Xylan, constitution

First, as noted above, xylans constitute a major renewable resource which is largely untapped though they are employed in the production of furfural and as a replacement for exudate gums. 

Xylan-type polysaccharides are the main hemicellulose components of secondary cell walls constituting about 20-30% of the biomass of dicotyl plants (hardwoods and herbaceous plants). In some tissues of monocotyl plants (grasses and cereals) xylans occur up to 50% [6j. Xylans are thus available in huge and replenishable amoimts as by-products from forestry, the agriculture, wood, and pulp and paper industries. Nowadays, xylans of some seaweed represent a novel biopolymer resource [4j. The diversity and complexity of xylans suggest that many useful by-products can be potentially produced and, therefore, these polysaccharides are considered as possible biopolymer raw materials for various exploitations. As a renewable resource, xylans are... 

Other workers140 have also examined the products derived by hydrolysis of periodate-oxidized xylan. Both wheat straw and com cob xylan, after oxidation and hydrolysis, yield small amounts of L-arabinose and D-xylose. These sugars are obtained even after the xylans have been subjected to extended periods of oxidation. It is concluded that the D-xylose constituted branch points in the xylan. Likewise, the L-arabi-nose molecules must not have been terminal units in a xylan chain but must have been either interior units in the xylan molecule or have constituted an araban-like polysaccharide which is in combination or admixture with the xylan polysaccharide. 

The hetero-l,4-linked xylans (or heteroxylans) constitute a well-characterized group of polysaccharides which form the major components of the hemicellulosic fractions of terrestrial plants (1-4). Softwoods are an exception, where the heteroxylans can be present as a minor component of the total hemicelluloses. They have been isolated from grasses, legumes, ferns, softwoods and hardwoods, and collectively may constitute up to 35% of the total dry weight of higher land plants (4). As such the heteroxylans rank second to cellulose in abundance as naturally occurring organic chemicals in the biosphere. The heteroxylans are closely associated with other... 

For xylanase applications which require xylanases of high selectivity (e.g., biopulping), any contaminating cellulases can be detrimental to the treatments. The constitutive levels of cellulases in yeasts is generally very low in relation to xylanase levels thus indicating yeast xylanase preparations are promising for the selective hydrolysis of xylans. However, extracellular yeast xylanases are typically produced in the order of 1 unit per millilitre... 

Xylanases act on the P-l,4-linked xylan, the most abundant of the hemicelluloses that constitutes over 30% of the dry weight of terrestrial plants.116 They resemble cellulases and cooperate with cellulases and xylosidases117 in digestion of plant cell walls.110116118-121 Galactanase digests the P-l,4-linked component of pectins.122... 

The production of D-xylanase activity by fungi cultured on cellulose may also be due to the action of a cellulase component having multisubstrate activity. Cellulases have always been reported to be adaptive enzymes,226 and the multiplicity of such enzymes may imply that this pseudo-xylanase activity is produced constitutively when fungi are grown on cellulose as the sole source of carbon. Cellulases of this type have been shown203 also to attack D-xylan even when they are highly purified (for example, cellulase F-2 from Trichoderma viride203). 

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. 

The foregoing structure for (1—>3)-/3-D-xylan should receive further examination by polysaccharide crystallographers. If it passes the test, it will probably constitute a model of the way in which many of the /3-d-(1— 6)- and /3-d-(1—>3)-linked polysaccharides crystallize. It also opens new concepts of structure in the polysaccharide field, and relates the crystallography of polysaccharides to that of the polypeptides and polynucleotides. 

---