Human cytosolic β-glucosidase (hCBG) is a xenobiotic-metabolizing enzyme that hydrolyses certain flavonoid glucosides, with specificity depending on the aglycone moiety, the type of sugar and the linkage between them. Based upon the X-ray structure of Zea mays β-glucosidase, we generated a three-dimensional model of hCBG by homology modelling. The enzyme exhibited the (β/α)8-barrel fold characteristic of family 1 β-glucosidases, with structural differences being confined mainly to loop regions. Based on the substrate specificity of the human enzymes, sequence alignment of family 1 enzymes and analysis of the hCBG structural model, we selected and mutated putative substrate (aglycone) binding site residues. Four single mutants (Val168→Tyr, Phe225→Ser, Tyr308→Ala and Tyr308→Phe) were expressed in Pichia pastoris, purified and characterized. All mutant proteins showed a decrease in activity towards a broad range of substrates. The Val168→Tyr mutation did not affect Km on p-nitrophenyl (pNP)-glycosides, but increased Km 5-fold on flavonoid glucosides, providing the first biochemical evidence supporting a role for this residue in aglycone-binding of the substrate, a finding consistent with our three-dimensional model. The Phe225→Ser and Tyr308→Ala mutations, and, to a lesser degree, the Tyr308→Phe mutation, resulted in a drastic decrease in specific activities towards all substrates tested, indicating an important role of those residues in catalysis. Taken together with the three-dimensional model, these mutation studies identified the amino-acid residues in the aglycone-binding subsite of hCBG that are essential for flavonoid glucoside binding and catalysis.
- binding subsite
- flavonoid glycosides
- glycosyl hydrolase family 1
- site-directed mutagenesis
- three-dimensional model
↵1 Present address: Nestlé Research Centre, Vers-Chez-Les-Blanc, PO Box 44, CH-1000 Lausanne 26, Switzerland
Abbreviations used: BMGY, buffered minimal glycerol-complex medium; BMMY, buffered minimal methanol-complex medium; CBG, cytosolic β-glucosidase; cbg-1, cDNA encoding hCBG; hCBG, human CBG; LPH, lactase-phlorizin hydrolase; pNP, p-nitrophenyl; pNPAra, pNP-β-l-arabinopyranoside; pNPFuc, pNP-β-d-fucopyranoside; pNPGal, pNP-β-d-galactopyranoside; pNPGlc, pNP-β-d-glucopyranoside; reCBG, recombinant CBG; ZMGlu, Zea mays (maize) β-glucosidase.
- The Biochemical Society, London ©2003