Reactivity and catalysis in reactions of the serine hydroxyl group and of O-acyl serines.

Certain acyl transfer and hydrolytic reactions catalyzed by chymotrypsin and related enzymes proceed with the formation of an isolable enzyme-substrate intermediate, in which the acyl portion of the substrate is covalently bound to the enzyme. A considerable body of evidence now exists which suggests that the site of enzyme acylation, as well as phosphorylat,ion, is the hydroxyl group of a serine residue in the active site. The acylation and deacylation of this serine residue may be aided by the catalytic effect of the imidazole group of a histidine residue, which is held in a suitable position for catalysis by the tertiary structure of the protein, but is not adjacent to the serine in the peptide chain (l-9). It has recently been shown that imidazole can catalyze certain acyl transfer reactions without the formation of acetylimidazole, by removing a proton from the attacking reagent in the transition state, i.e. by classical general base catalysis (lo), and it has been suggested that imidazole may act in a similar manner in the enzymic acylation and deacylation of the serine hydroxyl group (1,7,10-13). The experiments reported here were carried out to obtain information on the nucleophilic properties of the serine hydroxyl group and on catalysis of reactions of 0-acyl serines, in order to provide a more complete background for consideration of the enzyme-catalyzed reactions. Preliminary reports of portions of this work have been published (10, 14).