Reaction kinetics of immobilized α chymotrypsin in organic media 2. Effects of substrate partition

The reaction kinetics of αchymotrypsin (EC 3.4.21.1.) catalyzed esterification of N-protected phenylalanine with ethanol were studied. The enzyme was deposited on Chromosorb and reactions were performed mainly in water-saturated mixtures of ethyl acetate and heptane, but also other media were used, such as mixtures of ethyl acetate and acetonitrile. The hydrophobicity of the substrate was varied by using different N-protecting groups (acetyl, Cbz and Fmoc). The apparent Km obtained for the three substrates were 1.1, 7.8 and 17 mM, respectively. The apparent Vmax decreased as the hydrophobicity of the substrates increased. The reaction medium greatly affected the apparent kinetic parameters, Km and Vmax. Nonpolar media (increasing proportion of heptane in mixtures of ethyl acetate and heptane) increased the apparent Kmax and decreased the apparent Km. The effects on the apparent Km values could be correlated with the partitioning of the substrates between the reaction medium and an aqueous phase. In mixtures of acetonitrile and ethyl acetate both the apparent Km and the apparent Vmax decreased as the proportion of acetonitrile increased. The apparent Km and Vmax were also dependent on the water content in the reaction media as well as the buffer concentration and buffer pH