INTERACTION OF HYDROXYLAMINE WITH THE WATER‐OXIDIZING ENZYME INVESTIGATED VIA PROTON RELEASE*

Abstract

Abstract— Photosynthetic water oxidation is a four‐step redox process which is driven by a one‐quantum‐one‐electron reaction center. Stepwise electron Abstract—ion from the water‐oxidizing enzyme is accompanied by stepwise proton release with the following stoichiometric pattern at given half‐rise times: 1 H⁺ (250 μs, S₀→ S₁):0 H⁺(S₁→ S₂): 1 H⁺ (200 μs, S₂→ S₃): 2 H⁺ (1.2 ms, S₃→ S₄→ S₀). (Förster and Junge, 1985, preceding article in this issue). Hydroxylamine at low concentrations (≲100 μ M) appears to compete with water at the active site of the water‐ oxidizing enzyme. Its interference shifts the dark state of the water‐oxidizing enzyme by two steps backwards (Bouges, 1971). We found that the hydroxylamine‐induced shift was also reflected in the stoichiometric pattern and in the kinetics of proton‐release. In the presence of hydroxylamine, two protons per reaction center were released with a half‐rise time of ≅ 2 ms upon the first exciting flash given to dark adapted thylakoids. This was slower than observed for each of the protons released during unperturbed water oxidation. One proton was released upon the second flash. The half‐rise time of the main component observed upon the second flash in hydroxylamine‐treated samples agreed with the one observed upon the fourth flash in the absence of hydroxylamine, which had been attributed to the S₀→ S₁ transition. The two protons which were observed upon the first flash in hydroxylamine‐treated thylakoids may be due to hydroxylamine oxidation or to the association of water to the catalytic manganese center after detachment of oxidized hydroxylamine from its binding site.