PHOTOSYNTHETIC OXYGEN EVOLUTION - H D ISOTOPE EFFECTS AND THE COUPLING BETWEEN ELECTRON AND PROTON-TRANSFER DURING THE REDOX REACTIONS AT THE OXIDIZING SIDE OF PHOTOSYSTEM-II/

The oxygen evolving complex (OEC) of Photosystem II (PS TI) incorporat es a Mn-cluster and probably a further redox cofactor, X. Four quanta of light drive the OEC through the increasingly oxidized states S-0 do uble right arrow S-1 double right arrow S-2 double right arrow S-3 dou ble right arrow S-4 to yield O-2 during the transition S-4-->S-0. It h as been speculated that the oxidation of water might be kinetically fa cilitated by the abstraction of hydrogen. This implied that the respec tive electron acceptor is deprotonated upon oxidation. Whether Y-Z and X fulfill this expectation is under debate. We have previously inferr ed a 'chemical' deprotonation of X based on the kinetics of proton rel ease (Haumann M, Drerenstedt W, Hundelt M and Junge W (1996) Biochim B iophys Acta 1273: 237-250. Here, we investigated the rates of electron transfer and proton release as function of the D2O/H2O ratio, the pH, and the temperature both in thylakoids and PS II core particles. The largest kinetic isotope effect on the rate of electron transfer (facto r of 2.1-2.4) and the largest pH-dependence (factor of about 2 between pH 5 and 8) was found on S-2 double right arrow S-3 where X is oxidiz ed. During the other transitions both factors were much smaller (less than or equal to 1.4). Electron transfer is probably kinetically steer ed by proton transfer only during S-2 double right arrow S-3. These re sults corroborate the notion that X-. serves as a hydrogen acceptor fo r bound water during S-4-->S-0. We propose a consistent scheme for the final reaction with water to yield dioxygen: two two-electron (hydrog en) transfers in series with a peroxide intermediate.