Active-site Mapping of a Populus Xyloglucan endo-Transglycosylase with a Library of Xylogluco-oligosaccharides

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 XXXG moiety (G = Glcβ(1→4) and X = Xylα(1→6)Glcβ(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 kcat/Km 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′ (for Xyl substitution at Glc in subsite +2) have the largest contribution to transition state stabilization. GalGXXXGXXXG (Gal = Galβ(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.