STEPWISE DISINTEGRATION OF THE PHOTOSYNTHETIC OXYGEN-EVOLVING COMPLEX

Photosynthetic water oxidation catalyzed by Photosystem II takes place at a site that comprises a redox-active tyrosine, Y-z, a tetramangane se cluster, and, in addition to its redox components, two inorganic co factors, calcium and chloride. Recent work suggests that Y-z and the m etal site are intimately linked in the oxidation and deprotonation rea ctions of substrate water. The metal cluster stores oxidizing equivale nts and provides binding sites for the substrate from which Y-z(.) is proposed to abstract hydrogen atoms during the catalytic cycle of phot osystem II. Intrinsic to this hydrogen-abstraction mechanism for water oxidation is an intimate structural and functional relationship betwe en the metal site and Y-z, which predicts that the local Y-z environme nt will be sensitive to the composition and integrity of the metal clu ster. To test this postulate, we have examined the Y-z site and its st atus with respect to solvent exposure under varying degrees of disasse mbly of the oxygen-evolving complex. H-1/H-2-isotope exchange was carr ied out for various times in samples devoid of Mn, Ca2+, and Cl-, and in samples depleted exclusively of Ca2+. The Y-z(.) andS(2)Y(z)(.) spe cies were cryotrapped to high yield in these two preparations, respect ively, and the radical site was characterized by using electron spin-e cho envelope modulation spectroscopy. The isotope exchange at the Yz s ite was completed with an upper limit on the minutes time scale in bot h the (Mn)(4)-depleted and the Ca-depleted samples. The number of isot ope-exchangeable protons in the site and their distances to Yi were fo und 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, sugg est that Y-z(.) in samples that retain the (Mn)(4) cluster, but lack C a2+, is involved in a bifurcated hydrogen bond. The data for both clas ses of samples are consistent with a hydrogen-abstraction function for Y-z in water oxidation and provide insight into the light-driven asse mbly of the (Mn)4 cluster.