The role of calcium in the oxygen evolving center of photosystem II
- Univ. of California, Berkeley, CA (United States)
The photosynthetic oxygen evolving complex (OEC) contains a cluster of four manganese atoms and requires both Ca and Cl for activity. Ca can be replaced by Sr with retention of activity. The role of Ca in the OEC has been investigated by performing Mn X-ray absorption experiments on Ca-depleted samples of photosystem II (PS II) and on PS II samples depleted of Ca and reconstituted by either Ca or Sr. Mn X-ray K-edge spectra exhibit no significant differences in oxidation state or symmetry between Ca- and Sr-reactivated preparations, but differences are observed in the extended X-ray absorption fine structure (EXAFS). The amplitude of a Fourier transform peak arising from scatterers at distances greater than 3 A is larger for samples reactivated with strontium relative to calcium. Curve-fitting analyses of the EXAFS data using FEFF 5-calculated parameters favor a model where both manganese and calcium (or strontium) scatterers contribute to the ~3 Å Fourier peak (Mn-Mn at 3.3Å and Mn-Ca(Sr) at 3.4--3.5 Å). Possible structural arrangements for a calcium binding site are discussed. Analysis of Mn K-edge spectra from Ca-depleted samples in the S1, S2, and S3 states shows an edge shift on the S1-S2 transition, but no edge shift on the S2-S3 transition, supporting a model where the oxidizing equivalent from the S2 to S3 transition is stored on a ligand or nearby protein residue rather than on the Mn cluster. Parallels between Ca-depleted and native samples are discussed.
- Research Organization:
- Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- AC03-76SF00098
- OSTI ID:
- 199108
- Report Number(s):
- LBL-37897; ON: DE96004211
- Resource Relation:
- Other Information: TH: Thesis (Ph.D.); PBD: May 1995
- Country of Publication:
- United States
- Language:
- English
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