The reactivity of hydrazine with photosystem II strongly depends on the redox state of the water oxidizing system

Abstract

The decay kinetics of the redox states S 2 and S 3 of the water-oxidizing enzyme have been analyzed in isolated spinach thylakoids in the absence and presence of the exogenous reductant hydrazine. In control samples without NH 2NH 2 a biphasic decay is observed. The rapid decline of S 2 and S 3 with y D as reductant exhibits practically the same kinetics with t 1/2 = 6-7 s at pH = 7.2 and 7°C. The slow reduction (order of 5-10 min at 7°C) of S 2 and S 3 with endogenous electron donors other than y D is about twice as fast for S 2 as for S 3 under these conditions. In contrast, the hydrazine-induced reductive shifts of the formal redox states S i (i = 0⋯3) are characterized by a totally different kinetic pattern: (a) at 1 mMNH 2NH 2 and incubation on ice the decay of S 2 is estimated to be at least 25 times faster ( t 1/2⩽0.4 min) than the corresponding reaction of S 3 ( t 1/2≈13 min); (b) the NH 2NH 2-induced decay of S 2 is even slower (about twice) than the transformation of S 1 into the formal redox state ‘S −1’ ( t 1/2≈6 min), which gives rise to the two-digit phase shift of the oxygen-yield pattern induced by a flash train in dark adapted thylakoids. (c) the NH 2NH 2-induced transformation S 0→‘S −2’ [Renger, Messinger and Hanssum (1990) in: Curr. Res. Photosynth. (Baltscheffsky, M., ed). Vol. 1, pp. 845-848, Kluwer, Dordrecht] is about three times faster ( t 1/2≈ min) than the reaction ▪. Based on these results, the following dependence on the redox state S i of the reactivity towards NH 2NH 2 is obtained: S 3 < S 1 < S 0 ⪡ S 2. The implications of this surprising order of reactivity are discussed.