INHIBITION OF THE CATALASE REACTION OF PHOTOSYSTEM-II BY ANIONS

A new binding site for anions which inhibit the water oxidizing comple x (WOC) of Photosystem II in spinach has been identified. Anions which bind to this site inhibit the flash-induced S-2/S-0 catalase reaction (2H(2)O(2) --> 2H(2)O + O-2) of the WOC by displacing hydrogen peroxi de. Using a mass spectrometer and gas permeable membrane to detect the O-32(2) product, the yield and lifetime of the active state of the fl ash-induced catalase (to be referred to simply as 'flash-catalase') re action were measured after forming the S-2 or S-0-states by a short fl ash. The increase in flash-catalase activity with H2O2 concentration e xhibits a K-m = 10-20 mM, and originates from an increase in the lifet ime by 20-fold of the active state. The increased lifetime in the pres ence of peroxide is ascribed to formation of the long-lived S-0-state at the expense of the unstable S-2-state. The anion inhibition site di ffers from the chloride site involved in stimulating the photolytic wa ter oxidation reaction (2H(2)O --> O-2 + 4e(-) + 4H(+)). Whereas water oxidation requires Cl- and is inhibited with increasing effectiveness by F- << CN- << N-3(-), the flash-catalase reaction is weakly inhibit ed by Cl-, and with increasing effectiveness by F- << CN-, N-3(-). Unl ike water oxidation, chloride is unable to suppress or reverse inhibit ion of the flash-catalase reaction caused by these anions. The inhibit or effectiveness correlates with the pK(a) of the conjugate acid, sugg esting that the protonated species may be the active inhibitor. The re duced activity arises from a shortening of the lifetime of the flash-i nduced catalase active state by 3-10 fold owing to stronger anion bind ing in the flash-induced states, S-2 and S-0, than in the dark S-state s, S-1 and S--1. To account for the paradoxical result that higher ani on concentrations are required to inhibit at lower H2O2 concentrations , where S-2 forms initially after the flash, than at higher H2O2 conce ntrations, where S-0 forms initially after the flash, stronger anion b inding to the S-0-state than to the S-2-state is proposed. A kinetic m odel is given which accounts for these equilibria with anions and H2O2 . The rate constant for the formation/release of O-2 by reduction of S -2 in the WOC is < 0.4 s(-1).