Hydrolysis of substrate analogues catalysed by beta-D-glucosidase from Aspergillus niger. Part II: Deoxy and deoxyhalo derivatives of cellobiose.

The hydrolysis of sixteen mainly deoxy and deoxyhalo derivatives of celloboise catalysed by beta-D-glucosidase from Aspergillus niger has been studied by means of 1H NMR spectroscopy and progress-curve enzyme kinetics in both single-substrate and competition experiments. In the non-reducing ring of cellobiose it was found that the hydroxy groups at positions 2', 3', and 4' are essential for the enzymatic hydrolysis. The primary hydroxy group on 6' in this ring is, although important for the hydrolysis, not essential. The analogues modified at positions 3' and 4' and the 6'-bromo-6'-deoxy derivative were not inhibitors, whereas the 2'-deoxy derivative inhibited the enzymatic hydrolysis of methyl beta-cellobioside to some extent. Of the analogues modified in the reducing ring, some were hydrolysed faster (e.g. the deoxy compounds) and some slower than methyl beta-cellobioside in single-substrate experiments, but all derivatives were hydrolysed at a lower rate than this reference substrate in direct competition and displayed relatively weak inhibitory effects. The results are interpreted qualitatively with respect to changes in the free binding energies of the substrates and catalytic transition states based on the Michaelis-Menten mechanism, and some mechanistic implications of these findings are discussed.