Kinetic properties and mechanism of action of an intracellular beta-glucosidase from Thermoascus aurantiacus Miehe.

Intracellular beta-glucosidase is strongly inhibited by its own substrate p-nitrophenyl-beta-glucoside which displays high affinity for two binding sites. A non-productive complex is formed also by cellobiose, but its lower affinity results in a much lower inhibition. As shown by inhibition experiments performed with glucono-delta-lactone, the hydrolytic reaction proceeds through the formation of a carbonium ion, very similar in its half-chair conformation to the delta-lactone. Carboxylic groups (pK = 3.19) appear involved in the catalytic process together with a histidine residue (pK = 5.64): while the carboxylate ions stabilize the carbonium ion, the displaced group accepts a proton from the protonated imidazole.