ANALYSES OF PH-INDUCED MODIFICATIONS OF THE PERIOD-4 OSCILLATION OF FLASH-INDUCED OXYGEN EVOLUTION REVEAL DISTINCT STRUCTURAL-CHANGES OF THE PHOTOSYSTEM-II DONOR SIDE AT CHARACTERISTIC PH VALUES

This study presents a thorough analysis of the reaction pattern of fla sh-induced oxygen evolution in spinach thylakoids as a function of pH (4.5 I pH I 9) and the redox state of tyrosine Y-D in polypeptide D2. Evaluation of the experimental data within the conventional Kok model [Kok, B., Forbush, B., and McGloin, M. (1970) Photochem. Photobiol. 11 , 457-475] led to the following results: (1) the probability of the mi ss factor is strongly pH dependent (with a pronounced minimum near neu tral pH) while the double hit factor is less affected; (2) a marked in crease of the apparent S-0 population arises at alkaline pH in dark-ad apted samples where most of the Y-D is reduced, but this effect is abs ent if the percentage of PS II containing the oxidized form Y-D(ox) is high; and (3) the lifetimes of S-2 and S-3 exhibit a characteristic p H dependence that is indicative of conformational changes of functiona l relevance within the water-oxidizing complex and its environment; (4 ) the kinetic interaction of redox states S-2 and S-3 With Y-D is char acterized by a change of its behavior at a threshold pH of 6.5-7.0; an d (5) at acidic pH values the extent of S-2 and S-3 reduction by Y-D d ecreases concomitant with the occurrence of a very fast decay kinetics . On the basis of a detailed discussion of these results and data from the literature, the water oxidase is inferred to undergo structural c hanges at pH values of 5-5.5 and 6.5-7.0. These transitions are almost independent of the redox state S-i and modify the reaction coordiates of the water oxidase toward endogenous reductants.