Xylans arabino-

Scientific Name Common Name Cellu- lose i oiai Hemi-cellu-lose Gluco man- nan Acetyl- glucurono- xylan Arabino- galactan Pectin Lig- nin" Total Extrac- tives Ash Loca- tion Si... 

Softwood xylans (arabino-4-O-methyl-glucuronoxylans) are composed of shorter chains with a degree of polymarization between 70 and 130 (Fig. 11.4-5). Unlike hardwood xylan, the softwood xylan has a higher content of4-0-methyl-glucuronic acid. The acetyl groups are replaced by a-L-arabinofuranose units, which are linked by a-l,3-glycosidic bonds to the C-3 position of xylose 143. ... 

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. 

Fig. 3 Primary structure of (L-arabino)-4-0-methyl-D-glucurono-D-xylan (AGX)...

Fig. 5 Primary structure of water-soluble L-arabino-D-xylan (AX)...

The second important group of hemicelluloses found in softwoods are the arabino-(4-0-methylglucurono)xylans, which may make up from 5 to 10% of the dry weight of the wood. These consist of /3-1,4-linked D-xylopyranose units, partially substituted by L-arabino-furanose (at the 3 position) and by 4-O-methyl D-glucuronic acid (at the 2 position). The frequency of these substituent groups is around 1 to 2 residues per 10 xylose units. 

The specificity of a xylanase preparation may be determined from the isolation and characterization of oligosaccharides formed from xylan hydrolysis. While the oligosaccharides released were mainly dependent on the specificity of the xylanase, the complex structure of the heteroxylan also determines the extent of hydrolysis and the site of cleavage. Two types of xylooligosaccharides are usually produced acidic (from glucuronoxylans) and neutral (from arabino- and arabinoglucurono-xylans). The specificity of the action of several purified xylanases on various heteroxylans is shown below ... 

The side-chain L-arabinosyl groups of wheat-flour L-arabino-D-xylan are almost completely split off by the purified a-L-arabinofuranosidase11 from A. niger Kl. Similar results were reported36 for an enzyme preparation from Pectinol R-10. In contrast, the a-L-arabinofuranosidase from Pectinol 59-L hydrolyzes only 18% of the L-arabinosidic linkages of wheat L-arabino-D-xylan.37... 

Unlike starch or cellulose, BSG hemicellulose has a complex structure, which is still present, in part, on its autohydrolysis products. Oligosaccharides from BSG hydrolysis consist mainly of branched arabino-xylo-glucurono oligosaccharides that are not highly acetylated when compared to other xylans, such as from Eucalyptus wood (31). The action of several enzymatic activities including endo-l,4-P-xylanase P-xylosidase and accessory activities such as acetyl xylanesterase, a-glucuronidase, and a-arabino-furanosidase is therefore required for the complete hydrolysis of OCL to monosaccharides. 

Hemicellulase Hydrolysis of (3-1,4-glucans, oi-L-arabino-sides, (3-D-mannosides, l,3-(3-D-xylans, and other polysaccharides, yielding polysaccharides of lower molecular weight. 

Xylan composed of D-xylopyranose linked /3-(1 4) and glucurono-arabino-xylan, which is composed of /3-(1 4) D-xylopyranose chain with 4-0-methyl-a-D-glucopyranosyluron-ic acid linked 1—>2 and a-L-arabinofuranosyl linked 1 3 to the xylan chain [66,67,68,69] ... 

The same purified preparations have also been found9-11 to hydrolyze the (1 — 3)-a-L-arabinofuranosyl side-chain linkages of L-arabino-D-xylans and gum arabic, as well as liberating L-arabinose from phenyl a-L-arabinofuranoside. Enzymes having the latter type of specificity are usually classified as glycosidases. However, as they... 

In this Section, the term xylanase will refer to those enzymes capable of hydrolyzing the (1 — 4)-/3-D-xylopyranosyl linkages of the (1 — 4)-/3-D-xylans, namely, arabinoxylan, arabinoglucuronoxylan, arabino-4-O-methyl-D-glucuronoxylan, and glucuronoxylan. D-Xylanases of this type have been assigned the Enzyme Commission numbers 3.2.1.8 [(1 — 4)-/3-D-xylan xylanohydrolase, endo-xylanase] and 3.2.1.37 [(1 —> 4)-/3-D-xylan xylohydrolase, exo-xylanase]. 

Several xylo-oligosaccharides of d.p. 2-7 have been isolated from enzymic hydrolyzates of terrestrial-plant D-xylans, arabinoxylans, arabino-4-O-methylglucuronoxylans, and 4-O-methylglucuronoxylans by the action of endo-xylanases,5,6,38-40,42-44,47,181,215-217,222,228 236,247-250, 254,256,258-261 an(j have been shown to constitute a series of /3-D-(l — 4)-linked D-xylo-oligosaccharides. 

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