Phosphate esters dissociation constants

With different aryl phosphates at a fixed substrate concentration an analogous set of curves is generated (155). A correlation between the second dissociation constant of the ester and the pH optimum, rate of hydrolysis, and Km value was noted With increasing pK of the substrate, the pH optimum became more neutral, the rate of hydrolysis decreased, and Km increased. With rat intestinal phosphatase and / -glycerophosphate a linear relationship was found between the logarithm of the substrate concentration and the pH optimum (158) and has since been confirmed for tissue phosphatases of several mammalian species and of poultry (156). 

Like other esters, phosphates undergo hydrolysis to the parent acid and alcohol. Here, the acidity of —OH attached to phosphorus has several effects. In the first place, since acidic phosphate esters can undergo ionization, there may be many species present in the hydrolysis solution. A monoalkyl ester, for example, could exist as dianion, monoanion, neutral ester, and protonated ester any or all of these could conceivably be undergoing hydrolysis. Actually, the situation is not quite that complicated. From the dissociation constants of these acidic esters, one can calculate the fraction of ester in each form in a given solution. The dependence of rate on acidity of the solution often shows which species is the principal reactant. 

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