The Kinetic Study of Enzyme Action on Substrate Monolayers

The enzymatic reaction of porcine pancreatic lipase with insoluble monolayers of either trioctanoin or 1,2-dioctanoin can be followed quantitatively by monitoring the considerable change in surface pressure which occurs during the course of the reaction. The rate of each reaction is proportional to the surface concentration of the substrate. The calculated pseudo-first order rate constants are proportional to the enzyme concentration in the bulk solution. At pH 7.6, the value of the second order rate constant for the hydrolysis of the primary ester function in trioctanoin was found to be 8.46 x 105 m-1 s-1, and that for dioctanoin, 2.65 x 104 m-1 s-1. The pH dependency of the rate constant for dioctanoin hydrolysis reveals an ionizable basic group at the active site of the enzyme with pKa of 6.38. The results are consistent with fully hydrated enzyme molecules acting upon substrate molecules lying within the insoluble mono-layer.