Substrate (aglycone) specificity of human cytosolic beta-glucosidase

Biochem J. 2003 Jul 1;373(Pt 1):41-8. doi: 10.1042/BJ20021876.


Human cytosolic beta-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 beta-glucosidase, we generated a three-dimensional model of hCBG by homology modelling. The enzyme exhibited the (beta/alpha)(8)-barrel fold characteristic of family 1 beta-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 (Val(168)-->Tyr, Phe(225)-->Ser, Tyr(308)-->Ala and Tyr(308)-->Phe) were expressed in Pichia pastoris, purified and characterized. All mutant proteins showed a decrease in activity towards a broad range of substrates. The Val(168)-->Tyr mutation did not affect K (m) on p -nitrophenyl ( p NP)-glycosides, but increased K (m) 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 Phe(225)-->Ser and Tyr(308)-->Ala mutations, and, to a lesser degree, the Tyr(308)-->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.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Amino Acid Sequence
  • Base Sequence
  • Cytosol / enzymology
  • DNA Primers
  • Humans
  • Kinetics
  • Models, Molecular
  • Molecular Sequence Data
  • Multigene Family
  • Protein Conformation
  • Protein Structure, Secondary
  • Recombinant Proteins / chemistry
  • Recombinant Proteins / metabolism
  • Sequence Alignment
  • Sequence Homology, Amino Acid
  • Substrate Specificity
  • Zea mays / enzymology
  • beta-Glucosidase / chemistry*
  • beta-Glucosidase / genetics
  • beta-Glucosidase / metabolism*


  • DNA Primers
  • Recombinant Proteins
  • beta-Glucosidase