Xylans glucuronoxylans

Hemicelluloses, Polysaccharides, Xylan, Glucuronoxylan, Arabinoxylan, Mannan, Glucomannan and galactoglucomannan. Polymer, Bios3uithesis, Biodegradation, Properties, Applications... 

Most of the glucuronoxylans have single 4-0-methyl-o -D-glucopyranosyl uronic acid residues (MeGlcA) attached always at position 2 of the main chain Xylp units (Fig. 2). This structural type is usually named as 4-0-methyl-D-glucurono-D-xylan (MGX). However, the glucuronic acid side chain may be present in both the 4-0-methylated and non-methylated forms (GlcA). 

Most frequently, SEC with dextran-, pullulan-, or polystyrene calibration standards has been used to characterize the molecular properties of xylans. However, as for viscometric studies [108], a sufficient solvent ionic strength is a prerequisite for useful SEC measurements of charged polysaccharides, including glucuronoxylans [111-113]. An advantage of the SEC technique is that the presence of protein and phenolic components or oxidative changes can be detected by simultaneous ultraviolet (UV) detection. 

The only major accessory enzyme purified thus far for the xylanase was an a-(l,3)-L-arabinosidase with a molecular weight of 66,(XX) and isoelectric point of 4.1 (publication in preparation). The enzyme was not capable of acting on intact xylans. However, it acted on the end-products of arabinoxylan (but not glucuronoxylan) degradation by the xylanase, doubling the total extent of degradation. 

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 ... 

Glucuronoxylan Even if hemicelluloses in various hardwood species differ from each other both quantitatively and qualitatively, the major component is an 0-acetyl-4-0-methylglucuronoi3-D-xylan, sometimes called glucuronoxylan. Often the xylose-based hemicelluloses in both softwoods and hardwoods are termed simply xylans. 

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]. 

Glucuronoxylan (15-30%) is the major hemicellulose of hardwoods such as birch, eucalyptus and poplar/aspen. It is similar to grass xylan, but lacks the arabinofuranosyl decoration (Figure 4.44). 

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. ... 

In several cases, separation of acidic xylans from neutral glucomannans has been achieved by way of their acetates, by a method originally suggested by Perlin. As mentioned earlier (see Part I, p. 295), the acetates of acidic xylans are seldom readily soluble in chloroform. On the other hand, acetylated hexosans dissolve readily. When a chloroform solution of a glucuronoxylan acetate and a glucomannan acetate is shaken with water, an emulsion is formed which contains the pentosan acetate while the hexosan acetate remains in the chloroform. This method has been successfully applied by other workers, but it is somewhat time-consuming. 

Acidic polysaccharides are also precipitated but at higher concentrations of Ba(OH)2. Meier (1958) found that birch glucuronoxylan was precipitated when the Ba(OH)2 concentration was 0.15 M, probably due to the formation of the insoluble barium salt of the acidic xylan. Pectic acid can be separated almost quantitatively from the neutral (pectic) polysaccharides as the insoluble barium salt. Therefore the use of Ba(OH)2 as a precipitant will provide a complement to the other methods. 

In hardwood, the xylan, or glucuronoxylan, consists mainly of D-xylose residues, most of which are acetylated, and 4-0-methyl-D-glucuronic acid residues (Fig. 2). Minor amounts of other constituents, such as L-rhamnose and D-galacturonic acid, can also be found. The molar ratio of 4-0-methyl-a-D-glu-curonic acid, acetyl groups, and xylose has been estimated to be 1 7 10 [59]. Hardwood also contains small amounts (2-5% dry weight (DW)) of gluco-mannan, composed of D-glucose and o-mannose. 

Birch glucuronoxylan, grass xylan, corn cob xylan Gluten [31]... 

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