Stepwise Disintegration of the Photosynthetic Oxygen-Evolving Complex

Abstract

Photosynthetic water oxidation catalyzed by Photosystem II takes place at a site that comprises a redox-active tyrosine, YZ, a tetramanganese cluster, and, in addition to its redox components, two inorganic cofactors, calcium and chloride. Recent work suggests that YZ and the metal site are intimately linked in the oxidation and deprotonation reactions of substrate water. The metal cluster stores oxidizing equivalents and provides binding sites for the substrate from which YZ is proposed to abstract hydrogen atoms during the catalytic cycle of photosystem II. Intrinsic to this hydrogen-abstraction mechanism for water oxidation is an intimate structural and functional relationship between the metal site and YZ, which predicts that the local YZ environment will be sensitive to the composition and integrity of the metal cluster. To test this postulate, we have examined the YZ site and its status with respect to solvent exposure under varying degrees of disassembly of the oxygen-evolving complex. 1H2H-isotope exchange was carried out for various times in samples devoid of Mn, Ca2+, and Cl-, and in samples depleted exclusively of Ca2+. The YZ and S2YZ species were cryotrapped to high yield in these two preparations, respectively, and the radical site was characterized by using electron spin-echo envelope modulation spectroscopy. The isotope exchange at the YZ site was completed with an upper limit on the minutes time scale in both the (Mn)4-depleted and the Ca-depleted samples. The number of isotope-exchangeable protons in the site and their distances to YZ were found to be different in the two systems, indicating that YZ is shielded from the solvent in the Ca-depleted system and, upon removal of the (Mn)4 cluster, becomes accessible to bulk water. The results from an electron spin-echo analysis of S2YZ, in the weak-coupling limit, suggest that YZ in samples that retain the (Mn)4 cluster, but lack Ca2+, is involved in a bifurcated hydrogen bond. The data for both classes of samples are consistent with a hydrogen-abstraction function for YZ in water oxidation and provide insight into the light-driven assembly of the (Mn)4 cluster. (Less)