Proton release during the redox cycle of the water oxidase

Abstract

Old and very recent experiments on the extent and the rate of proton release during the four reaction steps of photosynthetic water oxidation are reviewed. Proton release is discussed in terms of three main sources, namely the chemical production upon electron abstraction from water, protolytic reactions of Mn-ligands (e.g. oxo-bridges), and electrostatic response of neighboring amino acids. The extent of proton release differs between the four oxidation steps and greatly varies as a function of pH both, but differently, in thylakoids and PS II-membranes. Contrastingly, it is about constant in PS II-core particles. In any preparation, and on most if not all reaction steps, a large portion of proton transfer can occur very rapidly (<20 μs) and before the oxidation of the Mn-cluster by Yz (+) is completed. By these electrostatically driven reactions the catalytic center accumulates bases. An additional slow phase is observed during the oxygen evolving step, S3⇒S4→S0. Depending on pH, this phase consists of a release or an uptake of protons which accounts for the balance between the number of preformed bases and the four chemically produced protons. These data are compatible with the hypothesis of concerted electron/proton-transfer to overcome the kinetic and energetic constraints of water oxidation.