title: Time-resolved oxygen production by PSII: chasing chemical intermediates.
abstract: Photosystem II (PSII) produces dioxygen from water in a four-stepped process, which is driven by four quanta of light and catalysed by a Mn-cluster and tyrosine Z. Oxygen is liberated during one step, coined S(3)=>S(0). Chemical intermediates on the way from reversibly bound water to dioxygen have not yet been tracked, however, a break in the Arrhenius plot of the oxygen-evolving step has been taken as evidence for its existence. We scrutinised the temperature dependence of (i) UV-absorption transients attributable to the reduction of the Mn-cluster and tyrosine Z by water, and (ii) polarographic transients attributable to the release of dioxygen. Using a centrifugatable and kinetically competent Pt-electrode, we observed no deviation from a linear Arrhenius plot of oxygen release in the temperature range from -2 to 32 degrees C, and hence no evidence, by this approach, for a sufficiently long-lived chemical intermediate. The half-rise times of oxygen release differed between Synechocystis WT* (at 20 degrees C: 1.35 ms) and a point mutant (D1-D61N: 13.1 ms), and the activation energies differed between species (Spinacia oleracea, 30 kJ/mol versus Synechocystis, 41 kJ/mol) and preparations (PSII membranes, 41 kJ/mol versus core complexes, 33 kJ/mol, Synechocystis). Correction for polarographic artefacts revealed, for the first time, a temperature-dependent lag-phase of the polarographic transient (duration at 20 degrees C: 0.45 ms, activation energy: 31 kJ/mol), which was indicative of a short-lived intermediate. It was, however, not apparent in the UV-transients. Thus the "intermediate" was probably newly formed and transiently bound oxygen.
authors.affiliation: Abteilung Biophysik, Fachbereich Biologie/Chemie, Barbarastr. 11, Universität Osnabrück, D-49069 Osnabrück, Germany.
authors.forename: Jürgen
authors.initials: J
authors.lastname: Clausen
authors.forename: Richard J
authors.initials: RJ
authors.lastname: Debus
authors.forename: Wolfgang
authors.initials: W
authors.lastname: Junge
mesh_terms: D000458:Cyanobacteria / Q000235:genetics / Q000378:metabolism; D004579:Electron Transport; D007700:Kinetics; D008345:Manganese / Q000737:chemistry; D009154:Mutation; D010084:Oxidation-Reduction; D010100:Oxygen / Q000378:metabolism*; D045332:Photosystem II Protein Complex / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D011048:Polarography; D013056:Spectrophotometry, Ultraviolet; D013696:Temperature; D013816:Thermodynamics
chemical_list: D045332:Photosystem II Protein Complex; D008345:Manganese; D010100:Oxygen