Xylans

The second most abundant hemicelluloses in dicot primary cell walls are xylans, which have a backbone of (1^4)-linked P-Xyl residues. Dicot xylans carry side chains, especially a-Ara, a-GlcA and a-MeGlcA attached predominantly to position 2 of some Xyl residues - hence the use of fuller names such as glucuronoarabinoxylans (GAXs), but there is probably a continuum of compositions, and the term 'xylans' will generally be used here to cover all such hemicelluloses. O-Acetyl groups are also present, especially on the Xyl residues. Xylans can hydrogen-bond to cellulose, though generally more slowly and less strongly than do xyloglucans and MLGs.

When cell walls or AIR are digested with Driselase, the xylan backbone is cleaved to yield a mixture containing xylose and xylobiose, the yield of which is a valuable indication of xylan content. Xylobiose is a particularly useful diagnostic fragment which enables assay of xylan - as with isopri-meverose for xyloglucan. The yield is maximized by prior treatment with NaOH or mild acid. Xylobiose is completely stable in the presence of Driselase. The endo-xylanase activity of Driselase cleaves xylan's backbone to yield progressively smaller oligosaccharides, the last permitted reaction being hydrolysis of the trisaccharide:

Xyl-Xyl-Xyl + H2O * Xyl-Xyl + Xyl forming a stable disaccharide, xylobiose. Other enzymes in Driselase remove the Ara residues as monosaccharide. However, Driselase lacks a-d-glucuronidase; the GlcA and MeGlcA residues of xylans probably end up in oligosaccharides.

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