HIGH RESOLUTION MN X-RAY EMISSION SPECTROSCOPY OF THE OXYGEN-EVOLVING COMPLEX

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This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Oxygen, that supports all aerobic life, is abundant in the atmosphere because of its constant regeneration by photosynthetic water oxidation by green plants, algae, and cyanobacteria. This light-requiring reaction is catalyzed by a Mn4Ca cluster associated with photosystem II (PS II). In our ongoing investigations of the Mn oxidation-state changes, the structure and the associated cofactors Ca and Cl and the mechanism of water oxidation, we are using the BioCAT beamline to conduct X-ray absorption and emission spectroscopy experiments that are at present not possible elsewhere. We have shown the feasibility of the range-extended EXAFS method and collected data without interference from Fe in the PS II sample. The improvement in the k-space data and the Fourier transform is striking. We can now resolve two short Mn-Mn distances in the S (subscript)1 and S (subscript)2 states; while earlier we could discern only one distance of ~2.7 ¿. We have also collected range-extended EXAFS data using oriented PS II membranes, and it is quite clear that the heterogeneity is more obvious. These results which provide important information about the structure of the water oxidizing Mn complex are in press in J. Biol. Chem. and this publication has been selected as the paper of the week with a cover figure. We have extended these studies to the S (subscript)0 and S (subscript)3 intermediate states of the enzymatic cycle, and the preliminary results show that there is a significant change in the structure of the Mn complex in the S (subscript)3 state. This result has important implications for choosing between the many mechanisms that have been proposed for the water oxidation reaction.