The pH-dependence of enzymic ester hydrolysis

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Abstract
The change of hydrolytic rate with pH was studied for different types of esterases and with a variety of uncharged (non-cationic) substrates. From these measurements, the dissociation constants of the nucleophilic (pKa) and the electrophilic (pKb) group of the esteratic site were calculated for each enzyme -substrate system. The esteratic sites of all esterases studied possess approximately the same dissociation constants. The chemical structure of the active surfaces of various esterases is thus closely related. The dissociation constants of the negative sites of true cholinesterase and pseudocholinesterase were derived from the pH-dependence of the inhibitory effect of tetraethylammonium ion. The curious values obtained, 6.2-6.5, seem to indicate neutralization at acid pH of the anionic sites by the iminazolinium ion of the esteratic group. Possible structures of the esteratic sites are discussed, and explanations put forward for the inhibition by large substrate concentrations of the hydrolysis of cationic and uncharged esters and for the different behavior of true cholinesterase and pseudocholinesterase towards the homologous series of aliphatic choline esters.
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