the type of glycosidic linkages, etc. Based on the nature of substituents found on
the backbone, xylans are categorized as linear homoxylan, arabinoxylan,
glucuronoxylan, and glucuronoarabinoxylan. However, in each category there exists
microheterogeneity with respect to the degree and nature of branching.
Homoxylans are xylans which are linear and unsubstituted. Homoxylans with
β-1,3-linkages substitute cellulose in the cell wall structure of the green algae
Caulerpa sp. and Bryopsis maxima [32]. Similarly, homoxylans with mixed β-13-/β-1-4-linkages serve as cell wall components of red seaweeds of the Palmariales
and Nemaliales [33]. Higher plants possess xylans containing β-1-4-linked xylose
units as the backbone which is usually substituted with sugar units and O-acetyl
groups and are known as heteroxylans. The only exception of heteroxylans with
mixed β-1-3/β-1-4-linkages in the main chain has been isolated from the seeds of
Plantago species [34]. However, unsubstituted linear forms have been obtained from
guar seed husk, esparto grass, and tobacco stalks.
Arabinoxylans contain a higher amount of arabinose, whereas glucuronoxylans
contain more glucuronic acid attached to the backbone. The glucuronoxylans
exist as O-acetyl-4-O-methylglucuronoxylan in hardwoods and as arabino-4-Omethylglucuronoxylan in softwoods [35]. Xylans in cereals, grasses, and annual
plants are typically arabinoxylans [35]. Glucuronoarabinoxylans contain both
arabinose and glucuronic acid. Arabinoxylans, glucuronoxylans, and arabinoglucuronoxylans are branched, and the degree of branching, the type of side-chain
residues, the type of linkage, etc. result in diverse features that make these xylans
very heterogeneous.
Arabinose can be attached to the xylan backbone by α-1,2- or α-1,3-linkages.
This side-chain residue can also contain xylose and galactose which can be linked
either to arabinose or xylose. In various plants, xylan exists in acetylated form
[35]. Acetyl residues may be attached to the C-2 or C-3 of the xylose residue or to
both sites [36] in the backbone of xylan, and the degree of acetylation differs
strongly among xylans of different origin. Glucuronic acid and its 4-O-methyl
ether are attached to the xylan backbone via an α-1,2-linkage, whereas aromatic
(feruloyl and p-coumaroyl) residues have so far only been found attached to the C-5
of terminal arabinose residues.
2.2 Mannan
Mannan is another important hemicellulose which is found almost in all plant cell
wall. One of its distinguishing features is its backbone which can be entirely mannan
or a mix of mannose and glucose [37]. Like xylan, it is also diverse and exists in
different forms including as linear mannan, galactomannan, glucomannan, and
glucogalactomannan.
Linear mannan is a polymer containing more than 95% mannose and can be
considered as “pure” mannan. It occurs in some seeds such as tagua palm seed,
coffee beans, and orchid tubers. Galactomannan is characterized by β-(1–4)-mannan
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G. Mamo
the backbone, xylans are categorized as linear homoxylan, arabinoxylan,
glucuronoxylan, and glucuronoarabinoxylan. However, in each category there exists
microheterogeneity with respect to the degree and nature of branching.
Homoxylans are xylans which are linear and unsubstituted. Homoxylans with
β-1,3-linkages substitute cellulose in the cell wall structure of the green algae
Caulerpa sp. and Bryopsis maxima [32]. Similarly, homoxylans with mixed β-13-/β-1-4-linkages serve as cell wall components of red seaweeds of the Palmariales
and Nemaliales [33]. Higher plants possess xylans containing β-1-4-linked xylose
units as the backbone which is usually substituted with sugar units and O-acetyl
groups and are known as heteroxylans. The only exception of heteroxylans with
mixed β-1-3/β-1-4-linkages in the main chain has been isolated from the seeds of
Plantago species [34]. However, unsubstituted linear forms have been obtained from
guar seed husk, esparto grass, and tobacco stalks.
Arabinoxylans contain a higher amount of arabinose, whereas glucuronoxylans
contain more glucuronic acid attached to the backbone. The glucuronoxylans
exist as O-acetyl-4-O-methylglucuronoxylan in hardwoods and as arabino-4-Omethylglucuronoxylan in softwoods [35]. Xylans in cereals, grasses, and annual
plants are typically arabinoxylans [35]. Glucuronoarabinoxylans contain both
arabinose and glucuronic acid. Arabinoxylans, glucuronoxylans, and arabinoglucuronoxylans are branched, and the degree of branching, the type of side-chain
residues, the type of linkage, etc. result in diverse features that make these xylans
very heterogeneous.
Arabinose can be attached to the xylan backbone by α-1,2- or α-1,3-linkages.
This side-chain residue can also contain xylose and galactose which can be linked
either to arabinose or xylose. In various plants, xylan exists in acetylated form
[35]. Acetyl residues may be attached to the C-2 or C-3 of the xylose residue or to
both sites [36] in the backbone of xylan, and the degree of acetylation differs
strongly among xylans of different origin. Glucuronic acid and its 4-O-methyl
ether are attached to the xylan backbone via an α-1,2-linkage, whereas aromatic
(feruloyl and p-coumaroyl) residues have so far only been found attached to the C-5
of terminal arabinose residues.
2.2 Mannan
Mannan is another important hemicellulose which is found almost in all plant cell
wall. One of its distinguishing features is its backbone which can be entirely mannan
or a mix of mannose and glucose [37]. Like xylan, it is also diverse and exists in
different forms including as linear mannan, galactomannan, glucomannan, and
glucogalactomannan.
Linear mannan is a polymer containing more than 95% mannose and can be
considered as “pure” mannan. It occurs in some seeds such as tagua palm seed,
coffee beans, and orchid tubers. Galactomannan is characterized by β-(1–4)-mannan
252
G. Mamo