@article {11068,
	title = {Active-site mapping of a Populus xyloglucan endo-transglycosylase with a library of xylogluco-oligosaccharides},
	journal = {Journal of Biological Chemistry},
	volume = {283},
	number = {32},
	year = {2008},
	note = {Saura-Valls, Marc Faure, Regis Brumer, Harry Teeri, Tuula T. Cottaz, Sylvain Driguez, Hugues Planas, Antoni},
	pages = {21853-21863},
	abstract = {Restructuring the network of xyloglucan (XG) and cellulose during plant cell wall morphogenesis involves the action of xyloglucan endo-transglycosylases (XETs). They cleave the XG chains and transfer the enzyme-bound XG fragment to another XG molecule, thus allowing transient loosening of the cell wall and also incorporation of nascent XG during expansion. The substrate specificity of a XET from Populus (PttXET16-34) has been analyzed by mapping the enzyme binding site with a library of xylogluco-oligosaccharides as donor substrates using a labeled heptasaccharide as acceptor. The extended binding cleft of the enzyme is composed of four negative and three positive subsites (with the catalytic residues between subsites -1 and + 1). Donor binding is dominated by the higher affinity of the XXXGmoiety (G = Glc beta(1 -> 4) and X = Xyl alpha(1 -> 6)Glc beta(1 -> 4)) of the substrate for positive subsites, whereas negative subsites have a more relaxed specificity, able to bind (and transfer to the acceptor) a cello-oligosaccharyl moiety of hybrid substrates such as GGGGXXXG. Subsite mapping with k(cat)/K(m) values for the donor substrates showed that a GG-unit on negative and-XXG on positive subsites are the minimal requirements for activity. Subsites -2 and -3 (for backbone Glc residues) and +2{\textquoteright} (for Xyl substitution at Glc in subsite +2) have the largest contribution to transition state stabilization. GalGXXXGXXXG (Gal = Gal beta(1 -> 4)) is the best donor substrate with a "blocked" nonreducing end that prevents polymerization reactions and yields a single transglycosylation product. Its kinetics have unambiguously established that the enzyme operates by a ping-pong mechanism with competitive inhibition by the acceptor.},
	doi = {10.1074/jbc.M803058200},
	author = {Saura-Valls, M. and Faure, R. and Brumer, Harry and Teeri, T. T. and Cottaz, S. and Driguez, H. and Planas, A.}
}
