Cooperative and reversible action of three or four hydroxylamine molecules on the water‐oxidizing complex

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Abstract
Photosynthetic water oxidation is a four‐step oxidation process which is formally described by the transitions S0 → S1, S1 → S2, S2 → S3, S3 → S4 → S0. In dark‐adapted thylakoids S1 is the most stable and oxygen is liberated after abstraction of three oxidizing equivalents from the water‐oxidizing complex. If hydroxylamine is present at low concentrations (15–100 μM), five instead of three oxidizing equivalents have to be abstracted before dioxygen is liberated. We measured the concentration dependence of this two‐digit shift via the pattern of proton release which is associated with water oxidation. We obtained an apparent Hill coefficient of 2.43. This result could be explained by cooperative binding of 3 or 4 hydroxylamine molecules, and by the assumption that each center which was modified by >= 1 hydroxylamine molecule gave rise to the same modified pattern of proton release.