A reaction in which the hydron [International Union of Pure and Applied Chemistry (1986) Chem. Int. 8 (4), 21] is a reactant will have a more acidic pH optimum than the reverse reaction in which it is a product. Simple mechanistic models, made by postulating various ways in which the hydron is supplied for the forward reaction, lead to the prediction that the pK that characterizes the fall in activity on the alkaline side for the forward reaction will be equal to that characterizing the fall on the acid side for the reverse reaction. It is not possible to identify what is responsible for this pK without additional information; as is well known, different possible dispositions of hydrons in transition states cannot usually be distinguished solely by kinetics.

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