Evidence for the proximity of calcium to the Manganese cluster of photosystem II: Determination by X-ray absorption spectroscopy
- Lawrence Berkeley Lab., CA (United States); and others
The photosynthetic oxygen-evolving complex contains a cluster of four manganese atoms and requires both Ca and Cl for activity. The question of Ca proximity to the Mn cluster has been investigated by performing Mn X-ray absorption experiments on native samples of photosystem II (PS II) and on samples depleted of Ca and reconstituted by either Ca or Sr. Analysis of X-ray K-edge spectra demonstrates no significant differences in oxidation state or symmetry between Ca- and Sr-reactivated preparations. Differences are observed in the extended X-ray absorption fine structure (EXAFS). The amplitude of a Fourier transform peak due to scatterers at distances greater than 3 {angstrom} is larger for samples reactivated with strontium than for calcium-reactivated samples. Taking into account the stoichiometry of Mn and Ca atoms in PS II and considering physically reasonable structures, curve-fitting analyses of the EXAFS data using FEFF5-calculated parameters favor a model where both manganese and calcium (or strontium) scatterers contribute to the Fourier peak at approximately 3 {angstrom}. Other models for the {approximately}3 {angstrom} peak with multiple Mn-Mn interactions or multiple Mn-Ca(Sr) interactions can also be fit to the data, but are considered less likely. This result provides confirmation for the structural proximity of Ca to the Mn cluster suggested previously. Possible structural arrangements for a calcium-binding site are discussed. 53 refs., 7 figs., 5 tabs.
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- AC03-76SF00098
- OSTI ID:
- 245320
- Journal Information:
- Biochemistry (Eaton), Journal Name: Biochemistry (Eaton) Journal Issue: 34 Vol. 34; ISSN 0006-2960; ISSN BICHAW
- Country of Publication:
- United States
- Language:
- English
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