Oxidation of Water to Molecular Oxygen

Abstract

Numerous papers have been published on the topic of water oxidation; however, the light-driven water splitting enzyme still remains one of the enigmas of photosynthesis. In this chapter, we have summarized the advances made on the oxidation of H2O to O2 The water-oxidizing complex includes, at least, the reaction center proteins D1 and D2 and a 33 kilodalton extrinsic protein; other proteins seem also to be involved. The 33 kDa extrinsic protein may turn out to be dispensable for the molecular mechanism of O2 evolution. Chloride and calcium ions are also required, although their exact functions remain unknown. It is clear, however, that Mn undergoes dynamic changes as the oxygen clock moves from relaxed states So to S1 and S1 to S2 This is followed by conversion of S2 to S3 and S3 to S4 until O2 is evolved. It is accepted that Mn is the charge accumulator, but it is considered likely now that at one of the steps, a histidine residue may act as a redox active ligand, and store the charge. It was generally believed that Ih are released as 1,0, 1,2 during So → S1,S1 → S2 S2 → S3 and S3 → (S4) → So transitions. However, the currently accepted pattern is 1,0.5, I and 1.5. The nature of the intermediates of water oxidation form H20 to O2 is still unknown. Although the recent knowledge about the 3-D crystal structure of the reaction center complex from purple photosynthetic bacteria has led to a more precise picture of a portion of the water oxidizing complex than known before, further understanding will come after this complex is crystallized and its 3-D structure known and after rnethods are evolved to trap and monitor transient intermediates in water oxidation.