Heteroxylans, structure

The heteroxylans (CHX) present in cereals, seeds, gum exudates, and mucilages are structurally more complex [6]. They have a (1 4)-)0-d-xylopyranose backbone decorated, except of the single uronic acid and arabinosyl residues with various mono- and oligoglycosyl side chains. Reinvestigations of CHX isolated from corn bran [52] have confirmed that the xylan backbone is heavily substituted (at both positions 2 and 3) with y0-D-Xylp, y0-L-Ara/, of-D-GlcpA residues and oligosaccharide side chains (1), (3) and (4). 

Due to the extreme variety of xylan structures, it is obvious that many kinds of enzymes are needed for their complete hydrolysis in nature. Xylanases (EC 3.2.1.8.) are the polysaccharide hydrolases responsible for the attack of the polymer backbone itself. The total hydrolysis or modification of heteroxylans requires in addition several different exo-glycosidases and esterases. The present knowledge of these enzymes is reviewed in this paper. 

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 ... 

Small proportions of 4-linked Xylp residues were found in linkage analyses of potato cell-wall preparations (Ring and Selvendran, 1978) and of 1 M and 4 M KOH soluble extracts (Ring and Selvendran, 1981). At least some of these residues may be present in heteroxylans of primary cell walls. Such heteroxylans are known to occur in small proportions in the primary cell walls of other eudicotyledons, but their structure has been characterized in detail only for those in the walls of suspension-cultured sycamore (Acer pseudoplatanus) cells (Darvill et al., 1980). These heteroxylans are glucuronoarabinoxylans with a backbone of p-D-Xyl/ residues linked... 

Figure 3.4 Structures of the galactoglucomannan (a heteromannan) known to occur in the primary cell walls of several species of eudicotyledons and the glucuronoarabinoxylan (a heteroxylan) known to occur in the primary cell walls of sycamore (Acer pseudoplatanus). The structures of the heteromannans and heteroxylans in potato cell walls are unknown.

Wagner et al. 33) have shown that two distinct polysaccharide fractions from Echinacea purpurea exhibited pronounced activities characterized by a high rate of phagocytosis stimulation. One polysaccharide was shown to be a heteroxylan of molecular weight 35000 and an arabinorhamnogalactan of molecular weight 450000. The main characteristics of these polysaccharides from Echinacea were the optimal solubility in water, the high content of uronic acids, and the very complex structure. 

Ebringerova, A., Hromadkova, Z., Alfodi, J., Hribalova, V., (1995). Structure and mitogenic activities of corn cob heteroxylans. 

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