Tryptophan Fluorescence and Time-Lag Hydrolysis of Peptide Bonds during Degradation of β-Lactoglobulin by Trypsin

Abstract

The opening of protein globules and corresponding exposure of their internal peptide bonds, the so-called demasking effect, is required for successful hydrolysis of peptide bonds by proteases. Under the proteolytic action of trypsin on β-lactoglobulin (β-LG), the evolution of tryptophan fluorescence spectra showed that the demasking process consists of two stages with different demasking rate constants for each stage. It was found that the ratio of these constants depends on the concentration of trypsin and changes are approximately threefold when the concentration of trypsin changes in the range of 0.3–15 mg/L. Simulation of hydrolysis taking into account the demasking effect demonstrated how the apparent first-order rate constants obtained experimentally are related to the true hydrolysis rate constants and demasking parameters. The lag phase in the kinetic curves corresponding to the hydrolysis of various peptide bonds in β-LG was also analyzed. The increased lag times indicated sites that are hydrolyzed by a two-stage demasking mechanism.