National Center for
4B9Z: Crystal Structure Of Agd31b, Alpha-transglucosylase, Complexed With Acarbose
Structural Enzymology of Cellvibrio japonicus Agd31B Protein Reveals alpha-Transglucosylase Activity in Glycoside Hydrolase Family 31
J. Biol. Chem. (2012) 287 p.43288-43299
The metabolism of the storage polysaccharides glycogen and starch is of vital importance to organisms from all domains of life. In bacteria, utilization of these alpha-glucans requires the concerted action of a variety of enzymes, including glycoside hydrolases, glycoside phosphorylases, and transglycosylases. In particular, transglycosylases from glycoside hydrolase family 13 (GH13) and GH77 play well established roles in alpha-glucan side chain (de)branching, regulation of oligo- and polysaccharide chain length, and formation of cyclic dextrans. Here, we present the biochemical and tertiary structural characterization of a new type of bacterial 1,4-alpha-glucan 4-alpha-glucosyltransferase from GH31. Distinct from 1,4-alpha-glucan 6-alpha-glucosyltransferases (EC 126.96.36.199) and 4-alpha-glucanotransferases (EC 188.8.131.52), this enzyme strictly transferred one glucosyl residue from alpha(1-->4)-glucans in disproportionation reactions. Substrate hydrolysis was undetectable for a series of malto-oligosaccharides except maltose for which transglycosylation nonetheless dominated across a range of substrate concentrations. Crystallographic analysis of the enzyme in free, acarbose-complexed, and trapped 5-fluoro-beta-glucosyl-enzyme intermediate forms revealed extended substrate interactions across one negative and up to three positive subsites, thus providing structural rationalization for the unique, single monosaccharide transferase activity of the enzyme.