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Chemical Composition of Biomass
Published in Jean-Luc Wertz, Philippe Mengal, Serge Perez, Biomass in the Bioeconomy, 2023
Jean-Luc Wertz, Philippe Mengal, Serge Perez
Hemicelluloses-related molecules are pectins, which occur in plant cell walls, mainly as gel matrices. They are heterogeneous and highly hydrated polysaccharides that contain α-1,4-linked-D-galacturonic acid residues.20 Three pectic polysaccharides have been structurally characterized: homogalacturonan, substituted galacturonan (especially xylogalacturonan) and Rhamnogalacturonan (Figure 5.9). Homogalacturonan (HG) is a linear chain of α-1,4-linked-D-galacturonic acid residues where some carboxyl groups are methyl esterified or partially O-acetylated. Xylogalacturonan (XG) has β-D-xylose residues attached to the C3 of the galacturonan backbone. Rhamnogalacturonan I (RG-I) is a group of pectic polysaccharides that contain a backbone of the repeating rhamnose-galacturonic acid disaccharide. From rhamnose, side chains of various neutral sugars branch off. The neutral sugars are mainly D-galactose and L-arabinose. Rhamnogalacturonan II (RG-II) is a highly branched pectin that contains at least 11 different glycosyl residues.21 Its backbone contains at least eight α-1,4-linked galacturonic acid residues.
Guided dietary fibre intake as a means of directing short-chain fatty acid production by the gut microbiota
Published in Journal of the Royal Society of New Zealand, 2020
Hemicelluloses provide the matrix in which cellulose fibrils are embedded within the plant cell wall. These non-cellulosic, structural polysaccharides are composed of a variety of different carbohydrate monomers. Examples are provided by galacto-glucomannan, xylan, arabinogalactan, mixed-link (β1-3,1-4) glucans, xyloglucan, and glucomannan. Many side-chain constituents, including arabinofuranosyl, acetyl, feruloyl, and methylglucuronyl groups, branch off the main backbone of non-cellulosic structural polysaccharides. Thus, the chemical complexity of hemicelloses is much greater than that of cellulose (White et al. 2014). As summarised by Kim et al. (2019), pectin is the most complex polysaccharide found in plant cell walls, consisting of structurally heterogeneous components, such as homogalacturonan (HG), rhamnogalacturonan-I (RG-I), and rhamnogalacturonan-II (RG-II) (Mohnen 2008). HG is a homogenous polymer of α-1,4-linked-d-galacturonic acid (d-GalpA) which constitutes the majority of uronic acid contents of pectin, and 65%–70% of the total pectin mass (Ridley et al. 2001; Mohnen 2008). Approximately half of d-GalpA residues present in HG are either methyl-esterified at C-6 or acetyl-esterified at O-2 and/or O-3 (Ridley et al. 2001). Non-esterified d-GalpA residues carry a negative charge, enabling the formation of a gel-like texture by chelating Ca2+ ions (Caffall and Mohnen 2009). The RG-I and RG-II regions of pectin are compositionally heterogeneous, containing diverse neutral sugars. Depending on the plant species, up to 20%–80% of l-rhamnose (l-Rhap) residues in RG-I are branched by arabinan (polymers of α-l-1,5-arabinofuranose [l-Araf] units branched at O-2 and O-3 with α-l-Araf residues), galactan (unbranched polymers of β-d-1,4-galactopyranose [d-Galp] residues), and arabinogalactan (a linear β-1,4-galactan substituted with α-l-1,5-Araf oligosaccharides) (Mohnen 2008). Some arabinan and galactan are substituted with ferulic acid side chains, which can dimerise to strengthen the pectin network (Zykwinska et al. 2005). The backbone of RG-I consists of alternating diglycosyl units of α-d-GalpA and α-l-Rhap G-II is made of a linear α-1,4-l-GalpA residues, and does not contain l-Rhap units as a part of the basal structure (Yapo 2011a). The RG-II is less abundant than RG-I, but shows a higher degree of structural complexity as it contains at least 13 glycosyl residues covalently linked together by more than 21 different types of glycosidic linkages (O’Neill et al. 2004; Ndeh et al. 2017). In the primary cell wall, RG-II predominantly occurs as a dimer crosslinked by a borate diester (Yapo 2011b).