Widespread Occurrence of a Covalent Linkage Between Xyloglucan and Acidic Polysaccharides in Suspension-cultured Angiosperm Cells

Abstract

Alkali-stable, anionic complexes of xyloglucan (reported in the case of Rosa to be xyloglucan-RG-I covalent complexes) are widespread in the cell walls of angiosperms, including gramineous monocots.

Mechanism-based Labeling Defines the Free Energy Change for Formation of the Covalent Glycosyl-enzyme Intermediate in a Xyloglucan endo-Transglycosylase

“…This could occur only if the concentrations of free acceptor substrates (xyloglucan oligo-or polysaccharides) were essentially nil, in which case, donor substrate hydrolysis would be extremely slow (for comparison, the archetypal GH16 endoxyloglucanase T. majus NXG1 exhibits hydrolytic rates (v 0 / [E] t ) of ϳ5 min Ϫ1 for xyloglucan oligo-and polysaccharides at or near enzyme saturation) (8). 6 Unfortunately, attempts to obtain crystallographic data on PttXET16 -34 glycosyl-enzymes have thus far been unsuccessful. This appears to be due to the fact that the preferred crystal packing of this enzyme places a second molecule of the asymmetric unit in the negative enzyme subsites (16).…”

“…The problem is further complicated by the observation that strict XETs such as PttXET16 -34 (formerly PttXET16A (13,(15)(16)(17) and BobXET16A (18)) do not use activated aryl ␤-glycosides or ␤-glycosyl fluorides of xylogluco-oligosaccharides as glycosyl donors (15,16). 6 This effectively precludes the steady-state accumulation of a glycosyl-XET complex using Withers' fluorosugar methodology or by manipulation of the relative rates of formation and breakdown of the intermediate using activated donor substrates in combination with mutation of the catalytic acid/base residue (19 -23).…”

Mutations in Multiple XXT Genes of Arabidopsis Reveal the Complexity of Xyloglucan Biosynthesis

“…Driselase is a mixture of exoglycosidases and endoglycosidases that will hydrolyze most cell wall polysaccharides. Because Driselase does not contain a-xylosidase activity, the hydrolysis of xyloglucan releases a mixture of monosaccharides plus a characteristic disaccharide, isoprimeverose [IP; Xyl-a-(1-6)-Glc], that is a diagnostic indicator of xyloglucan (Hayashi and Maclachlan, 1984;Popper and Fry, 2005). The levels of IP provide an estimate of the levels of xyloglucan present in a sample.…”

Revolutionary Times in Our Understanding of Cell Wall Biosynthesis and Remodeling in the Grasses

“…There have also been suggestions that pectic polysaccharides might be covalently linked with xyloglucans in plant cell walls (Keegstra et al, 1973;Thompson and Fry, 2000;Cumming et al, 2005;Popper and Fry, 2005). However, the HvXET5 enzyme purified by Hrmova et al (2007) did not link pectinrelated polysaccharides such as polygalacturonan or b-D-galactans to xyloglucan, nor did it link arabinoxylans to xyloglucans, despite suggestions based on molecular modeling (Strohmeier et al, 2004) that this was a possibility.…”