Abstract: The reaction kinetics of lipase-catalyzed esterification of lactic acid and ethanol was studied. The reaction rate could be described in terms of a ping-pong bi-bi mechanism and double-substrate inhibition. No evidence of any diffusion limitation that affected the kinetics was detected by the comparison between the time constants for the reaction (tR) and diffusion (tD). The predicted values of the corresponding mechanism reaction fitted the experimental data very well, and the rate constants were obtained by non-linear fitting analysis. The inhibition constants for lactic acid (A) and ethanol (B) were KiA=10.7 mmol/L and KiB=275.0 mmol/L, respectively, which suggested that lactic acid, being a short-chain polar acid, caused the enzyme inactivation more seriously than ethanol. The lactic acid was concentrated in the micro-aqueous layer and caused a pH drop, leading to the enzyme inactivation in its microenvironment, and the competitive inhibition of enzyme by lactic acid existed during the lipase-catalyzed esterification reaction.

Key words: reaction kinetics, lipase, enzyme catalysis, lactic acid, ethanol, ethyl lactate, competitive inhibition, diffusion limitation