ReviewPectic substances: From simple pectic polysaccharides to complex pectins—A new hypothetical model
Highlights
► Pectins are viewed as multi-block copolymers of at least homogalacturonan and rhamnogalacturonan-I. ► It is a matter of debate as to the way in which blocks of the two polymers are interlinked. ► A first hypothetical model suggests a linear connection between homogalacturonan and the rhamnogalacturonan-I backbone. ► A challenging model proposes homogalacturonan as a side chain of rhamnogalacturonan I. ► A more complicated model in which the previous two are accounted for is here proposed.
Introduction
Among the polysaccharides which higher plant cell walls comprise, pectic substances are by far and undoubtedly the most complex and interesting with respect to structural organisation and functionality. The pectic substances (sometimes just termed pectins) were first discovered in the eighteenth century (Vauquelin, 1790) within tamarind fruit as ‘a peculiar substance’, but no such a specific name was given at that time until the nineteenth century, where a crude characterisation revealed that it gels, thereby becoming the ‘active fruit component’ responsible for gel formation (Braconnot, 1825). As a consequence, this worker suggested the word ‘pectin’, in reference to the Greek word ‘pektikos’, which means to ‘congeal, solidify or curdle’ (Nussinovitch, 1997), and also predicted that it would have important functions in all plants and many applications in the art of the ‘confiseur’. On all points he was quite correct and the study of ‘this remarkable macromolecule’ has been pursued vigorously by both plant and food scientists ever since (Willats, Knox, & Mikkelsen, 2006). The chemistry of pectic substances actually began in 1917 when d-galacturonic acid (d-GalA), an isomer of d-glucuronic acid (d-GlcA), was discovered to be a basic constituent of all pectic substances so far examined with some of the d-GalA being partially esterified with methyl alcohol (Ehrlich, 1917). Since then, extensive structural and functional studies have been carried out on pectic substances, which have resulted in the discovery of other important functional properties in vivo as well as after extraction as predicted by Sir Hennri Braconnot. To date, pectins are thought to be composed of at least 17 kinds of monosaccharides, of which d-GalA is typically the most profuse, followed by d-galactose or l-arabinose (Vincken et al., 2003, Yapo, 2009a). In planta, pectins are principally structural components, fulfilling important biological functions such as the protection of plants against withering and drought, in the growth and development of cells and in the mechanical and physical properties of the cell wall. Non-extracted pectins in fruits and vegetables consumed daily are a part of dietary fibre, which may help prevent the occurrence of diseases such as diabetes and colecteral cancer (Yapo and Koffi, 2008a, Yapo and Koffi, 2008b. Extracted pectins from citrus peel and apple pomace are mainly used as gelling agents and secondarily as thickening agents, whereas those from sugar beet pulp are intended for oil-in-water (O/W) emulsification in different food and non-food formulations (Yapo, Wathelet, & Paquot, 2007). More recently, chemically altered (citrus) pectins, known under the generic term of ‘modified citrus pectins’ (MCPs), have been reported to possess health benefit properties, especially anti-metastatic properties, though the structural features of these bioactive macromolecules remain by far controversial (Glinsky & Raz, 2009). Despite this, the fine structures of pectic polysaccharides and complex pectins are neither plainly defined nor fully known, which unfortunately reduces their scope of application. Pectic substances were primarily thought to be a triad of homopolymers, viz. homogalacturonan (HG), arabinan and galactan (Hirst & Jones, 1939), before being shown to be heteropolysaccharides in which neutral sugars (NSs) such as l-rhamnose (l-Rha), l-arabinose (l-Ara), d-galactose (d-Gal), and d-xylose (d-Xyl) appeared to be incorporated in galacturonan macromolecules (McCready & Gee, 1960). Moreover, the reports of several pectic structural elements other than HG, namely xylogalacturonan (XGA), apiogalacturonan (ApGA), rhamnogalacturonan-I (RG-I), rhamnogalacturonan-II (RG-II), galactogalacturonan (GGA), arabinogalacturonan (ArGA) and galacturonogalacturonan (GaGA) (Aspinall and Fanous, 1984, Beck, 1967, Bouveng, 1965, Darvill et al., 1978, De Vries et al., 1983, Ovodova et al., 2006, Talmadge et al., 1973) from various plant materials allowed pectic substances to be viewed as a group of extremely complex and structurally diversified polysaccharides from all land plant cell walls and some mucilages (Aspinall, 1970, Ovodov, 2009, Voragen et al., 1995, Yapo, 2009a). Owing to a common galacturonan backbone, RG-II, XGA, ApGA, GGA, ArGA and GaGA are often gathered under the generic term of ‘substituted galacturonans’ (Ridley et al., 2001, Yapo, 2009a). It is widely believed that HG, RG-I, and RG-II or XGA are covalently interconnected to one another, thereby forming complex pectin composites in muro (O’Neill and York, 2003, Schols and Voragen, 1996, Voragen et al., 1995). However, compelling evidence for supporting such a structure has been rather light and sufficiently rare until recently, where structural studies with modern techniques have enabled a group of workers to identify the connecting linkage between HG or XGA and RG-I in apple pectin (Coenen, Bakx, Verhoef, Schols, & Voragen, 2007). It remains a matter of debate as to the way in which the different structural blocks are arranged to form the pectin composite(s), inasmuch as the well-known model of alternating HG and NS-branched RG-I blocks, the so-called ‘smooth’ (SR) and ‘hairy’ (HR) regions of pectins, respectively (Schols and Voragen, 1996, Voragen et al., 1995), has recently been challenged by the proposal of a strikingly different hypothetic model (Vincken et al., 2003) that might be called ‘the RG-I backbone model’. Furthermore, recent structural data, obtained from different new sources of pectins, suggest that both models may be a partial representation of complex pectin as it would natively exist in muro. The present review aims at focusing on the most recent and important work on pectic substances, dealing with the structural and functional features of the possible building blocks and hypothetic models of pectin-complex thereof proposed. Also examined is the possibility that the pectin-complex could be more composite than the existing two most striking models, which is expressed by the proposal of a new hypothetical model where both are accounted for.
Section snippets
Unsubstituted homogalacturonans
Unsubstituted homogalacturonans (HGs), also referred to as linear galacturonans, polygalacturonic acids, polygalacturonides or ‘smooth regions (SRs)’ of complex pectins are the first pectic polysaccharide type isolated from plant cell wall pectins. Structural studies, using methylation analysis after partial acid hydrolysis (PAH), anomeric configuration, and specific optical rotation allowed to delineate unsubstituted HGs as polymeric chains of (1 → 4)-linked-α-d-GalpA residues, irrespective of
Pectins as a macromolecular complex of different structural elements
Pectins are generally viewed as polysaccharide composites, resulting from the covalent inter-linkage of linear HG, branched RG-I and RG-II and/or XGA elements (O’Neill et al., 1990, Schols and Voragen, 1996, Voragen et al., 1995) though there is not sufficiently compelling evidence showing, for instance, that the first three structural elements are covalently inter-linked to form a macromolecular pectin-complex (Ishii & Matsunaga, 2001). The idea of the in muro existence of compositionally
Concluding remarks
Pectic substances represent an outstanding family of the cell wall polysaccharides, which are extraordinary versatile but not yet fully known on a structural as well as a functional point of view. This group comprises at least eight different pectic polysaccharide types, of which homogalacturonan, types I and II rhamnogalacturonans, and to lesser extent xylogalacturonan are the most common. However, the unavailability of analytical tools capable of directly exploring pectins in muro appears to
Acknowledgement
The author is profoundly indebted to Yeshua Ewrade for his assistance and precious help with this review.
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