Dual role of imidazole as activator/inhibitor of sweet almond (Prunus dulcis) β-glucosidase

The activity of Prunus dulcis (sweet almond) β-glucosidase at the expense of p-nitrophenyl-β-D-glucopyranoside at pH 6 was determined, both under steady-state and pre-steady-state conditions. Using crude enzyme preparations, competitive inhibition by 1–5 mM imidazole was observed under both kinetic conditions tested. However, when imidazole was added to reaction mixtures at 0.125–0.250 mM, we detected a significant enzyme activation. To further inspect this effect exerted by imidazole, β-glucosidase was purified to homogeneity. Two enzyme isoforms were isolated, i.e. a full-length monomer, and a dimer containing a full-length and a truncated subunit. Dimeric β-glucosidase was found to perform much better than the monomeric enzyme, independently of the kinetic conditions used to assay enzyme activity. In addition, the sensitivity towards imidazole was found to differ between the two isoforms. While monomeric enzyme was indeed found to be relatively insensitive to imidazole, dimeric β-glucosidase was observed to be significantly activated by 0.125–0.250 mM imidazole under pre-steady-state conditions. Further, steady-state assays revealed that the addition of 0.125 mM imidazole to reaction mixtures increases the Km of dimeric enzyme from 2.3 to 6.7 mM. The activation of β-glucosidase dimer by imidazole is proposed to be exerted via a conformational transition poising the enzyme towards proficient catalysis.