The Mechanism of Salt Activation of Thermolysin: Relation with Pressure Activation and Implications of Hydration Change Coupled with Rate Process

Effects of ionic strength on the enzymatic hydrolysis of diluted and concentrated whey protein isolate

To identify the parameters that affect enzymatic hydrolysis at high substrate concentrations, whey protein isolate (1-30% w/v) was hydrolyzed by Alcalase and Neutrase at constant enzyme-to-substrate ratio. No changes were observed in the solubility and the aggregation state of the proteins. With increasing concentration, both the hydrolysis rate and the final DH decreased, from 0.14 to 0.015 s(-1) and from 24 to 15%, respectively. The presence of 0.5 M NaCl decreased the rate of hydrolysis for low concentrations (to 0.018 s(-1) for 1% WPI), resulting in similar rates of hydrolysis for all substrate concentrations. The conductivity increase (by increasing the protein concentration, or by addition of NaCl) has significant effects on the hydrolysis kinetics, but the reason for this is not yet well understood. The results show the importance of conductivity as a factor that influences the kinetics of the hydrolysis, as well as the composition of the hydrolysates.

Kinetic characterization of the neutral protease vimelysin from Vibrio sp. T1800

The kinetics of the hydrolysis of dipeptide and tripeptide substrates by the recently discovered neutral protease from Vibrio species T1800 (vimelysin) were studied. In the pH dependence of the apparent second-order rate constant, the pKa2 value of vimelysin (approximately 6.5) was significantly lower than thermolysin (8.3), although the pKa1 (approximately 5.1) values were comparable (5.0). The Kcat/Km(lim) parameter for hydrolysis of Fua-Gly-PheNH2 (Fua = furylacryloyl) was more than sevenfold greater than for Fua-Gly-LeuNH2. This higher specificity for Fua-Gly-PheNH2 was deduced for both Kcat and Km parameters. Fua-Phe-PheNH2 showed the highest Kcat/Km(app) value of the six substrates studied. The discrimination between Phe/Leu at the P1' site was most evident when the P1 site was not sufficiently filled. Reflecting the characteristically high proteolytic activity of vimelysin at lower temperatures [Oda, K., Okayama, K., Okutomi, K., Shimada, M., Sato, R. & Takahashi, S. (1996) Biosci. Biotech. Biochem. 60, 463-467], the Arrhenius plot of the apparent second-order rate constant for the hydrolysis of Fua-Gly-LeuNH2 showed an inverse temperature dependence; higher reaction rates were observed at lower temperatures. This was not merely due to the pKa shift nor to thermal denaturation of the enzyme coupling, but rather to the Kcat(app) parameter, which alone showed an inverse temperature dependence. A model containing two temperature-dependent forms of the active enzyme was postulated to explain this unique temperature dependence.

Observation of the pre-steady state process in thermolysin catalysis with a fluorescent displacement probe at low pH

The pre-steady state process in the thermolysin-catalyzed hydrolysis of Cbz-Gly-Phe-Ala was observed at pH 4.5 by fluorescence stopped-flow method using Dns-Phe as a displacement probe. After the confirmation of the pre-equilibrium hypothesis for the binary interaction, the nonlinear substrate concentration dependence of the apparent kinetic constant for the pre-steady state process was analyzed and an existence of multi-intermediates was proposed.

Effect of high pressure on EcoRI reactions

The effect of pressure on reactions of restriction endonucleases was investigated. No obvious irreversible (after) effect was observed for EcoRI, while a considerable irreversible inactivation was found for BamHI. Thus the EcoRI reactions against lambda DNA, pBR322 and pBluescript were studied under high pressure and little effect was observed on the overall reactions. The DNA concentration dependence of the kinetic data apparently fits the Michaelis-Menten type equation and the evaluated rate parameters were: Vmax = 6.2 +/- 0.24 and 7.0 +/- 0.22 (x 10(-2) nM/min) at 0.1 and 200 MPa, respectively; Km = 19 +/- 1.8 and 28 +/- 1.7 nM at 0.1 and 200 MPa, respectively. The apparent activation volume corresponding to kcat/Km was ca +1 mL/mol. A characteristic effect of pressure on the sequence specificity of these enzymes was seen in their star activity. Relaxed specificity was tightened by increasing pressure (200 MPa) with respect to that induced by low salt concentration or by the presence of organic solvent.