Chemical And Enzymatic Reactions

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The reactions of pectin are those characteristic of polysaccharides, esters, and organic acids (7). A select few of the more important reactions will be discussed briefly. Pectin undergoes acid-, base-, and enzyme-catalyzed depolymer-izations. At acidic pH, glycosidic bonds other than those of (l->4) self-linked a-D-galacturonic acid are hydrolyzed preferentially. Thus commercial pectin is extracted from plant matrices by controlled acid hydrolysis. Prolonged acid hydrolysis can produce polygalacturonic acid with about 25 residues, whereas milder hydrolysis conditions have produced homogalacturonans with 70 to 100 galac-turonan residues. Base-catalyzed depolymerizations occur at neutral and higher pH. These are beta-elimination reactions, which proceed with concurrent endodepolymeri-zation, deesterification, and double bond formation (Fig. 2). The relatively high susceptibility to enzymatic and acid-catalyzed hydrolysis of the neutral sugar side chains and rhamnoglacturonan glycosidic bonds in the backbone of pectin may play an important role in plant metabolic processes. Evidence is accumulating that various pectic fragments act as biochemical messengers that initiate various biochemical reactions in plant development, senescence, and the defense of plants against pathogens (4). For example, evidence suggests that fragments from pectin-containing galactose may elicit ethylene production, which is important in the process of senescence. Endogenous en-dopolygalacturonases have been associated with fruits that ripen rapidly (eg, pears and freestone peaches), whereas those that contain only exopolygalacturonases (eg, apples and clingstone peaches) ripen more slowly. Polygalacturonases depolymerize deesterified pectins only. Most of the common pectin lyases depolymerize pectin in a fashion similar to beta-elimination reactions (Fig. 2). Commonly, pectin lyases are not found endogenously in plant cell walls but are produced by microorganisms. In addition to their biological importance, bacterial pectolytic enzymes have gained importance as probes to elucidate the neutral sugar side chain structure of pectin (11). For example, the action of /?-(l,4)-galactanase has shown that apple pectin contains arabinogalactan side chains high in arabinose. More recently, fungal enzymes have been used to investigate the structure of the hairy regions in pectin (8). Pectin methyl esterase activity has been found in the plant cell wall, but the biological function of this enzyme is not clear. Reactions specific to the reducing sugar end group are important in that they permit determination of the number-average molecular weight and provide a method of following the course of depolymerization reactions. Chlorite oxidation of end groups has been used to determine pectin molecular weight, whereas several color reactions specific for reducing sugars have been used to assay for the galacturonic acid produced in depolymerization reactions (6). Ammonia partially amidates pectin through displacement of methyl ester groups. Amidated low methoxy pectic substances gel more readily than corresponding nonamidated low methoxy pectic substances (7). Pectin from sugar beet pulp has been gelled by intermolecular, oxidative coupling of feruloyl ester groups

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