2.2.4 Pectic Polysaccharides
Pectic polysaccharides often know as pectin were first time discovered within tamarind fruits in the 18th century. Due to the peculiar nature no specific name was assigned to it. Until, the 19th century it was revealed that this substance is an active fruit component and responsible for gel formation, therefore they named it ‘pectin’ which mean congeal, curdle or solidify. Pectin is heterogeneous and there are more complex polysaccharides in nature that consist of α (1–4) D-galacturonic acid and rhamnose containing backbone, branched with side chains that have enrich with arabinose, galactose and trace amount of other sugars. Being complex in nature, pectin consists of seventeen different types of monosaccharides with more than twenty different types of linkages. Furthermore, pectin performs many biological functions like, cell to cell adhesion, cell wall expansion, cell wall porosity, bonding of ions, seed hydration, fruit development, pollen tube growth, lead abscission and modulation of growth regulators [7, 23, 24].
2.2.4.1 Homogalacturonan (HG)
HG is frequently existing pectin, nearly 60% present in plant cell wall. Structurally, it comprises of α (1–4) linked D-Glucuronic acid with linear chain. In the HG, the –COOH at C-6 of the α-D-Glucuronic acid are methyl-esterified, which conventionally known as high methyl-esterified HG [24, 25]. Homogalacturonan have further two types such as, Rhamnogalacturonan-I (RG-I) which represent 20–35% of pectin. The backbone of RG-I is linear, homologues and composed of the repeating units of galacturonic acid and rhamnose. At position O-3 and O-4 the rhamnose sugars are partially substituted with neutral glucosyl units with side-chain that frequently contain of α (1–5) L-arabinans, arabinogalactans and β (1–4) D-galactans [24, 26]. The second type of homogalacturonan is Rhamnogalacturonan II (RG-II). It is a branched pectic polysaccharides whose backbone is made of repeating units of α (1–4) D-Galacturonic acid α (1–2) L-Rhamnose. The branched side chains of the RG-II are consisting of 12 different glycosyl residues which are bonded together through 22 different glycosidic bonds. So, the covalent cross-linkage between RG-I, RG-II an HG in the cell wall and responsible for flexibility, toughness and dynamicity of the plant cell wall [24, 27].
2.2.4.2 Arabinan
Arabinan is also branched pectic sugar that has a backbone of α (1–5) L-Arabinose with helical structure. The branch consists of single chain of arabinose residues which are linked via (1–2) or (1–3) bonds. The branches of arabinan often degraded during cell expansion and fruit ripening [28, 29]. Arabinan have further two classes such as, Arabinogalactan I and Arabinogalactan II. AG-I is the most common arabinan which abundantly found in several fruits as well as in the cell wall of dicot plants. The backbone of AG-I is made of (1–4) linked β-D-galacturonic acid with small side chain having α (1–5) linked arabinose attached at the O3 position [30]. AG-II is highly complexed pectic polysaccharides that oftenly conjugated with proteins known as arabinogalactan proteins (AGPs). These proteins have 90% polysaccharides and less than 10% of amino acids. Structurally, AG-II has branched backbone which consisting of β (1–3), (1–6) linked D-Galacturonic acid subunits. AG-II has a small branch of one to two residues that also β (1–6) linked D-Galacturonic acid linkages [30, 31].
2.3 Algal Cell Wall Polysaccharides
Algae possess carbohydrates-rich cell walls which composed highly coordinated network of different polysaccharides, water and metals. All polysaccharides that present in algae, either they will be in the form of reserve food or structural component of their cell wall. Polysaccharides that present in algae, act as main representative of their corresponding taxa, and it could be speculated that algal cell wall polysaccharides also act as taxonomical and structural marker. Structurally, the algal cell wall has crystalline and fibrous polysaccharides like, cellulose, hemicellulose and xylans which have embedded in to jelly matrix of carboxylic and sulfated polysaccharides. Alongside the proteins, proteoglycan and phenolic also help in cell wall formation. Similar, different algae produce different polysaccharides such as, the cell wall of green algae have structurally different sulfated or carboxylic polysaccharides. The cell wall of red algae has composite structures of different sugars such as, xylans fibrils, mannan, cellulose, carrageenan and ager. The cell wall of brown algae is made up of cellulosic fibrils, alginates and fucoidans as matrix polysaccharides [32, 33].
Algal cell wall polysaccharides have significant diversity in structure, molecular mass, composition, glycosidic linkages, the arrangement of the sugar residues and the existence or configuration of the functional moieties such as, sulfate semi-esters, methyl-ether, carboxylates, acetyl-ester, etc. Also, the cell wall polysaccharides have linear or branched structure and it may be varied due to the size of side chains or the numbers of branching to internal or peripheral side [33, 34].
2.3.1 Alginates
It is matrix polysaccharides of the algal cell wall, especially in the cell wall of brown algae. Alginate have a linear structure and made of (1–4)-linked β-D-mannuronic acid subunits with epimerically attached α-L-guluronic acid at C-5 position. During biosynthesis of algal cell wall alginates synthesized as the polymeric chains of homomannuronan. Later on, the β-D-mannuronic acid subunits are partly change into α-L-guluronic acid units through C-5 epimerase enzyme (Figure 2.2A). So, these homopolymeric chains form helical structures which stabilized through intramolecular forces such as, hydrogen bonding, etc. [32].
The ratio between mannuronic acid and guluronic acid in algal cell wall alginates depends upon the sources and geographical distribution of algal raw materials, seasonal collection, anatomical part of the algal thalli and the method of its extraction. Algal cell wall alginate could be classified on the basis of high and low mannuronic acid contents, high guluronic acid contents or their intermediates. It also reported that the tissue of young algal thalli have high content of mannuronic acid that compared to different tissue of older thalli of the algae [32, 35].
2.3.2 Sulfated Galactans
Sulfated galactans is matrix polysaccharides with sulfate group and found in algae cell wall, especially, in the cell wall of green and red algae. It is a polymer that composed of sulfated α-D(L)-galactopyranosyl β-D-sand subunits (Figure 2.2B). Sulfated galactans in the cell wall of green algae have both types of hetero and homopolysaccharides and it could be varying among the species. The red algae cell wall contains unbranched, alternative linked (1–3) β and (1–4) α-D-galactans. There are also some functional moieties been attached to algal polysaccharides such as, sulfate hemiesters, pyruvic acid and methyl ester. Methyl and sulfate moieties occasionally attached to the position O-4 and O-2 of β (1–3) galactans and pyruvic acid attached to the β (1–3) D-Galactans at position C-6. In algal polysaccharides, there are good balance between hydrophilic moiety such as, sulfate and hydrophobic like, methoxyl group. The algal enzymes have these abilities to equilibrate the proportion between these two functional groups prior to their biological deigning [36, 37].
Figure 2.2 Figure illustrates different polysaccharides molecules in the algal cell wall. (A) is alginate, having β (1–4) linked D-mannuronic acid subunits with epimerically attached α-L-guluronic acid at C-5 position. (B) Fucoidan is sulfated α (1–3) (1–4)-linked un sulfated and 2-sulfated-α-L-fucose residues. (C) Sulfated galactans which composed of 3-sulfated with 4-linked α-D(L)-galactopyranosyl β-D-sand subunits [62, 63].
Red algae
