The role of calcium in the oxygen evolving center of photosystem II

Abstract

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 {approximately}3 {Angstrom} Fourier peak (Mn-Mn at 3.3{Angstrom} and Mn-Ca(Sr) at 3.4--3.5 {Angstrom}). Possible structural arrangements for a calcium binding site are discussed. Analysis of Mn K-edge spectra from Ca-depleted samples in the S{sub 1}, S{sub 2}, and S{sub 3} states shows an edge shift on the S{sub 1}-S{sub 2} transition, but no edge shift on the S{sub 2}-S{sub 3} transition, supporting a model where the oxidizing equivalent from the S{sub 2} to S{sub 3} 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.