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Pectin

Pectin is a heterosaccharide in the cell wall of plants. Pectins are very variable in composition; chain lengths are variable and there is a high complexity in the combination and order of each of the monosaccharide derivative units. It is synthesised in the plant Golgi apparatus and forms a matrix in which the hemicellulose polysacharides of the plant cell are embedded. An important part of fruit walls, pectin is broken down to pectinic acid and finally pectic acid. During this chemical breakdown process, the fruit gets softer as the cell walls degenerate.

Pectin is composed of three main polysaccharide types; polygalacturonan (composed of repeated galacturonic acid monosaccharide subunits), rhamnogalacturonan I (composed of alternating rhamnose and galacturonic acid subunits) and rhamnogalacturonan II (a complex, highly branched polysaccharide).

Apples, plums and oranges contain much pectin, and pectin is sometimes found in yogurt, while soft fruits like cherries and strawberries contain little pectin. For commercial utilisation, pectin is extracted from shredded fruit peel or pulp by adding hot water. The pectin dissolves into the hot water, and may then precipitated as a gel by adding ethanol.

Pectin is commonly used as the active ingredient in cough drops because it coats the upper trachea and prevents the spasms which precede coughing. Under acidic conditions, pectin forms a gel. This effect is used for making jams and jellies.

More general information

Structural unit

Pectin has a complex structure. Preparations consist of substructural entities that depend on their source and extraction methodology. Commercial extraction causes extensive degradation of the neutral sugar-containing sidechains.

The majority of the structure consists of homopolymeric partially methylated poly-a-(1®4)-D-galacturonic acid residues ('smooth', see right) but there are substantial 'hairy' non-gelling areas (see below) of alternating a -(1®2)-L-rhamnosyl-a -(1®4)-D-galacturonosyl sections containing branch-points with mostly neutral side chains (1 - 20 residues) of mainly L-arabinose and D-galactose (rhamnogalacturonan I). Pectins may also contain rhamnogalacturonan II sidechains containing other residues such as D-xylose, L-fucose, D-glucuronic acid, D-apiose, 3-deoxy-D-manno-2-octulosonic acid (Kdo) and 3-deoxy-D-lyxo-2-heptulosonic acid (Dha) attached to poly-a-(1®4)-D-galacturonic acid regions [478].

Molecular structure

Generally, pectins do not possess exact structures [328]. D-galacturonic acid residues form most of the molecules, in blocks of 'smooth' and 'hairy' regions. The molecule does not adopt a straight conformation in solution, but is extended and curved ('worm like') with a large amount of flexibility. The `hairy' regions of pectins are even more flexible and may have pendant arabinogalactans. The carboxylate groups tend to expand the structure of pectins as a result of their charge, unless they interact through divalent cationic bridging (their pKa of about 2.9 [326] ensuring considerable negative charge under most circumstances). Methylation of these carboxylic acid groups forms their methyl esters , which take up a similar space but are much more hydrophobic and consequently have a different effect on the structuring of the surrounding water. The properties of pectins depend on the degree of esterification, which is normally about 70%. Low methoxyl-pectins (< 40% esterified) gel by calcium di-cation bridging between adjacent two-fold helical chains forming so-called 'egg-box' junction zone structures so long as a minimum of 14-20 residues can cooperate [326]. Gel strength increases with increasing Ca2+ concentration but reduces with temperature and acidity increase (pH < 3) [463]. It may well be that the two carboxylate groups have to cooperate together in prizing the bound water away from the calcium ions to form the salt links that make up these junction zones. The gelling ability of the di-cations is similar to that found with the alginates ( Mg2+ << Ca2+, Sr2+ < Ba2+) with Na+ and K+ not gelling. If the methoxyl esterified content is greater than about 50%, calcium ions show some interaction but do not gel. The similarity to the behavior of the alginates is that poly-a-(1®4)-D-galacturonic acid is almost the mirror image of poly-a-(1®4)-L-guluronic acid, the only difference being that the 3-hydroxyl group is axial in the latter. The controlled removal of methoxyl groups, converting high methoxyl pectins to low-methoxyl pectins, is possible using pectin methylesterases but the reverse process is not easily achieved.

High methoxyl-pectins (> 43% esterified, usually ~67%) gel by the formation of hydrogen-bonding and hydrophobic interactions in the presence of acids (pH ~3.0, to reduce electrostatic repulsions) and sugars (e.g. about 62% sucrose by weight, to reduce polymer-water interactions) [664]. Low methoxy-pectins (~35% esterified), in the absence of added cations, gel by the formation of cooperative 'zipped' associations at low temperatures (~10°C) to form transparent gels [684]. This hydrogen-bonded association is likely to be similar to that of alginate (see above).

