Galacturonic polygalacturonic acid

Polygalacturonases were strongly induced in FORL by apple pectin, citric pectin and polygalacturonic acid. When FORL was grown on ucose as carbon source no extracellular PG activity could be detected. Galacturonic acid did not look like a good inducer of PG since the increase of enzyme activity was delayed in comparison with the other carbon sources, after the 5th day of incubation, and also showed lower values (fig.l). 

Hexuronic acids are decarboxylated in the presence of refluxing, aqueous acid to form 5, reductic acid (2,3-dihydroxy-2-cyclopenten-l-one 47) and very small amounts of 48 (5-formyl-2-furoic acid). The yields of these products decrease when dilute acid solutions are used. Most of this work was performed with hydrochloric acid the use of phosphoric acid was not nearly so effective. For instance, D-galacturonic acid quantitatively lost CO2 within 4 h with 3.5 A/ HCl, whereas only 12 mole% of the CO2 was recovered after 4 h with 1.2 M H3PO4. The yields of CO2 from several hexuronic acids (including polygalacturonic acid) were comparatively determined. The acidic decarboxylation of hexuronic acids is bi-molecular and dependent on both the hexuronic acid and hydrochloric acid concentrations. Prior labeling work had established that C-6 of the hexuronic acid is the source of the CO2. 

Figure 1. Schematic representation of pectin structure indicating stabilization of catenated polygalacturonic acid chains through Ca + (O) cross-bridging. Non-bridging sequences of a(l-4) linked /S-galacturonic acid methylester extend from L-rhamnose via (1-4) linkage to another rhamnose via a(l-2) linkages. Arabinogalactan side chains are linked to rhamnose residues and couple the RG structure to hemicellulose.

Citrus pectin ( Polygalacturonic Acid Methyl Ester from Citrus Fruits, Grade I ) was obtained from the Sigma Chemical Company. It had a galacturonic acid content of 89% and a degree of esterification of 57. Separate aqueous solutions of citrus pectin were freeze-dried and air-dried in deionized water. These samples were replicated with 9.8A Pt/C and backed with 148A of carbon. The replicas for these samples were picked up without a carbon support film (38). 

Pectins. The basic building unit of pectins is known to be a-1,4 linked D-galacturonic acid which forms the polygalacturonic acid chain. The carboxyl groups of the D-galacturonic acid in the chain are methylated to various degrees. 

The substrate prescribed for these assays is commercially available polygalacturonic acid. It was chosen based on its availability and on the observation that PGase enzymes are typically specific for glycosidic linkages between de-esterified galacturonic acid units... 

Chemically, pectins can be regarded as a polygalacturonic acid - the pectin molecule is a polymer with galacturonic acid (3) monomers linked... 

The pectic substances are polysaccharides composed principally of a-1,4 linked galacturonic acid or its methyl ester. The term pectin refers to material with a significant amount of esterification in contrast to unesterified polymers called pectic or polygalacturonic acids (Figure 1). The esterification degree of a particular pectin depends upon its source and extraction method. Some pectic substances are partially acetylated at the C2 and C3 positions of galacturonide residues. 

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