Studies on the interaction between hydroxylamine and hydrazine as substrate analogues and the water-oxidizing enzyme system in isolated spinach chloroplasts

Abstract

The functional interaction between the photosynthetic water-oxidizing enzyme system and the substrate analogues hydroxylamine and hydrazine has been analyzed in isolated class II chloroplasts by measuring the effect of these species on the characteristic oscillation pattern of oxygen yield induced by a flash train. The following was found. (1) At concentrations where both substances cause the pronounced two-flash phase shift (Bouges, B. (1971) Biochim. Biophys. Acta 234, 103–112) the dark equilibration is rather slow with half-times of approx. 1 min. (2) The numerical evaluation of the oscillation patterns reveals quantitative differences between hydroxylamine and hydrazine. The interaction with hydroxylamine is complex. It involves one- and two-electron processes as well as fast reaction steps during the flash sequence. The fast reactions take place only with redox states S 2 and S 3 of the water-oxidizing enzyme. Furthermore, the redox turnover in the presence of hydroxylamine leads to an S 1-state that differs markedly in its susceptibility to hydroxylamine from that of S 1 in control chloroplasts. (3) Below a threshold concentration which varies for different preparations the hydrazine effect can be quantitatively described by the assumption that after dark equilibration the agent becomes consumed irreversibly via a reaction with two oxidizing redox equivalents produced by PS II. This process is accomplished during the first two flashes. No further interaction occurs during the flash sequence, so that besides the two-flash phase shift the water-oxidizing enzyme system reveals the normal oxygen-evolution pattern. (4) Based on the analysis of the concentration dependence hydrazine is inferred to interact with the catalytic center of the water-oxidizing enzyme system via a cooperative mechanism including two binding sites. The data are discussed in terms of the kinetics of the dark interaction and its possible rate limitation. Mechanistic aspects (ligand-ligand exchange at the functional manganese cluster and transport step) are considered. Furthermore, possible mechanisms for the redox reaction of hydrazine at the catalytic site are briefly discussed.