title: Structural basis for broad substrate specificity in higher plant beta-D-glucan glucohydrolases.
abstract: Family 3 beta-D-glucan glucohydrolases are distributed widely in higher plants. The enzymes catalyze the hydrolytic removal of beta-D-glucosyl residues from nonreducing termini of a range of beta-D-glucans and beta-D-oligoglucosides. Their broad specificity can be explained by x-ray crystallographic data obtained from a barley beta-D-glucan glucohydrolase in complex with nonhydrolyzable S-glycoside substrate analogs and by molecular modeling of enzyme/substrate complexes. The glucosyl residue that occupies binding subsite -1 is locked tightly into a fixed position through extensive hydrogen bonding with six amino acid residues near the bottom of an active site pocket. In contrast, the glucosyl residue at subsite +1 is located between two Trp residues at the entrance of the pocket, where it is constrained less tightly. The relative flexibility of binding at subsite +1, coupled with the projection of the remainder of bound substrate away from the enzyme's surface, means that the overall active site can accommodate a range of substrates with variable spatial dispositions of adjacent beta-D-glucosyl residues. The broad specificity for glycosidic linkage type enables the enzyme to perform diverse functions during plant development.
authors.affiliation: Department of Plant Science, University of Adelaide, Waite Campus, Glen Osmond, South Australia 5064, Australia.
authors.forename: Maria
authors.initials: M
authors.lastname: Hrmova
authors.forename: Ross
authors.initials: R
authors.lastname: De Gori
authors.forename: Brian J
authors.initials: BJ
authors.lastname: Smith
authors.forename: Jon K
authors.initials: JK
authors.lastname: Fairweather
authors.forename: Hugues
authors.initials: H
authors.lastname: Driguez
authors.forename: Joseph N
authors.initials: JN
authors.lastname: Varghese
authors.forename: Geoffrey B
authors.initials: GB
authors.lastname: Fincher
mesh_terms: D000595:Amino Acid Sequence; D001665:Binding Sites; D002240:Carbohydrate Sequence; D002384:Catalysis; D002475:Cellobiose / Q000737:chemistry / Q000378:metabolism; D004187:Disaccharides / Q000737:chemistry / Q000378:metabolism; D004693:Glucan Endo-1,3-beta-D-Glucosidase / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D005936:Glucans / Q000737:chemistry / Q000378:metabolism; D005959:Glucosidases / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D006026:Glycoside Hydrolases / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D007700:Kinetics; D008958:Models, Molecular; D008969:Molecular Sequence Data; D010802:Phylogeny; D010944:Plants / Q000201:enzymology* / Q000235:genetics; D011485:Protein Binding; D017386:Sequence Homology, Amino Acid; D013329:Structure-Activity Relationship; D013379:Substrate Specificity; D014312:Trisaccharides / Q000737:chemistry / Q000378:metabolism; D001617:beta-Glucosidase / Q000737:chemistry / Q000235:genetics / Q000378:metabolism
chemical_list: C463464:4-nitrophenyl S-glucopyranosyl-(1-3)-3-thioglucopyranosyl-1-3-glucopyranoside; D004187:Disaccharides; D005936:Glucans; D014312:Trisaccharides; C014637:laminaribiose; D002475:Cellobiose; D005959:Glucosidases; D006026:Glycoside Hydrolases; C446280:beta-D-glucan glucohydrolase; D001617:beta-Glucosidase; D004693:Glucan Endo-1,3-beta-D-Glucosidase; C100052:gentiobiose; C003328:sophorose