The effect of high pressure on the rates of proteolytic hydrolysis. II. Trypsin

Effect of high hydrostatic pressure on the enzymic hydrolysis of beta-lactoglobulin B by trypsin, thermolysin and pepsin

Hydrolysis of beta-lactoglobulin B (beta-lg B) by pepsin, a process slow at ambient conditions, is facilitated at a moderately high hydrostatic pressure such as 300 MPa, corresponding to an apparent volume of activation delta V# = -63 ml mol-1 at pH 2.5, 30 degrees C and gamma/2 = 0.16. Digestion of beta-lg by trypsin and thermolysin is likewise enhanced by pressure, and the pressure effect has been traced to pressure denaturation of beta-lg B, which by high-pressure fluorescence spectroscopy has been shown to have a large negative volume of reaction, delta V(o) = -98 ml mol-1, at pH 6.7, 30 degrees C and gamma/2 = 0.16. Pressure denaturation is only slowly reversed following release of pressure and the enhanced digestibility is maintained at ambient pressure for several hours.

Pressure dependence of trypsin-catalyzed hydrolyses of specific substrates

The effects of pressure on the trypsin-catalyzed hydrolyzed hydrolyses of three specific substrates, N-benzoyl-L-arginine ethyl ester (BzArgOEt), amide (BzArgNH2) and p-nitroanilide (BzArgNA), have been examined. The volume of the activation (delta V++) for kcat was -2.4 ml/mol for BzArgOEt and +3 - +6 ml/mol for BzArgNH2. Because of different rate-determining steps in the steady-state kinetics, the delta V++ value for BzArgOEt would indicate the activation volume of the deacylation step, whereas that for BzArgNH2 the delta V++ for the acylation step. The activation volumes were accounted for in terms of the difference in the mechanisms on the formation and decomposition of the tetrahedral-like intermediates during the acylation and deacylation steps. The delta V values for the formation of BzArgNH2- and thionine-trypsin complexes were several ml/mol, consistent with the fact that the main driving force of the substrate binding to this enzyme is electrostatic interaction, and in contrast to the delta V values of alpha-chymotrypsin complex formation with indole (approximately 0 ml/mol) or 2-furylacryloyl-D-tryptophan methyl ester (approximately 0 ml/mol), for which the hydrophobic interaction is the dominant force of the substrate binding. For the hydrolysis of BzArgNA, which showed a distinct substrate activation at high substrate concentrations, the pressure dependence of the four parameters, ks, Ks, (the catalytic rate and dissociation constant of the normal enzyme-substrate complex, respectively), Kss and Kss (those of the complex activated by the binding of the second substrate molecule), were measured at 1 atm and 1000 atm (25 degrees C). All of the four parameters increased with increase in pressure.