STUDIES ON THE MECHANISM OF PHOTOSYNTHETIC OXYGEN FORMATION

Abstract

This chapter presents the studies on the mechanism of photosynthetic oxygen formation. Photosynthetic water cleavage by visible light is the cornerstone of the evolutionary development of solar radiation exploitation as unique free energy source of biological organisms. It is energetically driven by two light reactions that are connected via a plastoquinone pool. The reaction sequence in system II leads to water cleavage into O2 and hydrogen bound to plastoquinone (PQH2). Subsequent reactions supported by photosystem I enhance the electronic energy of metabolically bound hydrogen from the level of PQH2 up to that of nicotinamide adenine dinucleotide phosphate hydride. A high quantum efficiency of the reaction center processes requires a delicate balance of the rate constants for the forward and back reaction of each individual transfer step. This essential problem is obviously solved by selection of appropriate redox groups that are incorporated into a special protein matrix, so that a definite mutual electronic coupling arises. A complete understanding of the molecular mechanism of photosynthetic water oxidation can be only achieved if dynamic structural changes occurring in the apoenzyme are unraveled, because in well characterized enzymes redox changes are known to be coupled with functional significant structural changes.