The rutinosidase-encoding gene Aorut has been expressed in Pichia pastoris with its native signal sequence from Aspergillus oryzae. Biochemical and structural investigation of the purified recombinant mature AoRut, designated as rAoRutM, was performed in this study. A 1.7 Å resolution crystal structure of rAoRutM was determined, which is an essential step forward in the utilization of AoRut as a potential catalyst. The crystal structure of rAoRutM was represented by a (β/α)8 TIM barrel fold with structural similarity to rutinosidase from Aspergillus niger (AnRut) and an exo-β (1, 3)-glucanase from Candida albicans. The crystal structure revealed that the catalytic site was located in a deep cleft, similar to AnRut, and internal cavities and water molecules were also present. Purified rAoRutM hydrolyzed not only 7-O-linked and 3-O-linked flavonoid rutinosides, but also 7-O-linked and 3-O-linked flavonoid glucosides. rAoRutM displayed high catalytic activity toward quercetin 3-O-linked substrates such as rutin and isoquercitrin, rather than the 7-O-linked substrate, quercetin-7-O-glucoside. Unexpectedly, purified rAoRutM exhibited increased thermostability after treatment with endo-β-N-acetylglucosaminidase H. Circular dichroism (CD) spectra of purified intact rAoRutM and the enzyme after N-deglycosylation showed a typical α-helical CD profile, however, the molar ellipticity values of the peaks at 208 nm and 212 nm varied. The Km and kcat values for the substrates modified by rutinose were higher than those for substrates modified by β-D-glucose.
Importance Flavonoid glycosides constitute a class of secondary metabolites widely distributed in nature. These compounds are involved in the bitter taste or clouding in plant-based foods or beverages, respectively. Flavonoid glycoside degradation can proceed through two alternative enzymatic pathways: one that is mediated by monoglycosidases, and the other catalyzed by a diglycosidase. The present study on the biochemical and structural investigation of A. oryzae rutinosidase provides a potential biocatalyst for industrial applications of flavonoids.
Production of alpha-amylase from Aspergillus oryzae for several industrial applications in a single step