backbone having single galactose branches linked α-(1–6) [38]. The mannose-togalactose ratio and the structure vary from source to source [39]. It is a common
hemicellulose in legumes seeds. Fenugreek, guar gum, tara gum, and locust bean
gums are some of the most known galactomannan hemicelluloses. Guaran (or guar
gum) and locust bean gums are widely used as thickeners in the food industry.
Glucomannan has a backbone composed of mannose and glucose. The mannoseto-glucose ratio and the presence of side group vary among glucomannans from
different botanical sources. Hardwood glucomannan has no attached side group, and
the most common mannose-to-glucose ratio is about 2:1. On the other hand,
glucomannan from softwood contains side groups, and the common mannose-toglucose ratio is about 3:1. The side group is galactose which is attached to mannose
residue at position C-6 or at C-3 of glucose. Glucomannans are found in iris seeds,
lily bulbs, bluebell seeds, orchids, etc. Galactoglucomannan is basically a lower DP
glucomannan which consists of about ten times more galactose. Glucomannans and
galactoglucomannans are the main hemicelluloses in softwoods and hardwoods.
Hardwoods contain about 3–5% glucomannans, whereas softwoods contain mainly
galactoglucomannans [40].
2.3 Other Hemicelluloses
Xyloglucan is a common hemicellulose in non-graminaceous plants and accounts for
up to 20% of the dry weight of the cell wall [41]. It has celluloselike backbone
consisting of β-1-4-linked glucose residues to which xylose units are attached at
position 6 of some of the backbone glucose residues and are highly branched unlike
cellulose [41]. To the xylose residues attached to backbone glucose, residues such as
arabinose and galactose can be attached and form di- or tri-glycosyl side chains
[42]. Other hemicelluloses which consist of hexose backbones are also known.
Depending on the nature of the sugar backbone, these hemicelluloses are divided
into different groups. Among these groups, arabinogalactans, β-glucan, lichenan,
and laminarin are well-characterized. Arabinogalactan has a backbone of
β-1,3-linked D-galactose residues and is substituted with β-1,6 side chains. It is the
dominant hemicellulose in larchwood, an exception among softwoods where
galactoglucomannan is the dominant hemicellulose [27]. β-glucan and lichenan are
glucan-based hemicelluloses. The gramineae seed β-glucan consists of linear polymer of mixed β-1,3-/β-1,4-linked glucan in a DP range of 25–75 [43]. On the other
hand, lichenan has a similar structure to that of β-glucan, but with DP in the range of
100–200. Lichenan constitutes about 10% of the lichen Icelandic moss Cetraria
islandica [44]. Laminarin possesses β-1,3-linked glucose backbone with an average
DP of 20–25 [33]. The brown alga Phaeophyta spp. is the commercial source of
this hemicellulose. A microbial hemicellulose, curdlan (β-1,3-glucan) has been
reported from Agrobacterium sp. and the alkali-tolerant Alcaligenes faecalis
[45, 46].
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253
hemicellulose in legumes seeds. Fenugreek, guar gum, tara gum, and locust bean
gums are some of the most known galactomannan hemicelluloses. Guaran (or guar
gum) and locust bean gums are widely used as thickeners in the food industry.
Glucomannan has a backbone composed of mannose and glucose. The mannoseto-glucose ratio and the presence of side group vary among glucomannans from
different botanical sources. Hardwood glucomannan has no attached side group, and
the most common mannose-to-glucose ratio is about 2:1. On the other hand,
glucomannan from softwood contains side groups, and the common mannose-toglucose ratio is about 3:1. The side group is galactose which is attached to mannose
residue at position C-6 or at C-3 of glucose. Glucomannans are found in iris seeds,
lily bulbs, bluebell seeds, orchids, etc. Galactoglucomannan is basically a lower DP
glucomannan which consists of about ten times more galactose. Glucomannans and
galactoglucomannans are the main hemicelluloses in softwoods and hardwoods.
Hardwoods contain about 3–5% glucomannans, whereas softwoods contain mainly
galactoglucomannans [40].
2.3 Other Hemicelluloses
Xyloglucan is a common hemicellulose in non-graminaceous plants and accounts for
up to 20% of the dry weight of the cell wall [41]. It has celluloselike backbone
consisting of β-1-4-linked glucose residues to which xylose units are attached at
position 6 of some of the backbone glucose residues and are highly branched unlike
cellulose [41]. To the xylose residues attached to backbone glucose, residues such as
arabinose and galactose can be attached and form di- or tri-glycosyl side chains
[42]. Other hemicelluloses which consist of hexose backbones are also known.
Depending on the nature of the sugar backbone, these hemicelluloses are divided
into different groups. Among these groups, arabinogalactans, β-glucan, lichenan,
and laminarin are well-characterized. Arabinogalactan has a backbone of
β-1,3-linked D-galactose residues and is substituted with β-1,6 side chains. It is the
dominant hemicellulose in larchwood, an exception among softwoods where
galactoglucomannan is the dominant hemicellulose [27]. β-glucan and lichenan are
glucan-based hemicelluloses. The gramineae seed β-glucan consists of linear polymer of mixed β-1,3-/β-1,4-linked glucan in a DP range of 25–75 [43]. On the other
hand, lichenan has a similar structure to that of β-glucan, but with DP in the range of
100–200. Lichenan constitutes about 10% of the lichen Icelandic moss Cetraria
islandica [44]. Laminarin possesses β-1,3-linked glucose backbone with an average
DP of 20–25 [33]. The brown alga Phaeophyta spp. is the commercial source of
this hemicellulose. A microbial hemicellulose, curdlan (β-1,3-glucan) has been
reported from Agrobacterium sp. and the alkali-tolerant Alcaligenes faecalis
[45, 46].
Alkaline Active Hemicellulases
253