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
Autor(en): | Haumann, M Bogershausen, O Cherepanov, D Ahlbrink, R Junge, W |
Stichwörter: | CENTER CORE PREPARATION; electron transfer; EPR SPECTROSCOPY; EVOLVING COMPLEX; FLASHING LIGHT; H/D-isotope effect; HISTIDINE OXIDATION; MANGANESE CLUSTER; MEMBRANE-FRAGMENTS; peroxide; photosystem II; Plant Sciences; proton release; REDUCTION KINETICS; TYROSINE Y-Z; WATER OXIDATION | Erscheinungsdatum: | 1997 | Herausgeber: | KLUWER ACADEMIC PUBL | Journal: | PHOTOSYNTHESIS RESEARCH | Volumen: | 51 | Ausgabe: | 3 | Startseite: | 193 | Seitenende: | 208 | Zusammenfassung: | The oxygen evolving complex (OEC) of Photosystem II (PS TI) incorporates a Mn-cluster and probably a further redox cofactor, X. Four quanta of light drive the OEC through the increasingly oxidized states S-0 double right arrow S-1 double right arrow S-2 double right arrow S-3 double right arrow S-4 to yield O-2 during the transition S-4-->S-0. It has been speculated that the oxidation of water might be kinetically facilitated by the abstraction of hydrogen. This implied that the respective electron acceptor is deprotonated upon oxidation. Whether Y-Z and X fulfill this expectation is under debate. We have previously inferred a `chemical' deprotonation of X based on the kinetics of proton release (Haumann M, Drerenstedt W, Hundelt M and Junge W (1996) Biochim Biophys 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 (factor 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 oxidized. During the other transitions both factors were much smaller (less than or equal to 1.4). Electron transfer is probably kinetically steered by proton transfer only during S-2 double right arrow S-3. These results corroborate the notion that X-. serves as a hydrogen acceptor for bound water during S-4-->S-0. We propose a consistent scheme for the final reaction with water to yield dioxygen: two two-electron (hydrogen) transfers in series with a peroxide intermediate. |
ISSN: | 01668595 | DOI: | 10.1023/A:1005861917596 |
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