Substrate (aglycone) specificity of human cytosolic β-glucosidase

Jean Guy Berrin, Mirjam Czjzek, Paul A. Kroon, W. Russell McLauchlan, Antoine Puigserver, Gary Williamson, Nathalie Juge

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57 Citations (Scopus)

Abstract

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.

Original languageEnglish
Pages (from-to)41-48
Number of pages8
JournalBiochemical Journal
Volume373
Issue number1
DOIs
Publication statusPublished - 1 Jul 2003
Externally publishedYes

Keywords

  • Binding subsite
  • Flavonoid glycosides
  • Glycosyl hydrolase family 1
  • Site-directed mutagenesis
  • Three-dimensional model

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