Rapid proton transfer under flashing light at both functional sides of dark-adapted Photosystem II particles

Abstract

By exposing dark-adapted Photosystem II core particles to a series of light flashes, we aimed at kinetic resolution of proton release during the four steps of water oxidation. The signal-to-noise ratio was improved by averaging under repetitive dark adaptation. The previously observed kinetic damping of pH-transients by particle aggregation was prevented by detergent. The complicating superimposition of protolytic events at the donor side (water oxidation) and at the acceptor side (quinone oxido-reduction) was unravelled by characterizing the rate constants of electron and proton transfer at the acceptor side ( Q A − · n H + + DCBQ → Q A + DCBQ − + n H + : k = 1.7 · 10 6 M −1 //2 DCBQ − + 2H + → DCBQ + DCBQH 2: k = 4 · 10 8 M −1 s −1). Contrasting with the pronounced period of four oscillations of the oxygen-evolving centre, the extent of proton release was practically constant. The apparent half-rise time of the stepped acidification was shortened upon lowering of the pH (250 μs at pH 7.5, 70 μs at pH 6.0 and 12 μs at pH 5.2). This kinetic behaviour was independent of the nature and the concentration of the added pH-indicator. We conclude that this reflects the protolysis of several electrostatically interacting acids at the surface of the protein in response to a new positive charge on Y Z +, and persisting upon electron transfer from the manganese cluster to Y Z +.