TY - JOUR
T1 - Substrate (aglycone) specificity of human cytosolic β-glucosidase
AU - Berrin, Jean Guy
AU - Czjzek, Mirjam
AU - Kroon, Paul A.
AU - Russell McLauchlan, W.
AU - Puigserver, Antoine
AU - Williamson, Gary
AU - Juge, Nathalie
PY - 2003/7/1
Y1 - 2003/7/1
N2 - 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.
AB - 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.
KW - Binding subsite
KW - Flavonoid glycosides
KW - Glycosyl hydrolase family 1
KW - Site-directed mutagenesis
KW - Three-dimensional model
UR - http://www.scopus.com/inward/record.url?scp=0038343935&partnerID=8YFLogxK
U2 - 10.1042/BJ20021876
DO - 10.1042/BJ20021876
M3 - Article
C2 - 12667141
AN - SCOPUS:0038343935
VL - 373
SP - 41
EP - 48
JO - Biochemical Journal
JF - Biochemical Journal
SN - 0264-6021
IS - 1
ER -