Structure and Function of the Manganese Complex Involved in Photosynthetic Oxygen Evolution Determined by X-Ray Adsorption Spectroscopy and Electron Paramagnetic Resonance Spectroscopy.

摘要

Water is the terminal electron donor in the linear light-driven electron transport chain used by higher plants, cyanobacteria and green algae to fix carbon dioxide. The involvement of a membrane-bound manganese-containing protein complex has been demonstrated at the site of water oxidation within the photosystem II (PSII) reaction center. The photosynthetic oxidation of water to molecular oxygen is believed to involve intermediate S-states (S (sub 0)/hor ellipsis/S (sub 4)), of the oxygen evolving complex (OEC). The use of multiline EPR signal associated with Mn and assigned to the S sub 2 state has greatly facilitated structural characterization of the OEC. This thesis contains a description of methods used to cryogenically stabilize PSII preparations suitable for x-ray absorption spectroscopy in the S sub 1 , S (sub 2) and S (sub 3) states as well as a state induced by hydroxylamine resembling the S sub 0 state of the OEC. Studies of the Mn K-edges of PSII preparations indicate that a light-induced oxidation of Mn occurring during the S sub 1 implies S (sub 2) state transition corresponds to a formal valence change from Mn(III) to Mn(IV). An analysis of the extended x-ray absorption fine structure (EXAFS) of the Mn complex within PSII preparations poised in the S sub 1 , S (sub 2), S (sub 3) and hydroxylamine-induced S sub 0 states indicates that the four manganese present are organized as two di- mu -oxo bridged binuclear managanese complexes. An essential component of the analysis of the EXAFS was a parallel analysis of a set of crystallographically characterized multinuclear mu -oxo bridged manganese complexes. Based on conclusions drawn from the analysis of the Mn K-edge and EXAFS of PSII preparations cryogenically stabilized in the S-states described above, a model for the mechanism of photosynthetic water oxidation is presented. 274 refs., 46 figs., 17 tabs. (ERA citation 13:039508)