S-3 STATE OF THE WATER OXIDASE IN PHOTOSYSTEM-II

The effect of the reductant hydrazine on the flash-induced oxygen osci llation patterns of spinach thylakoids was used to characterize a new super-reduced redox state of the water oxidase in photosystem II. The formation of a discrete S-3 state is evident from the shift of the fir st maximum of oxygen evolution from the 3rd flash through the 5th flas h to the 7th flash during a 90 min incubation of dark-adapted thylakoi ds with 10 mM hydrazine sulfate at pH 6.8 on ice. A distinct period fo ur oscillation with further maxima on the 11th and 15th flashes is sti ll observed at this stage of the incubation. The data analysis within the framework of an extended Kok model reveals that a S-3 state popula tion of almost 50% can be achieved by this treatment. A prolonged incu bation of the S-3 sample with 10 mM hydrazine (and even 100 mM) does n ot lead to a further shift of the first maximum toward the 9th flash t hat could reflect the formation of the S-5 state. Instead, a slow oxid ation of S-3 to S-2 takes place by an as yet unidentified electron acc eptor. A consistent simulation of all the measured oxygen oscillation patterns of this study could, however, only be achieved by including t he formal redox states S-4 and S-5 in the fits (S-4 + S-5 up to 35%). The implications of these findings for the oxidation states of the man ganese in the tetranuclear cluster of the water oxidase are discussed.