Читать книгу Polysaccharides - Группа авторов - Страница 40

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].