title: Kinetic analysis using low-molecular mass xyloglucan oligosaccharides defines the catalytic mechanism of a Populus xyloglucan endotransglycosylase.
abstract: Plant XETs [XG (xyloglucan) endotransglycosylases] catalyse the transglycosylation from a XG donor to a XG or low-molecular-mass XG fragment as the acceptor, and are thought to be important enzymes in the formation and remodelling of the cellulose-XG three-dimensional network in the primary plant cell wall. Current methods to assay XET activity use the XG polysaccharide as the donor substrate, and present limitations for kinetic and mechanistic studies of XET action due to the polymeric and polydisperse nature of the substrate. A novel activity assay based on HPCE (high performance capillary electrophoresis), in conjunction with a defined low-molecular-mass XGO {XG oligosaccharide; (XXXGXXXG, where G=Glcbeta1,4- and X=[Xylalpha1,6]Glcbeta1,4-)} as the glycosyl donor and a heptasaccharide derivatized with ANTS [8-aminonaphthalene-1,3,6-trisulphonic acid; (XXXG-ANTS)] as the acceptor substrate was developed and validated. The recombinant enzyme PttXET16A from Populus tremula x tremuloides (hybrid aspen) was characterized using the donor/acceptor pair indicated above, for which preparative scale syntheses have been optimized. The low-molecular-mass donor underwent a single transglycosylation reaction to the acceptor substrate under initial-rate conditions, with a pH optimum at 5.0 and maximal activity between 30 and 40 degrees C. Kinetic data are best explained by a ping-pong bi-bi mechanism with substrate inhibition by both donor and acceptor. This is the first assay for XETs using a donor substrate other than polymeric XG, enabling quantitative kinetic analysis of different XGO donors for specificity, and subsite mapping studies of XET enzymes.
authors.affiliation: Laboratory of Biochemistry, Institut Químic de Sarrià, Universitat Ramon Llull, 08017 Barcelona, Spain.
authors.forename: Marc
authors.initials: M
authors.lastname: Saura-Valls
authors.forename: Régis
authors.initials: R
authors.lastname: Fauré
authors.forename: Sergi
authors.initials: S
authors.lastname: Ragàs
authors.forename: Kathleen
authors.initials: K
authors.lastname: Piens
authors.forename: Harry
authors.initials: H
authors.lastname: Brumer
authors.forename: Tuula T
authors.initials: TT
authors.lastname: Teeri
authors.forename: Sylvain
authors.initials: S
authors.lastname: Cottaz
authors.forename: Hugues
authors.initials: H
authors.lastname: Driguez
authors.forename: Antoni
authors.initials: A
authors.lastname: Planas
mesh_terms: D002384:Catalysis; D019075:Electrophoresis, Capillary; D005936:Glucans / Q000737:chemistry* / Q000378:metabolism*; D006031:Glycosylation; D016695:Glycosyltransferases / Q000378:metabolism*; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008970:Molecular Weight; D009281:Naphthalenes / Q000138:chemical synthesis; D032107:Populus / Q000201:enzymology*; D013379:Substrate Specificity; D013696:Temperature; D014990:Xylans / Q000737:chemistry* / Q000378:metabolism*
chemical_list: D005936:Glucans; D009281:Naphthalenes; D014990:Xylans; C029353:xyloglucan; C046465:8-amino-1,3,6-naphthalenetrisulfonic acid; D016695:Glycosyltransferases; C073693:xyloglucan endotransglycosylase