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a-Chymotrypsin-catalyzed acyl tranfer was studied using three acyl-group donors (Mal-~-Ala-L-Ala-L-PheOMe, Bz-L-TyrOEt and Ac-L-TrpOEt ; Mal, maleyl; Bz, benzoyl ; OMe, methyl ester; OEt, ethyl ester) and a series of amino-acid amides. Most of the reactions studied can be described by the simplest kinetic model without the nucleophile binding to the acyl-enzyme. The a-chymotrypsin-catalyzed transfer of the Mal-L-Ah-L-Ala-L-Phe group to the amides of L-Phe and L-Tyr showed a linear dependence of the partition constant, p , on the nucleophile concentration which can be interpreted by the hydrolysis of the acyl-enzyme-nucleophile complex. The a-chymotrypsincatalyzed transfer of the Bz-L-Tyr and A c -L -T~~ groups to several amino-acid amides showed unusual behavior which can be interpreted by the kinetic model involving formation of a complex of the acyl-enzyme with two nucleophile molecules. These observations can explain the conflicting conclusions concerning the kinetics of a-chymotrypsin-catalyzed acyl transfer evident in previous studies.In proteinase-catalyzed acyl-transfer reactions, the acyl group of a substrate can be transferred both to water and to the added nucleophile. As a result, the hydrolytic product donor, N is the acyl-group acceptor (added nucleophile, amino component), P is the peptide synthesized, P, and P, are the products of the hydrolysis of the acyl-group donor and ES is the enzyme complex with S and EA denoting the acyl-enzyme. The kinetic constants are indicated for the appropriate reactions in the scheme. where E represents the enzyme, S denotes the acyl-group