxygen-evolving photosystem II from Thermosynechococcus elongatus. We used time-resolved absorption spectroscopy to measure the kinetics of P680(•+) reduction by tyrosine after the first flash given to dark-adapted PS II as a function of temperature and pH. The half-life of TyrZ oxidation by P680(•+) increases from 20ns at 300K to about 4µs at 150K. Analyzing the temperature dependence of the rate, one obtains a reorganization energy of about 770meV. Between 260K and 150K, the reduction of P680(•+) by TyrZ is increasingly replaced by charge recombination between P680(•+) and QA(•-). We propose that the driving force for TyrZ oxidation by P680(•+) decreases upon lowering the temperature. TyrZ oxidation cannot be excluded in a minority of PS II complexes at cryogenic temperatures. TyrD oxidation by P680(•+) with a half-life of about 30ns was observed at high pH. The pH dependence of the yield of TyrD oxidation can be described by a single protonable group with a pK of approximately 8.4. The rate of TyrD oxidation by P680(•+) is virtually identical upon substitution of solvent exchangeable protons with deuterons indicating that the rate is limited by electron transfer. The rate is independent of temperature between 5K and 250K. It is concluded that TyrD donates the electron to P680(•+) via PD2.