Study on the role of tyrosine side-chains at the active centre of emulsin β-d-glucosidase

The role of exposed tyrosine side-chains in enzyme-catalyzed reaction by sweet almond emulsion β-d-glucosidase (β-d-glucoside glucohydrolase, EC 3.2.1.21) has been studied using N-acetylimidazole as the specific reagent. The changes in activity, binding affinity and kinetic parameters (K_m, V) as a result of acetylation of the phenolic hydroxyl groups have been determined. The acetylation increased the K_m values of both β-glucosidase and β-galactosidase activities, whereas V remained unchanged. Similarly, the binding affinity for immobilized phenyl β-d-glucopyranoside decreased appreciably. After the removal of the acetyl groups the enzyme regained 96% of the original activity. It is concluded that the tyrosine moieties, located in the active centre of the enzyme, have both glucoside and galactoside binding functions.