Studies on the reaction coordinates of the water oxidase in PS II membrane fragments from spinach

Abstract

The temprature dependence of the rate constants of the univalent redox steps Y,"'S, + Y,S,+, (i = 0.1,21 and Y.""S, 3 (Y&) + YSO + Oz in the water oxidase was investigated by measuring time resolved absorption changes at 355 nm induced by a laser flash train in dark adapted PS II membrane fragments from spinach. Activation energies of 5.0. 12.0 and 36.0 kJ/mol were obtained for the reactions Y,"*S, + YrSr+, with i = 0.1 and 2, respectively. The reaction Yp'S, + (Y&) 3 Y$, + 0: exhibits a temperature dependence with a characteristic break point at 279 K with activation energies of 2L1 kJ/mol (T > 279 K) and 46 kJ/mol (T < 279 K). Evaluation of the data within the framework of the classical Marcus theory of nonadiabatic electron transfer :(1985) Biochim. Biophys. Acta 811,2&L3221 leads to the conclusion that the SI! oxidation to S, is coupled with significant structural changes. Furthermore, the water oxidase in S3 is inferred to attain two different conformational states with populations that markedly change at a chrtracteristic transition temperature, PS II membrane fragment; Water oxidasc; Activation energy; Nonadiabatic electron transfer; S, conformation 1, INTRODUCTIONPhotosynthetic water oxidation to dioxygen and protons released into the thylakoid lumen takes place via a sequence of four univalent oxidation steps, comprising redox transitions at a manganese-containing unit (for recent reviews see [1,2]) Although the kinetics of the individual univalent redox steps have been unraveled by time resolved EPR-and W-spectroscopic measurements [3-51, the electronic configuration and the nuclear geometry of the intermediary redox states of the water oxidase are still unknown. To address this problem, two questions of central mechanistic relevance have to be answered: (i) are the univalent electron abstractions, symbolized by St + Sr+l, metal-or ligandcentered processes; and (ii) at which redox level does the key step of the overall reaction take place, i.e. the formation of an oxygen-oxygen bond? Based on our current knowledge it seems established that the S, + S2 redox transition is a manganese centered Mn(II1) + Mn(IV) oxidation whereas much less information is available on the other steps (for discussion see last univalent oxidation step although alternative mechanisms cannot be excluded [1,6]. Apart from spectral analyses, recent studies on the interaction of small redox active agents (NHzOH, NHzNH2) with the water oxidase led to the conclusion that the redox transition S1 + S3 is accompanied by a significant nuclear rearrangement [7,8].In this report results are presented on an attempt to gather further information on the reaction coordinates of the univalent redox steps in the water oxidase by determining the temperature dependence of their rate constants from measurements of 355 nm absorption changes induced by a train of laser flashes in dark adapted PS II membran fragments. The data obtained reveal: (i) the formation of S3 is coupled with significant structural changes; and (ii) the water oxidase in S3 und...

Photosynthetic oxygen evolution: Net charge transients as inferred from electrochromic bandshifts are independent of proton release into the medium

“…Electron transfer and electrochromism were recorded after adding even more, namely 10 mM imidazole, which fully and rapidly quenched all pH-indieating transients of Neutral red and further accelerated proton release. At 360 mu (left) a fast and unresolved jump, mainly due to QA reduction [19], was induced by the third flash. It was followed by a decay with half-rise time of 1.1 ms. A small rise with half-rise time of 260 ps was superimposed on the decay.…”

“…With thylakoids the media contained no CaCI,. Electrochromism was recorded at 443 nm (no differences between two wavelengths) and electron transfer (Mn to Y3 at 360 mn [19].…”

Effects of methanol on the S i -state transitions in photosynthetic water-splitting

“…S 0 transition and the second one may bind during the S 2 ? S 3 transition that is known to be coupled with significant structural changes (Boussac and Rutherford 1988;Boussac et al 1990;Dau et al 2001;Guiles et al 1990;Haumann et al 2005;Liang et al 2000;Messinger et al 1991;Noguchi and Sugiura 2002;Pushkar et al 2008;Renger and Hanssum 1992;Siegbahn 2008). To decide between both possibilities further advanced EPR, FTIR/Raman, and time-resolved membrane-inlet mass spectrometry measurements will be required.…”

Structural Change of the Mn Cluster during the S2→S3 State Transition of the Oxygen-Evolving Complex of Photosystem II. Does It Reflect the Onset of Water/Substrate Oxidation? Determination by Mn X-ray Absorption Spectroscopy

“…[67][68][69] The activation energy increases by a factor of about 3 for S 2 →S 3 compared to that for the S 1 →S 2 transition. The authors attribute this marked difference in activation energy to the involvement of a significant structural change in the S 2 →S 3 transition that is absent in the S 1 →S 2 transition.…”