2008 Volume 55 Issue 2 Pages 111-118
Glycoside hydrolase family 31 (GH 31) is one of the most intriguing glycoside hydrolase families. This family contains α-glucosidase, α-xylosidase, α-glucan lyase and isomaltosyltransferase. Escherichia coli YicI (α-xylosidase) is a representative enzyme of GH 31 because its biochemical and structural studies have been thoroughly carried out. YicI is a strict α-xylosidase, which rigidly recognizes α-xyloside at the non-reducing terminal end, even though its amino acid sequence apparently displays similarity with α-glucosidases. Phe277, Cys307, Trp345 and Lys414 at the subsite-1 are important for α-xylosidase activity. The mutant YicI enzymes, which possesses Ile307/Asp308 instead of Cys307/Phe308 and which has a shorter β→α loop 1 of (β/α)8 barrel in place of the original longer loop, respectively, possess α-glucosidase activity. In the transxylosylation of YicI, glucose, mannose and allose are able to act as acceptors, but galactose, talose and gulose never do, implying that equatorial OH-4 of the aldopyranose is crucial for acting as an acceptor. YicI transfers α-xylosyl moieties to a specific hydroxy group in the acceptor sugar (except fructopyranose) showing 1,6 regioselectivity, which is in agreement with the structural feature of the aglycone-biding site. Among the transxylosylation products of YicI, α-D-xylopyranosyl-(1→6)-D-mannopyranose, α-D-xylopyranosyl-(1→6)-D-fructofuranose, and α-D-xylopyranosyl-(1→3)-D-fructopyranose are novel sugars. α-D-Xylopyranosyl-(1→6)-D-mannopyranose and α-D-xylopyranosyl-(1→6)-D-fructofuranose have the ability to inhibit rat intestinal α-glucosidases.