Kinetics and specificity of serine proteases in peptide synthesis catalyzed in organic solvents

Initial rates of peptide-bond synthesis catalyzed by poly(ethylene glycol)-modified chymotrypsin in benzene were determined using high-performance liquid chromatography. Enzymatic synthesis of N-benzoyl-L-tyrosyl-L-phenylalanine amide from N-benzoyl-L-tyrosine ethyl ester and L-phenylalanine amide was found to obey Michaelis-Menten kinetics an to be consistent with a ping-pong mechanism modified by a hydrolytic branch. The catalytic activity of modified chymotrypsin was dependent on both water concentration and type of organic solvent, the highest synthesis rate being obtained in toluene. Since the chymotrypsin specificity in the organic phase was actually altered, the enzyme's apparent kinetic parameters were determined for different substrates and compared to those obtained with other serine proteases in benzene. Both N-benzoyl-L-tyrosine ethyl ester and N-alpha-benzoyl-L-lysine methyl ester were comparable acyl donors in benzene and the (kcat/Km)app value of modified chymotrypsin was only 10-fold smaller than that obtained with poly(ethylene glycol)-modified trypsin in the synthesis of N-alpha-benzoyl-L-lysyl-L-phenylalanine amide. The change in chymotrypsin specificity was also confirmed through the binding of trypsin inhibitors in benzene. The overall results suggest that hydrophobic bonding between the enzyme and its substrate should not be taken into account during catalysis in the organic phase. In general, if hydrophobic interactions are involved in the binding of substrates to the active site in aqueous media, the replacement of water by hydrophobic solvents will induce some change in enzyme specificity. Moreover, secondary residues of enzyme-binding sites may also exert a significant influence on specificity since, as observed in this study, chymotrypsin exhibited high affinity for cationic substrates and cationic inhibitors as well in apolar solvents.