Slow inhibition of almond beta-glucosidase by azasugars: determination of activation energies for slow binding

The thermodynamic and activation energies of the slow inhibition of almond beta -glucosidase with a series of azasugars were determined. The inhibitor s studied were isofagomine ((3R,4R,51E)-3,4-dihydroxy-5-hydroxymethylpiperi dine, 1), isogalacto-fagomine ((3R,4S,5R)-3,4-dihydroxy-5-hydroxymethylpipe ridine, 2), (-)-1-azafagomine ((3R,4R,5R)-4,5-dihydroxy-3-hydroxymethylhexa hydropyridazine, 3), 3-amino-3-deoxy-1-azafagomine (4) and 1-deoxynojirimyc in (5). It was found that the binding of 1 to the enzyme has an activation enthalpy of 56.1 kJ/mol and an activation entropy of 25.8 J/molK. The disso ciation of the enzyme-1 complex had an activation enthalpy of -2.5 kJ/mol a nd an activation entropy of -297 J/molK. It is suggested that the activatio n enthalpy of association is due to the breaking of bonds to water, while t he large negative activation entropy of dissociation is due at least in par t to the resolvation of the enzyme with water molecules. For the associatio n of 1 DeltaH(0) is 58.6 kJ/mol and DeltaS(0) is 323.8 J/molK. Inhibitor 3 has an activation enthalpy of 39.3 ka/mol and an activation entropy of -17. 9 J/molK for binding to the enzyme, and an activation enthalpy of 40.8 kJ/m ol and an activation entropy of -141.0 J/molK for dissociation of the enzym e-inhibitor complex. For the association of 3 DeltaH(0) is -1.5 kJ/mol and DeltaS(0) is 123.1 J/molK. Inhibitor 5 is not a slow inhibitor, but its Del taH(0) and DeltaS(0) of association are -30 kJ/mol and -13.1 J/molK. The la rge difference in DeltaS(0) of association of the different inhibitors sugg ests that the anomeric nitrogen atom of inhibitors 1-4 is involved in an in teraction that results in a large entropy increase. (C) 2001 Elsevier Scien ce B.V. All rights reserved.