Photosynthetic Oxygen Evolution Studied by Electron Paramagnetic Resonance and X-Ray Absorption Spectroscopy

摘要

Photosystem II catalyzes the four electron oxidation of water to O sub 2 . A membrane-bound complex which contains Mn is thought to be the site of O sub 2 -evolution. During turnover, this complex cycles through a series of intermediate states, S/sub i/ (i = 0...4). In this thesis, EPR and X-ray absorption spectroscopy were used to probe the role of several proteins and cofactors in the O sub 2 -evolution reactions. The effects of alkaline pH on the O sub 2 -evolving system were investigated. Two modes of inhibition of electron transfer and O sub 2 production were revealed. EPR studies revealed that the two types of alkaline inhibition affected electron transfer reactions at different sites. Mechanisms for the reversible and irreversible inhibition are proposed. The flash number dependence of the reduction kinetics of Z sup + , an intermediate electron carrier, was studied as a direct probe of charge accumulation at the O sub 2 -evolving complex. Characteristic period four oscillations in the kinetics occurred in active preparations. Upon depletion of the extrinsic proteins, or upon reversible alkaline inhibition, these oscillations were absent, indicating that even partial advancement of the S-states was blocked. Addition of Ca sup 2+ and Cl sup - reconstituted charge accumulation in some of these preparations. The structure of the Mn complex was investigated in preparations depleted of the extrinsic proteins. The Mn X-ray absorption edge and extended X-ray absorption fine structure (EXAFS) spectra are consistent with an oxo-bridged Mn complex, and are very similar to those obtained in intact preparations. Analysis of these data indicates that the extrinsic proteins do not ligate the Mn. In preparations containing only two Mn, the shape and position of the edge, and the EXAFS spectra are dramatically changed, suggesting that the structure of the complex is disrupted. (ERA citation 13:011213)