Active-site mapping of a Populus xyloglucan endo-transglycosylase with a library of xylogluco-oligosaccharides

J Biol Chem. 2008 Aug 8;283(32):21853-63. doi: 10.1074/jbc.M803058200. Epub 2008 May 28.

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 XXXG moiety (G=Glcbeta(1-->4) and X=Xylalpha(1-->6)Glcbeta(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' (for Xyl substitution at Glc in subsite +2) have the largest contribution to transition state stabilization. GalGXXXGXXXG (Gal=Galbeta(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.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Binding Sites
  • Electrophoresis, Capillary
  • Glycosyltransferases / metabolism*
  • Kinetics
  • Naphthalenes / metabolism
  • Oligosaccharides / metabolism*
  • Populus / enzymology*
  • Small Molecule Libraries / metabolism*
  • Time Factors

Substances

  • Naphthalenes
  • Oligosaccharides
  • Small Molecule Libraries
  • 8-amino-1,3,6-naphthalenetrisulfonic acid
  • Glycosyltransferases
  • xyloglucan endotransglycosylase