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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