Proton Coupled Electron Transfer and Redox-Active Tyrosine Z in the Photosynthetic Oxygen-Evolving Complex

摘要

Proton coupled electron transfer (PCET) reactions play an essential role in many enzymatic processes. In PCET, redox-active tyrosines may be involved as intermediates when the oxidized phenolic side chain deprotonates. Photosystem II (PSIl) is an excellent framework for studying PCET reactions, because it contains two redoxactive tyrosines, YD and YZ, with different roles in catalysis. One of the redox-active tyrosines, YZ, is essential for oxygen evolution and is rapidly reduced by the manganese-catalytic site. In this report, we investigate the mechanism of YZ PCET in oxygen-evolving PSH. To isolate YZ* reactions, but retain the manganese-calcium duster, low temperatures were used to block the oxidation of the metal duster, high microwave powers were used to saturate the YD* EPR signal, and YZ* decay kinetics were measured with EPR spectroscopy. Analysis of the pH and solvent isotope dependence was performed. The rate of YZ* decay exhibited a significant solvent isotope effect, and the rate of recombination and the solvent isotope effect were pH independent from pH 5.0 to 7.5. These results are consistent with a ratelimiting, coupled proton electron transfer (CPET) reaction and are contrasted to results obtained for YD* decay kinetics at low pH. This effect may be mediated by an extensive hydrogen-bond network around YZ. These experiments imply that PCET reactions distinguish the two PSII redoxactive tyrosines.