Abstact

β-Glucosidases play essential roles in nature as well as in industrial applications. Glycoside hydrolase family 116 (GH116) is a relatively sparsely characterized family of β-glucosidases. We describe the functional and structural characterization of Thermosynechococcus elongatus BP-1 TeGH116, the one β-glucosidase in this model cyanobacterium. TeGH116 was expressed in Escherichia coli and purified by heating, immobilized metal affinity chromatography, and size exclusion chromatography. TeGH116 hydrolyzes p-nitrophenyl β-d-glucopyranoside most rapidly at pH 5.5 and 55 °C, and other p-nitrophenyl glycosides 3% or less of this rate. TeGH116 also hydrolyzes natural phenolic glucosides and β-1,3- and β-1,4-linked gluco-oligosaccharides. Kinetic analysis suggests that TeGH116 binds three glucosyl residues in these oligosaccharides. TeGH116 is inhibited by glucose but is only weakly inhibited by δ-gluconolactone. The X-ray crystallographic structures of TeGH116 and its complex with glucose were similar to those of Thermoanaerobacterium xylanolyticum TxGH116. However, a long loop extending from the noncatalytic N-terminal domain helps form the mouth of the TeGH116 active site, making it narrower. TxGH116 was found to hydrolyze barley β-glucan, laminarin, and lichenan, but not carboxymethylcellulose, while TeGH116 hydrolyzed the same polysaccharides at relatively lower rates. Deletion of the loop in TeGH116 resulted in poor stability and low activity, but higher relative activity on laminarin compared to p-nitrophenyl β-d-glucoside. The narrowed active site and lower activity on large substrates highlight the contribution of N-terminal domain loops to the substrate specificity of GH116 enzymes.