Trans-α-xylosidase, a widespread enzyme activity in plants, introduces (1→4)-α-d-xylobiose side-chains into xyloglucan structures

Phytochemistry. 2012 Jun;78:29-43. doi: 10.1016/j.phytochem.2012.02.003. Epub 2012 Mar 16.

Abstract

Angiosperms possess a retaining trans-α-xylosidase activity that catalyses the inter-molecular transfer of xylose residues between xyloglucan structures. To identify the linkage of the newly transferred α-xylose residue, we used [Xyl-(3)H]XXXG (xyloglucan heptasaccharide) as donor substrate and reductively-aminated xyloglucan oligosaccharides (XGO-NH(2)) as acceptor. Asparagus officinalis enzyme extracts generated cationic radioactive products ([(3)H]Xyl·XGO-NH(2)) that were Driselase-digestible to a neutral trisaccharide containing an α-[(3)H]xylose residue. After borohydride reduction, the trimer exhibited high molybdate-affinity, indicating xylobiosyl-(1→6)-glucitol rather than a di-xylosylated glucitol. Thus the trans-α-xylosidase had grafted an additional α-[(3)H]xylose residue onto the xylose of an isoprimeverose unit. The trisaccharide was rapidly acetolysed to an α-[(3)H]xylobiose, confirming the presence of an acetolysis-labile (1→6)-bond. The α-[(3)H]xylobiitol formed by reduction of this α-[(3)H]xylobiose had low molybdate-affinity, indicating a (1→2) or (1→4) linkage. In NaOH, the α-[(3)H]xylobiose underwent alkaline peeling at the moderate rate characteristic of a (1→4)-disaccharide. Finally, we synthesised eight non-radioactive xylobioses [α and β; (1↔1), (1→2), (1→3) and (1→4)] and found that the [(3)H]xylobiose co-chromatographed only with (1→4)-α-xylobiose. We conclude that Asparagus trans-α-xylosidase activity generates a novel xyloglucan building block, α-d-Xylp-(1→4)-α-d-Xylp-(1→6)-d-Glc (abbreviation: 'V'). Modifying xyloglucan structures in this way may alter oligosaccharin activities, or change their suitability as acceptor substrates for xyloglucan endotransglucosylase (XET) activity.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Asparagus Plant / enzymology*
  • Asparagus Plant / metabolism*
  • Disaccharides / chemistry
  • Disaccharides / metabolism
  • Glucans / chemistry*
  • Glucans / metabolism
  • Molecular Structure
  • Oligosaccharides / chemistry
  • Oligosaccharides / metabolism
  • Xylans / chemistry*
  • Xylans / metabolism
  • Xylosidases / metabolism*

Substances

  • Disaccharides
  • Glucans
  • Oligosaccharides
  • Xylans
  • xyloglucan
  • isoprimeverose
  • Xylosidases
  • xylobiose