L-Arabinopyranosyl unit

The L-arabino-(4-0-methyl-D-glucurono)xylans are found in softwoods and annual plants. The L-arabinose is present primarily as a-L-arabinofuranosyl units, although P-L-arabinopyranosyl units may also be present. In either case, the arabinosyl units are often, but not always, present as single-unit side chains, as are the uronic acid units. 

The predominant hemicelluloses in secondary cell walls of both monocotyledons and dicotyledons are xylans. These polysaccharides have a xylan main chain that is decorated with 4-0-methyl-D-glucopyranosyluronic acid, D-glucopyranosyluronic acid, L-arabinofuranosyl, and/or L-arabinopyranosyl units. These related polysaccharides are usually acetylated to various degrees. Gymnosperms contain glucomannans, also acetylated, as the principal secondary cell-wall hemicellulose [26]. 

Mild, acid hydrolysis of European-larch arabinogalactan results in the formation of 3-0-/3-L-arabinopyranosyl-L-arabinose (XXXIX). 26 Although this disaccharide has been isolated as an acid-reversion product from L-arab-inose,131 it is accompanied under these conditions by two other arabinose-containing disaccharides, neither of which was detected in the larch polysaccharide hydrolyzate. Since hydrolysis of methylated -galactan gave approximately equimolar proportions of 2,3,4-tri-O-methyl-L-arabinose and 2,5-di-O-methyl-n-arabinose, traces of 2,3,5-tri-O-methyl-L-arabinose, and no mono-O-methyl-L-arabinose, it is clear that the L-arabinose units must arise from an arabinogalactan rather than from an arabinan, and that the majority of these must be present in 3-0-/3-L-arabinopyranosyl-L-arabino-furanose side-chains (XLV) linked, in some way as yet unknown, to the framework of D-galactose units (XLI). 

D-galactopyranosyl residues carrying (1 — 3)-a-L-arabino-furanosyl and (1 — 6)-/3-D-galactopyranosyl groups as side chains, as well as an occasional, interposed, (1 - 6)-/3-D-galactopyranosyl entity as illustrated. The frequency of occurrence of additional L-arabinopyranosyl side-branch units in the polysaccharide chain is not yet known. 

Virgilia oroboides gum contains a complex array of peripheral units, and controlled, partial hydrolysis with acid affords the disaccharides 5-O-a-L-arabinopyranosyl-L-arabinofuranose (29), 5-O-a-L-arabino-furanosyl-L-arabinofuranose (57), and 5-0-/3-D-xylopyranosyl-L-ara-binofuranose (35), together with traces of oligosaccharides to which the structures 3-O-a-L-arabinofuranosyl-L-arabinose (25) and O-a-L-arabinopyranosyl-(l - 5)-0-L-arabinofuranosyl-(l - 5)-L-arabino-furanose (58) have been assigned. 

D-mannose. Smith degradation destroyed the side-chain residues and left an a-(1 3)-linked mannan backbone, [a]j)+43°. Partial acid hydrolysis liberated g-xylose and the disaccharide 4-0 -a-L-arabinopyranosyl-D-xylose. These results, together with those of methylation analyses, suggested a branched structure wherein some of the (1 3)-linked mannosyl backbone residues are substituted with D-xylopyranosyl, g-mannopyranosyl and 4-0-a-L-arabinopyra-nosyl-g-xylopyranosyl units. 

Larch arabinogalactan has a backbone of (l-3)-linked (3-D-galactopyranosyl units each of which contains a side chain at position C-6. Most of these side chains are galactohiosyl units containing a (l-6)-p-D-linkage. Another side chain type that occurs is a single L-arabinofuranose unit or 3-0-(3-L-arabinopyranosyl)-a-L-arabinofuranosyl units. The preliminary X-ray fibre diffraction data [16] proved that western larch arabinogalactan can adopt a triple helix conformation. 

The most extensively studied exudate gum from this plant family is that from Anogeissus latifolia which is known as gum ghatti or Indian gum (Chap. 10.2.6). This polysaccharide has a main chain of alternating )8-D-glucopyranosyluronic acid units substituted at 0-4 and D-mannopyranosyl units substituted at 0-2 (30, 162). The D-mannopyranosyl units constitute double branch points. The side chains contain L-arabinopyranosyl, L-arabinofuranosyl, D-galactopyranosyl, and D-glucopyranosyluronic acid units. 

The D-arabino-D-galactan of cells of the insect protozoon Crithidia fasciculata contains /3-D-(l—>3)-linked D-galactopyranosyl main-chain units, some of which are unsubstituted others are substituted at 0-2 by (single-unit) D-arabinopyranosyl groups. The 13C-n.m.r. spectrum shows that these are only partial structures, as the C-l region contained 8 signals.125... 

Little is known about the L-arabinofuranosyl transfer machinery. A gene in the genome of A. thaliana has been identified as a membrane-bound arabino-furanosyl transferase by its membrane-insertion sequence and the fact that mutants in the gene led to expression of mRNA, but phenotypes that were nonetheless defective in arabinan side-chains of their pectin. Attempts to use UDP-a-L-arabinopyranose as arabinofuranose donor in crude extracts, presumably in an attempt to uncover any mutase similar to UDP galactopyranose mutase (see Section 5.12.4), merely resulted in the incorporation of pyranose units into the arabinan. However, an enzyme using this donor has been shown to add the terminal arabinopyranosyl residue to the 3-position of the terminal arabinose of the 1,5-arabininan of rhamnogalacturonan 1. [By contrast, the o-arabinofuranosyl transfer machinery that produces the... 

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