The temperature dependence of P680+ reduction in oxygen-evolving photosystern

The temperature dependence for the reduction of the oxidized primary electron donor P680(+) by the redox active tyrosine Yz has been studied in oxygen-evolving photosystem H preparations from spinach. The observed temperature dependence is found to vary markedly with the S-state of the manganese cluster. In the higher oxidation states, S-2 and S-3, sub-microsecond P680(+) reduction exhibits activation energies of about 260 meV. In contrast, there is only a small temperature dependence for the sub-microsecond reaction in the S-0 and S-1 states (an activation energy of approximately 50 meV). Slower microsecond components of P680(+) reduction show an activation energy of about 250 meV which, within experimental error, is independent of the oxidation state of the Mn cluster. By combining these values with measurements of DeltaG for electron transfer, the reorganization energies for each component of P680(+) reduction have been calculated. High activation and reorganization energies are found for sub-microsecond P680(+) reduction in S-2 and S-3, demonstrating that these electron transfers are coupled to significant reorganization events which do not occur in the presence of the lower S-states. One interpretation of these results is that there is an increase in the net charge on the manganese cluster on the S-1 to S-2 transition which acts as a barrier to electron transfer in the higher S-states. This argues against the electroneutrality requirement for some models of the function of the manganese cluster and hence against a role for Yz as a hydrogen abstractor on all S-state transitions. An alternative or additional possibility is that there are proton (or other ion) motions in the sub-microsecond phases in S-2 and S-3 which contribute to the large reorganization energies observed, these motions being absent in the S-0 and S-1 states. Indeed charge accumulation may directly cause the increased reorganization energy.