Heteroxylans

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). 

Several CHX samples were isolated from the leaves and barks of tropical dicots such as the Litsea species [3]. The mucilage-forming seeds of Plantago sp. contain very complex heteroxylans [53,54]. For the CHX from Plantago major seeds [53], a (1 3, 1 -> 4)-mixed-linkage xylan backbone has... 

Analysis of the products which become water-soluble after extrusioncooking showed that feroylated heteroxylans and glucans were solubilised from wheat bran (10), arabinans, heteroxyalans and pectins from pea hulls (11,12), and pectins from the other sources (13,14). 

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. 

This chapter is concerned with their specificity in degrading their substrate. The biodegradative pathways of the heteroxylans are also discussed. 

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... 

O-acetyl groups are readily removed when the heteroxylans are extracted with alkali (saponified), and the isolated product can therefore be misleadingly low in O-acetyl content. Other chemical groups may also be linked to the heteroxylans. There is now evidence to suggest that lignin is covalently attached to heteroxylans and other plant cell wall polysaccharides... 

Enzymes attacking the non-glycosidic appendages of the heteroxylans include several recently discovered enzymes, i.e., acetyl-xylan esterase (42) and ferulic acid esterase (33), which are specific for the hydroly-... 

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 purple coneflower Echinacea purpura, and its close relatives, E. angustifolia and E. pallida, are the source of the herb Echinacea, which is widely popular as a nonspecific immune stimulant. These perennials are native to the prairies of North America and are now widely grown garden ornamentals. The root and aerial parts of the plant are the portions used, and the preparation s potency can be verified by the transient tingling sensation produced when it is tasted. Echinacea contains alkamides, caffeic acid esters (echinacoside, cichoric acid, caftaric acid), polysaccharides (heteroxylan), and an essential oil. Some echinacea products are standardized for their echinacoside content. In the past, adulteration with American feverfew (Parthenium integri-folium) was common. Echinacea is now sold either by itself or in combination with golden seal or zinc for the treatment of colds and influenza. 

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... 

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