Studies on the Interaction between Tyrosine Yz and the Mn complex

Abstract

We have shown that YZ interacts with the Mn cluster of the WOC in native and modified samples to give split electron paramagnetic resonance (EPR) signals at cryogenic temperatures, and that this places YZ close to the Mn cluster. This EPR signal can therefore be used in native samples to investigate YZ in normal turnover. Our latest results show that YZ in oxygen-evolving preparations can function down to at least 5K and suggest that this arises from tyrosinate-like behaviour of YZ. If YZ is effectively deprotonated, or if it were involved in a permanent hydrogen bond that is modified between YZ redox states, YZ would be able to be oxidised at temperatures where proton movements are prevented, as observed. YZ would be effectively negative in the reduced form and neutral when oxidised, accounting for EPR and electrochromic shift data. We therefore favour an electrostatically controlled electron transfer mechanism, where at physiological temperatures the changes in pKa induced by the initial oxidation of P680 cause a wave of deprotonation and progressive reduction in redox potential, firstly of YZ and then the WOC. YZ is therefore rapidly oxidised by P680+, and at the same time deprotonation from the WOC occurs to complete the oxidation of YZ and this deprotonation in turn lowers the redox potential in the Mn cluster, promoting its oxidation by YZ. Domino deprotonation through a hydrogen-bonding network leads to proton release into the lumen.