Co-solvent Effects on Reaction Rate and Reaction Equilibrium of an Enzymatic Peptide Hydrolysis

Abstract

This work presents an approach that expresses the Michaelis constant K_M^a and the equilibrium constant Kth of an enzymatic peptide hydrolysis based on thermodynamic activities instead of concentrations. This provides K_M^a and Kth values that are independent of present co-solvent. To this end, the hydrolysis reaction of N-succinyl-L-phenylalanine-p-nitroanilide catalysed by the enzyme α chymotrypsin was studied in pure buffer and in the presence of the co-solvents dimethyl sulfoxide, trimethylamine-N-oxide, and urea. A strong influence of the co-solvents on the measured Michaelis constant (KM) and equilibrium constant (Kx) was observed, which was found to be caused by molecular interactions expressed as activity coefficients. Substrate and product activity coefficients were used to calculate the activity-based values K_M^a and Kth for the co-solvent free reaction. Based on these constants, the co-solvent effect on KM and Kx was predicted in almost quantitative agreement with the experimental data. The approach presented here does not only reveal the importance of understanding the thermodynamic non-ideality of reactions taking place in biological solutions and in many technological applications, it also provides a framework for interpreting and quantifying the multifaceted co-solvent effects on enzyme-catalysed reactions that are known and have been observed experimentally for a long time.