Kinetic analysis of urea-inactivation of beta-galactosidase in the presence of galactose

Supramolecular enzyme engineering in complex nanometer-thin biomimetic organosilica layers

Enzyme shielding at the surface of silica nanoparticles was performed using different mixtures of biomimetic building blocks. The performances of the nanobiocatalysts are strongly impacted by the chemical composition of the shielding layer.

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

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 K_m and V_max 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.

Diverse clinical compounds alter the quaternary structure and inhibit the activity of an essential enzyme

An in vitro evaluation of the Johns Hopkins Clinical Compound Library demonstrates that certain drugs can alter the quaternary structure of an essential human protein. Human porphobilinogen synthase (HsPBGS) is an essential enzyme involved in heme biosynthesis; it exists as an equilibrium of high-activity octamers, low-activity hexamers, and alternate dimer configurations that dictate the stoichiometry and architecture of further assembly. Decreased HsPBGS activity is implicated in toxicities associated with lead poisoning and 5-aminolevulinate dehydratase (ALAD) porphyria, the latter of which involves hexamer-favoring HsPBGS variants. A medium-throughput native PAGE mobility-shift screen coupled with evaluation of hits as HsPBGS inhibitors revealed 12 drugs that stabilize the HsPBGS hexamer and inhibit HsPBGS activity in vitro. A detailed characterization of these effects is presented. Drug inhibition of HsPBGS in vivo by inducing hexamer formation would constitute an unprecedented mechanism for side effects. We suggest that small-molecule perturbation of quaternary structure equilibria be considered as a general mechanism for drug action and side effects.