Expression and characterization of a recombinant stevioside hydrolyzing β-glycosidase from Enterococcus casseliflavus

Abstract

The demand for steviol glycosides, non-caloric sweet components of Stevia rebaudiana Bertoni (stevia) leaves, has increased considerably as a benefit to enhance human health. However, the supply has remained challenging due to limited production, with the lack of a specific steviol glycoside hydrolyzing enzyme. In this study, a novel β-glucosidase (EcBgl) from Enterococcus casseliflavus was cloned and expressed in Escherichia coli. An EcBgl consists of 721 amino acids corresponding to a molecular mass of 79.37 kDa. The EcBgl was purified to homogeneity, followed by enzyme characterization. The enzyme showed optimum pH and temperature at 6.0 and 37 °C, and exhibited the kinetic constants k_cat/K_m for pNPG and k_cat/K_m for stevioside of 8583 mM^-1s^-1 and 95.41 mM^-1s^-1, respectively. When compared to the stevioside hydrolyzing β-glycosidases previously reported, EcBgl was found to be the most efficient enzyme. EcBgl also rendered hydrolysis of the stevioside to produce rubusoside, a rare steviol glycoside with a pharmaceutical solubilizing property, by cleaving at the glucose moiety. In addition, the enzyme demonstrated substantial resistance against amygdalin, so it served as a potential enzyme in agricultural and pharmaceutical applications.

Keywords: Enterococcus casseliflavus; Rubusoside; Steviol glycosides; Stevioside; β-Glucosidase.