The specificity of the aglycone-binding site of Escherichia colialpha-xylosidase (YicI), which belongs to glycoside hydrolase family 31, was characterized by examining the enzyme's transxylosylation-catalyzing property. Acceptor specificity and regioselectivity were investigated using various sugars as acceptor substrates and alpha-xylosyl fluoride as the donor substrate. Comparison of the rate of formation of the glycosyl-enzyme intermediate and the transfer product yield using various acceptor substrates showed that glucose is the best complementary acceptor at the aglycone-binding site. YicI preferred aldopyranosyl sugars with an equatorial 4-OH as the acceptor substrate, such as glucose, mannose, and allose, resulting in transfer products. This observation suggests that 4-OH in the acceptor sugar ring made an essential contribution to transxylosylation catalysis. Fructose was also acceptable in the aglycone-binding site, producing two regioisomer transfer products. The percentage yields of transxylosylation products from glucose, mannose, fructose, and allose were 57, 44, 27, and 21%, respectively. The disaccharide transfer products formed by YicI, alpha-d-Xylp-(1-->6)-d-Manp, alpha-d-Xylp-(1-->6)-d-Fruf, and alpha-d-Xylp-(1-->3)-d-Frup, are novel oligosaccharides that have not been reported previously. In the transxylosylation to cello-oligosaccharides, YicI transferred a xylosyl moiety exclusively to a nonreducing terminal glucose residue by alpha-1,6-xylosidic linkages. Of the transxylosylation products, alpha-d-Xylp-(1-->6)-d-Manp and alpha-d-Xylp-(1-->6)-d-Fruf inhibited intestinal alpha-glucosidases.
Aglycone specificity of Escherichia colialpha-xylosidase investigated by transxylosylation. Publishing Authors By Initials
