Influence of Organic Solvents on the Specificity of α-Chymotrypsin and Subtilisin from B. Subtilis Strain 72 in Acyl Transfer Reactions

Nucleophilic properties of amino acid amides were studied systematically in acyl-transfer reactions catalyzed by α-chymotrypsin and subtilisin from Bacillus subtilis strain 72 (subtilisin 72) using Mal-L-Ala-L-Ala-L-PheOMe and Bz-L-TyrOme as the acyl group donors. In α-chymotrypsin-catalyzed reactions in water the reactivity of the amino acid amides increases with hydrophobicity of the nucleophiles. Hydrophobic interactions in this case are responsible for the differences between the reactivity of the nucleophiles for amides of all the amino acids tested with the exception of D-AlaNH₂ and L-ArgNH₂. In a low water system (4% of H₂O, 47.5% of acetonitrile, 47.5% of dimethyl formamide, 1% of (C₂H₅)₃N) the specificity of α-chymotrypsin towards the amino acid amides in acyl transfer reactions decreases and does not depend on the amino acid side chain hydrophobicity. The specificity in this case correlates with the bulk characteristics of the amino acid side chains (normalized van der Waals volume, polarizability, molecular weight). The bulky amino acid amides are less efficient nucleophiles. In reactions catalyzed by subtilisin 72, amino acid side chain characteristics do not correlate with the nucleophile reactivities. The data obtained show that different factors may be responsible for the specificity of enzymes in water and in low water systems and in general, specificity patterns obtained in water can not be used for low water systems.