Kinetic Analysis of Guanidine Hydrochloride Inactivation of β-Galactosidase in the Presence of Galactose

Abstract EN

Inactivation of purified β-Galactosidase was done with GdnHCl in the absence and presence of varying [galactose] at 50°C and at pH 4.5.

Lineweaver-Burk plots of initial velocity data, in the presence and absence of guanidine hydrochloride (GdnHCl) and galactose, were used to determine the relevant Km and Vmax values, with p-nitrophenyl β-D-galactopyranoside (pNPG) as substrate, S.

Plots of ln([P]∞−[P]t) against time in the presence of GdnHCl yielded the inactivation rate constant, A.

Plots of A versus [S] at different galactose concentrations were straight lines that became increasingly less steep as the [galactose] increased, showing that A was dependent on [S].

Slopes and intercepts of the 1/[P]∞ versus 1/[S] yielded k+0 and k'+0, the microscopic rate constants for the free enzyme and the enzyme-substrate complex, respectively.

Plots of k+0 and k'+0 versus [galactose] showed that galactose protected the free enzyme as well as the enzyme-substrate complex (only at the lowest and highest [galactose]) against GdnHCl inactivation.

In the absence of galactose, GdnHCl exhibited some degree of non-competitive inhibition.

In the presence of GdnHCl, galactose exhibited competitive inhibition at the lower [galactose] of 5 mM which changed to non-competitive as the [galactose] increased.

The implications of our findings are further discussed.