β-Glucosidase catalyzing specific hydrolysis of an iridoid β-glucoside from Plumeria obtusa

Abstract

An iridoid β-glucoside, namely plumieride coumarate glucoside, was isolated from the Plumeria obtusa (white frangipani) flower. A β-glucosidase, purified to homogeneity from P. obtusa, could hydrolyze plumieride coumarate glucoside to its corresponding 13-O-coumarylplumieride. Plumeriaβ-glucosidase is a monomeric glycoprotein with a molecular weight of 60.6 kDa and an isoelectric point of 4.90. The purified β-glucosidase had an optimum pH of 5.5 for p-nitrophenol (pNP)-β-D-glucoside and for its natural substrate. The Km values for pNP-β-D-glucoside and Plumeriaβ-glucoside were 5.04±0.36 mM and 1.02±0.06 mM, respectively. The enzyme had higher hydrolytic activity towards pNP-β-D-fucoside than pNP-β-D-glucoside. No activity was found for other pNP-glycosides. Interestingly, the enzyme showed a high specificity for the glucosyl group attached to the C-7″ position of the coumaryl moiety of plumieride coumarate glucoside. The enzyme showed poor hydrolysis of 4-methylumbelliferyl-β-glucoside and esculin, and did not hydrolyze alkyl-β-glucosides, glucobioses, cyanogenic-β-glucosides, steroid β-glucosides, nor other iridoid β-glucosides. In conclusion, the Plumeriaβ-glucosidase shows high specificity for its natural substrate, plumieride coumarate glucoside. ©Institute of Biochemistry and Cell Biology, SIBS, CAS.