User:Minihaa/Pectin

Structural domains

Pectin is often described as having alternating ‘smooth’ and ‘hairy’ regions, with the ‘hairy’ regions representing the branched rhamnogalacturonan I and rhamnogalacturonan II, and the ‘smooth’ regions corresponding to the linear homogalacturonan backbone.^[1]

More specifically, pectin consists of different galacturonic acid–containing domains—mainly homogalacturonan (HG), rhamnogalacturonan I (RG-I), and rhamnogalacturonan II (RG-II)—which differ in their sugar composition and linkage patterns. Additionally, xylogalacturonan (XGA) and apiogalacturonan (APGA) are often considered to be pectin because they have the same backbone as homogalacturonan.

Homogalacturonan is a linear homopolymer of α-(1 → 4)-linked D-galacturonic acid residues that comprises ∼65 % of pectin.^[2] Generally, homogalacturonan comprises D-galacturonic acid residues monomers in long stretches of at least 72 to 100 residues linked together.

Rhamnogalacturonan I is a repeating disaccharide of [→4-α-D-GalA-(1 → 2)-α-L-Rha-(1→], i. e. an alternating copolymer of galacturonic acid and rhammnose, with many O-4 positions containing other neutral sugars, such as D-galactose or L-arabinose.^[3] The length of the backbone of rhamnogalacturonan I is about 100 to 300 repeating units. Side chains varying by plant sources, such as arabinan, β-(1 → 4)-galactan, type I arabinogalactan (AG-I), and type II arabinogalactan (AG-II) exist. Arabinan consist of α-(1 → 5)-linked L-arabinose backbone, which is usually substituted with α-L-arabinose in different linkages. AG-I is composed out of a β-(1 → 4)-linked D-Gal backbone with α-L-arabinose residues attached to the O-3 position. The terminal galactose of β-(1 → 4) galactan is frequently linked to L-arabinose by α-(1 → 5) glycoside bonds. AG-II is composed of a β-(1 → 3)-linked D-Gal backbone, containing short side chains of α-L-Ara-(1 → 6)-[β-D-Gal-(1 → 6)]n. The galactosyl residues of the side chains can be substituted with α-(1 → 3)-linked L-arabinose residues (Kaczmarska et al., 2022). AG-II is mainly associated with proteins (3–8 %), so called arabinogalactan proteins (AGPs), which are rich in proline/hydroxyproline, alanine, serine, and threonine (Leszczuk et al., 2023). D-galacturonic acid residues residues in the backbone of rhamnogalacturonan I may be highly O-acylated on O-2 and/or O-3, but they are not usually methyl esterified. Ferulic acid groups in rhamnogalacturonan I may be ester-linked to O-2 of the arabinose residues and to O-6 of the galactose residues.

Rhamnogalacturonan II, a highly conserved structure, is a homogalacturonic backbone with numerous complex side chains at C-2 and C-3 positions containing L-rhamnose and other neutral sugars, up to 13 types of sugar may be present, including the rarely observed sugars D-apiose (D-Api), aceric acid, 3-deoxy-lyxo-2-heptolusaric acid (DHA), 3-deoxy-manno-2-octulosonic acid (KDO), D-xylose (D-Xyl), and L-fucose (L-Fuc), in over 21 different linkages (Roman-Benn et al., 2023). As depicted above, rhamnogalacturonan II generally consists of seven branched chains (chains A-F) (Ndeh et al., 2017). Rhamnogalacturonan II can complex together with Boron, forming a borate–diol ester, which can crosslink two homogalacturonan molecules (Voragen et al., 2009).

Xylogalacturonan is an α-(1 → 4)-linked D-galacturonic acid residues chain, which is highly substituted with β-D-xylose (β-D-Xyl) at the C-3 position, which has occasionally been found to be further substituted at O-4 position with an additional β-D-Xyl and apiogalacturonan resulting from D-Apiofuranose substitution at O-2 or O-3 positions. The degree of xylosidation can vary between 25 % (watermelon) and 75 % (apple) (Voragen et al., 2009).

Methoxylation and Degree of Esterification (DE)

α-D-galacturonic acid residues units may be substituted by methyl residues, in which some of the C-6 carboxyl groups are methyl-esterified. In this context, the degree of methoxylation is defined as the percentage of esterified D-galacturonic acid residues units of the total number of D-galacturonic acid residues units in the molecule. In addition, D-galacturonic acid residues may be O-acetylated at the O-2 or O-3 position. The degree of esterification (DE) of pectin is important to the functional properties of the plant cell wall and also affects its physicochemical properties and applications. For instance, pectin is classified into two major groups depending on the DE: HMP (DE > 50 %) and LMP (DE < 50 %). HMP and LMP have different physicochemical properties, and thus diverse applications, such as the stability of pectin emulsion, formation and strength of pectin gel (Cui, Zhang, et al., 2023). Pillai et al. (2020) found that the critical pH of complex formation shifted significantly to higher pH as the DE of the pectin decreased. HMP displayed greater interactions with proteins at optimal mixing conditions compared to LMP (Warnakulasuriya et al., 2018). DE of pectin could influence the bioaccessibility of carotenoids loaded in the oil phase of emulsions (Verrijssen et al., 2014). What's more, it has been verified that the interfacial and emulsifying properties of citrus pectin changed with different DE, and HMP was characterized as smaller hydrodynamic radii and faster adsorption kinetics (Schmidt et al., 2017). The gelation of pectin was closely related to its DM value. In general, HMP generates a gel network at acidic pH (lower than 3.5) in the presence of large amounts of sugars (65 %, w/w) or other co-solutes, by stacking of the methoxyl groups through hydrophobic interactions and hydrogen bonding. Gelation of LMP is ionically mediated through divalent cations called “egg-box model” over a wide range of pH with and without sugar (Wan et al., 2019). Thus, LMP is more preferred in low calorie or dietetic food industry, meeting the increasing consumer demand for healthy diet. Finally, amidated pectin is synthesized through the reaction of ammonia with carboxymethyl groups (–COOCH3) on the pectin molecule. The degree of amidation (DA) is defined as the percentage of carboxylic acid groups of pectin present in amide form. The replacement of methoxyl groups with amide groups modifies some properties of the pectin gels; for instance, amidation increases the water solubility of pectin and enables it to be more thermoreversible and withstand more calcium variation (Chen et al., 2015).