Substrate specificity of α-chymotrypsin-catalyzed esterification in organic media

11 amino acid derivatives were tested as α-chymotrypsin substrates in the esterification reaction with methanol in organic media. The reactions were carried out in water-saturated ethyl acetate and in acetonitrile containing 4% water. α-Chymotrypsin adsorbed on Celite was used as a catalyst. From initial reaction rate measurements, the Michaelis-Menten parameters V_max and K_M were determine. All the amino acid derivatives tested were esterified and the highest values of k_cal K_M were obtained with the N-acylated aromatic amino acids. Correlations between Michaelis-Menten parameters and physical properties of the substrates such as molar refractivity (MR) and log P were deduced. The results show that the specificity of the α-chymotrypsin towards the side chain of the amino acids in organic media is the same as that in aqueous media. However, the specificity towards the N-protecting group is opposite to that in water, so the reaction medium affects the interactions of this part of the molecule with the enzyme to a large extent.