Reorganization and degradation of the wall crosslinking and seed storage polysaccharide xyloglucan by glycoside hydrolase family 16 (GH16) endo-transglycosylases and hydrolases are crucial to the growth of the majority of land plants, affecting processes as diverse as germination, morphogenesis, and fruit ripening. A high-resolution, three-dimensional structure of a nasturtium (Tropaeolum majus) endo-xyloglucanase loop mutant, TmNXG1-DeltaYNIIG, with an oligosaccharide product bound in the negative active-site subsites, has been solved by X-ray crystallography. Comparison of this novel complex to that of the strict xyloglucan endo-transglycosylase PttXET16-34 from hybrid aspen (Populus tremula x tremuloides), previously solved with a xylogluco-oligosaccharide bound in the positive subsites, highlighted key protein structures that affect the disparate catalytic activities displayed by these closely related enzymes. Combination of these "partial" active-site complexes through molecular dynamics simulations in water allowed modeling of wild-type TmNXG1, TmNXG1-DeltaYNIIG, and wild-type PttXET16-34 in complex with a xyloglucan octadecasaccharide spanning the entire catalytic cleft. A comprehensive analysis of these full-length complexes underscored the importance of various loops lining the active site. Subtle differences leading to a tighter hydrogen bonding pattern on the negative (glycosyl donor) binding subsites, together with loop flexibility on the positive (glycosyl acceptor) binding subsites appear to favor hydrolysis over transglycosylation in GH16 xyloglucan-active enzymes.
Glycosynthases are active-site mutants of glycoside hydrolases that catalyse glycosyl transfer using suitable activated donor substrates without competing product hydrolysis (S. M. Hancock, M. D. Vaughan and S. G. Withers, Curr. Opin. Chem. Biol., 2006, 10, 509-519). Site-directed mutagenesis of the catalytic nucleophile, Glu-85, of a Populus tremula x tremuloides xyloglucan endo-transglycosylase (PttXET16-34, EC 2.4.1.207) into alanine, glycine, and serine yielded enzymes with glycosynthase activity. Product analysis indicated that PttXET16-34 E85A in particular was able to catalyse regio- and stereospecific homo- and hetero-condensations of alpha-xylogluco-oligosaccharyl fluoride donors XXXGalphaF and XLLGalphaF to produce xyloglucans with regular sidechain substitution patterns. This substrate promiscuity contrasts that of the Humicola insolens Cel7B E197A glycosynthase, which was not able to polymerise the di-galactosylated substrate XLLGalphaF. The production of the PttXET16-34 E85A xyloglucosynthase thus expands the repertoire of glycosynthases to include those capable of synthesising structurally homogenenous xyloglucans for applications.
Glycosynthases, hydrolytically inactive mutant glycosidases that catalyze glycosylation reactions using glycosyl fluoride donor substrates, are emerging as useful tools for the synthesis of large, complex polysaccharides [Faijes, M.; Planas, A. Carbohydr. Res. 2007, 342, 1581-1594]. Guided by wild-type xyloglucanase activity, we have produced and characterized new glycosynthases for the synthesis of xyloglucan oligo- and polysaccharides, based on family GH7, GH12, and GH16 scaffolds. The Humicola insolens GH7 glycosynthase, HiCel7B E197S, is capable of synthesizing nongalactosylated, XXXG-based homoxyloglucan up to M(w) 60,000 [G = Glcβ(1→4); X = Xylα(1→6)Glcβ(1→4); L = Galβ(1→2)Xylα(1→6)Glcβ(1→4)], which is among the largest products so far obtained with glycosynthase technology. Novel glycosynthases based on the GH16 xyloglucan hydrolase from Tropaeolum majus (nasturtium), TmNXG1, are capable of synthesizing XLLG-based xyloglucan oligosaccharides at rates feasible for preparative synthesis, thus providing an essential expansion of product range. Finally, a new glycosynthase based on the recently characterized GH12 xyloglucanase from Bacillus licheniformis, BlXG12 E155A, can perform the condensation of xyloglucosyl fluorides, albeit at poor rates. Altogether, the high catalytic efficiency demonstrated by HiCel7B E197S and the extended product range provided by TmNXG1 E94A are key achievements toward a robust and versatile method for the preparative synthesis of homogeneous xyloglucans with regular substitution patterns not available in nature. Such compounds enable in vitro experimental studies regarding the role of particular structural elements for xyloglucan properties and its interaction with cellulose.
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