title: Trans-α-xylosidase, a widespread enzyme activity in plants, introduces (1→4)-α-d-xylobiose side-chains into xyloglucan structures.
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.
authors.affiliation: The Edinburgh Cell Wall Group, Institute of Molecular Plant Sciences, School of Biological Sciences, The University of Edinburgh, The King's Buildings, Mayfield Road, Edinburgh EH9 3JH, UK.
authors.forename: Lenka
authors.initials: L
authors.lastname: Franková
authors.forename: Stephen C
authors.initials: SC
authors.lastname: Fry
mesh_terms: D027761:Asparagus Plant / Q000201:enzymology* / Q000378:metabolism*; D004187:Disaccharides / Q000737:chemistry / Q000378:metabolism; D005936:Glucans / Q000737:chemistry* / Q000378:metabolism; D015394:Molecular Structure; D009844:Oligosaccharides / Q000737:chemistry / Q000378:metabolism; D014990:Xylans / Q000737:chemistry* / Q000378:metabolism; D014995:Xylosidases / Q000378:metabolism*
chemical_list: D004187:Disaccharides; D005936:Glucans; D009844:Oligosaccharides; D014990:Xylans; C029353:xyloglucan; C046205:isoprimeverose; D014995:Xylosidases; C004173:xylobiose