A new binding site for anions which inhibit the water oxidizing complex (WOC) ofPhotosystem II in spinach has been identified. Anions which bind to this site inhibit the flash-induced SJS 0 catalase reaction (2H202 --+ 2H20 + 02) of the WOC by displacing hydrogen peroxide. Using a mass spectrometer and gas permeable membrane to detect the 3202 product, the yield and lifetime of the active state of the flash-induced catalase (to be referred to simply as 'flash-catalase') reaction were measured after forming the S z or S0-states by a short flash. The increase in flash-catalase activity with H202 concentration exhibits a Km= 10-20 mM, and originates from an increase in the lifetime by 20-fold of the active state. The increased lifetime in the presence of peroxide is ascribed to formation of the long-lived S0-state at the expense of the unstable S2-state. The anion inhibition site differs from the chloride site involved in stimulating the photolytic water oxidation reaction (2H20 --+ 02 + 4e-+ 4H+). Whereas water oxidation requires CI-and is inhibited with increasing effectiveness by F-<< CN-<< N3, the flash-catalase reaction is weakly inhibited by C1-, and with increasing effectiveness by F-<< CN-, N 3. Unlike water oxidation, chloride is unable to suppress or reverse inhibition of the flash-catalase reaction caused by these anions. The inhibitor effectiveness correlates with the pK of the conjugate acid, suggesting that the protonated species may be the active inhibitor. The reduced activity arises from a shortening of the lifetime of the flash-induced catalase active state by 3-10 fold owing to stronger anion binding in the flash-induced states, S 2 and S 0, than in the dark S-states, S 1 and S v To account for the paradoxical result that higher anion concentrations are required to inhibit at lower H202 concentrations, where S 2 forms initially after the flash, than at higher H202 concentrations, where S O forms initially after the flash, stronger anion binding to the S0-state than to the S2-state is proposed. A kinetic model is given which accounts for these equilibria with anions and H202. The rate constant for the formation/release of 02 by reduction of S 2 in the WOC is < 0.4 s -1.
Abbreviations: ADRY -acceleration of the deactivation reactions of the water splitting enzyme system Y; BTP -bis [tris(hydroxymethyl)methylamino]-propane; CCCP -carbonylcyanide m-chlorophenylhyrazone; DCBQ -2,5-dichlorobenzoquinone; DMBQ -2,3-dimethylbenzoquinone; WOC -water oxidizing complex