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

Angiosperms possess a retaining trans-a-xylosidase activity that catalyses the inter-molecular transfer of xylose residues between xyloglucan structures. To identify the linkage of the newly transferred a-xylose residue, we used [Xyl-H-3]XXXG (xyloglucan heptasaccharide) as donor substrate and reductively-aminated xyloglucan oligosaccharides (XGO-NH2) as acceptor. Asparagus officinalis enzyme extracts generated cationic radioactive products ([H-3]Xyl center dot XGO-NH2) that were Driselase-digestible to a neutral trisaccharide containing an alpha-[H-3]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-axylosidase had grafted an additional alpha-[H-3]xylose residue onto the xylose of an isoprimeverose unit. The trisaccharide was rapidly acetolysed to an alpha-[H-3]xylobiose, confirming the presence of an acetolysis-labile (1 -> 6)-bond. The alpha-[H-3]xylobiitol formed by reduction of this alpha-[H-3]xylobiose had low molybdate-affinity, indicating a (1 -> 2) or (1 -> 4) linkage. In NaOH, the alpha-[H-3]xylobiose underwent alkaline peeling at the moderate rate characteristic of a (1 -> 4)-disaccharide. Finally, we synthesised eight non-radioactive xylobioses [alpha and beta; (1 <-> 1), (1 -> 2), (1 -> 3) and (1 -> 4)1 and found that the [H-3]xylobiose co-chromatographed only with (1 -> 4)-alpha-xylobiose. We conclude that Asparagus trans-alpha-xylosidase activity generates a novel xyloglucan building block, alpha-D-Xylp-(1 -> 4)-alpha-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. (C) 2012 Elsevier Ltd. All rights reserved.