title: Kinetic analyses of retaining endo-(xylo)glucanases from plant and microbial sources using new chromogenic xylogluco-oligosaccharide aryl glycosides.
abstract: A library of phenyl beta-glycosides of xylogluco-oligosaccharides was synthesized via a chemoenzymatic approach to produce new, specific substrates for xyloglucanases. Tamarind xyloglucan was completely hydrolyzed to four, variably galactosylated component oligosaccharides based on Glc 4 backbones, using a Trichoderma endo-glucanase mixture. Oligosaccharide complexity could be further reduced by beta-galactosidase treament. Subsequent per- O-acetylation, alpha-bromination, phase-transfer glycosylation, and Zemplen deprotection yielded phenyl glycosides of XXXG and XLLG oligosaccharides with a broad range of aglycon p K a values. Kinetic and product analysis of the action of the archetypal plant endo-xyloglucanase, Tropaeolum majus NXG1, on these compounds indicated that formation of the glycosyl-enzyme intermediate was rate-limiting in the case of phenol leaving groups with p K a values of >7, leading exclusively to substrate hydrolysis. Conversely, substrates with aglycon p K a values of 5.4 gave rise to a significant amount of transglycosylation products, indicating a change in the relative rates of formation and breakdown of the glycosyl-enzyme intermediate for these faster substrates. Notably, comparison of the initial rates of XXXG-Ar and XLLG-Ar conversion indicated that catalysis by TmNXG1 was essentially insensitive to the presence of galactose in the negative subsites for all leaving groups. More broadly, analysis of a selection of enzymes from CAZy families GH 5, 12, and 16 indicated that the phenyl glycosides are substrates for anomeric configuration-retaining endo-xyloglucanases but are not substrates for strict xyloglucan endo-transglycosylases (XETs). The relative activities of the GH 5, 12, and 16 endo-xyloglucanases toward GGGG-CNP, XXXG-CNP, and XLLG-CNP reflected those observed using analogous high molar mass polysaccharides. These new chromogenic substrates may thus find wide application in the discovery, screening, and detailed kinetic analysis of new xyloglucan-active enzymes.
authors.affiliation: School of Biotechnology, Royal Institute of Technology (KTH), AlbaNova University Centre, SE-106 91 Stockholm, Sweden.
authors.forename: Farid M
authors.initials: FM
authors.lastname: Ibatullin
authors.forename: Martin J
authors.initials: MJ
authors.lastname: Baumann
authors.forename: Lionel
authors.initials: L
authors.lastname: Greffe
authors.forename: Harry
authors.initials: H
authors.lastname: Brumer
mesh_terms: D005936:Glucans / Q000737:chemistry; D006026:Glycoside Hydrolases / Q000737:chemistry / Q000378:metabolism*; D006027:Glycosides / Q000138:chemical synthesis / Q000737:chemistry / Q000378:metabolism*; D007700:Kinetics; D015394:Molecular Structure; D009844:Oligosaccharides / Q000737:chemistry; D014990:Xylans / Q000737:chemistry
chemical_list: D005936:Glucans; D006027:Glycosides; D009844:Oligosaccharides; D014990:Xylans; C029353:xyloglucan; D006026:Glycoside Hydrolases; C072818:xyloglucan endo(1-4)-beta-D-glucanase