Kinetic characterization of the interaction of NO with the S2and S3-states of the oxygen-evolving complex of Photosystem II

The reactivity of the S 3 and S 2 states towards NO and NH 2 OH was studied and compared using the period-4 oscillations in the F 0 -value induced by a train of single turnover Xenon flashes spaced 100 ms apart to monitor the reaction kinetics. The flash frequency also determined the time resolution of the assay, i.e. 100 ms. The S 2 and S 3 -states were created by one and two single turnover pre-flashes, respectively. The NO-concentration-dependence of the S 3 -decay indicated that at low NO-concentrations an S 2 -state was formed as an intermediate, whereas at higher concentrations a seemingly monophasic decay to the S 1 -state was observed. The sigmoidal concentration dependence indicated that a fast interaction of the S 3 -state with (at least) two NO-molecules is necessary for the fast S 3 to S 1 decay (τ ~0.4 s at 1.2 mM NO). The pH-dependence of the S 3 -decay suggests that a protonation-reaction (pK ~6.9) is involved in the S 3 to S 1 decay. At intermediate NO-concentrations the protonation is only partially rate limiting, since the pH effect is more pronounced at high compared to intermediate NO-concentrations. A comparison of the reactivity of NO and hydroxylamine suggests that hydroxylamine reacts more efficiently with the S 1 and S 2 states, whereas NO reacts more efficiently with the S 3 -state. Based on our present knowledge of the oxygen evolving complex a possible reaction mechanism is proposed for the interaction between NO and the S 3 state.