Feb 28, 2002

An evaluation of structural models for the photosynthetic water-oxidizing complex derived from spectroscopic and X-ray diffraction signatures

Journal of Biological Inorganic Chemistry : JBIC : a Publication of the Society of Biological Inorganic Chemistry
G CarrellC Dismukes

Abstract

Four of the five intermediate oxidation states (S-states) in the catalytic cycle of water oxidation used by O2-evolving photoautotrophs have been previously characterized by EPR and/or ENDOR spectroscopy, with the first reports for the S0, S1, and S3 states available in just the last three years. The first electron density map of the Mn cluster derived from X-ray diffraction measurements of single crystals of photosystem II at 3.8-4.2 A resolution has also appeared this year. This wealth of new information has provided significant insight into the structure of the inorganic core (Mn4OxCa1Cl1-2), the Mn oxidation states, and the location and function of the essential Ca2+ cofactor within the water-oxidizing complex (WOC). We summarize these advances and provide a unified interpretation of debated structural proposals and Mn oxidation states, based on an integrated analysis of the published data, particularly from Mn X-ray absorption spectroscopy (XAS) and EPR/ENDOR data. Only three magnetic spin-exchange models for the inter-manganese interactions are possible from consideration of the EPR data for the S0, S1, S2 and S(-N) (NO-reduced) states. These models fall into one of three types denoted butterfly, funnel, or tetrahedron. A...Continue Reading

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Mentioned in this Paper

Manganese
Calcium [EPC]
Crystal - Body Material
Dioxygen
Calcium
Complex (molecular entity)
Electron Density
Electron Transport
Free Radicals
Calcium Binding

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