Functionality

Pectins are mainly used as gelling agents, but can also act as thickener, water binder and stabilizer. Low methoxyl pectins (< 50% esterified) form thermoreversible gels in the presence of calcium ions and at low pH (3 - 4.5) whereas high methoxyl pectins rapidly form thermally irreversible gels in the presence of sufficient (e.g. 65% by weight) sugars such as sucrose and at low pH (< 3.5); the lower the methoxyl content, the slower the set. The degree of esterification can be (incompletely) reduced using commercial pectin methylesterase, leading to a higher viscosity and firmer gelling in the presence of Ca2+ ions. Highly (2-O- and/or 3-O-galacturonic acid backbone) acetylated pectin from sugar beet is reported to gel poorly but have considerable emulsification ability due to its more hydrophobic nature, but this may be due to associated protein impurities [309].

As with other viscous polyanions such as carrageenan, pectin may be protective towards milk casein colloids, enhancing the properties (foam stability, solubility, gelation and emulsification) of whey proteins whilst utilizing them as a source of calcium.

Interactive structures are available (COW, 'smooth' [Plug-in, ActiveX], 14 KB; 'hairy' [Plug-in, ActiveX], 15 KB; both Chime, 8 KB).

Studies with Pectin

Although Pectin is perhaps best known for its use as a thickening agent in jams and jellies. Found in almost all plants, its concentration in citrus fruits is particularly high. A class of complex polysaccharides, pectin consists primarily of linear anhydrogalacturonic acid (AGA) repeating units with neutral sugar side chains. The carboxyl moeity of AGA units can exist unmodified, esterified with methanol, or neutralized with cations. Pectin's molecular composition varies not only from plant to plant, but from tissue to tissue in the same plant and at different stages in a plant's development.

Yan Liu of Texas A&M University and colleagues report in the Journal of Agricultural and Food Chemistry (ASAP article) on the characterization of pectin from the peels (flavedo/albedo) and segment membranes of four kinds of citrus fruit: Meyer lemon , Marsh White grapefruit , Dancy tangerine , and Marrs orange . They also examined the inhibitory effect of the different pectins on the ability of fibroblast growth factor 1 (FGF) to bind with its receptor (FGFR) in the presence of heparin.

Pectin is structurally quite similar to heparin. A sulfated polysaccharide , heparin must interact simultaneously with FGF and FGFR for the conformational activation of the FGF-FGFR signaling complex. The FGF system plays an important role in the development of the embryo and in the normal function of adult tissues. The authors hypothesized that pectin may bind competitively to FGF or FGFR, but not both at the same location on a pectin chain, inhibiting heparin binding and thus the activation of the complex.

The authors looked at pectin content, AGA content as a percentage of pectin, methoxyl content as a percentage of pectin, percent low-molecular-weight fraction of the pectin, and total neutral sugar content as a percentage of pectin. Pectins isolated from the tangerine peels and segment membranes had the highest levels of pectin. Pectins from the lemon and tangerine peels had the highest AGA content. Pectin from lemon segment membranes had the highest methoxyl content. The grapefruit and tangerine pectins had the highest amount of low-molecular-weight compounds. Tangerine pectin had the highest neutral sugar content.

Pectin clearly had an inhibitory effect on the binding of FGF to FGFR in the presence of heparin. Pectin from lemon membrane segments was able to completely inhibit binding of the complex; the concentration that inhibited 50% of the binding events (IC50) was <3mg/mL in the presence of 0.1 mg/mL of heparin. Pectin derived from all of the fruits studied was able to inhibit FGF binding. In general, the segment membrane pectins for each fruit had higher inhibitory activity than the peel. Lemon segment membrane pectin induced the greatest inhibition of all of the pectins studied, followed by the segment membranes from tangerines, oranges, and grapefruit.

Various studies have shown pectin to have potential direct health benefits such as a decrease in cholesterol, reduced serum glucose in people with diabetes, and inhibitory effects on cancer growth and metastasis. It also might have applications in wound-healing treatments.

The authors state that "although further studies on the absorption of pectin by the cells is needed, our results seem to suggest that the consumption of citrus fruits along with the segment membrane, the edible part, is expected to result in higher intake of pectin and may have enhanced health benefits." Because of its ability to inhibit FGF binding, the authors hypothesize that "lemon may provide the best beneficial effect in preventing diseases."

Last updated: 10-19-2005 17:20:13
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