Galacturonic acid decarboxylation

The sugar 2-amino-2-deoxy-D-gluconic acid inhibits reduction of iron(III) by D-galacturonic acid. Decarboxylation of oxalacetic acid is the final step in a sequence of reactions with the C-bound chromium(III) complex, [(H20)5CrCH2CN] ". The initial steps are coordination and ring closure of the oxalacetate, both by displacement of coordinated H2O with rate constants 0.16 M s and 1 x 10 s respectively at 25.0 °C and 1.0 M ionic strenght. Decarboxylation takes place after protonation of the coordinated substrate. 

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. 

The three acid-derived components of the hexuronic acids, namely, 5, 47, and 48, are essentially end products in the reaction. This is quite surprising, as 48 required only decarboxylation to form 2-furaldehyde. However, the rate of decarboxylation of the furoic acid was <2% of that of D-galacturonic acid under similar conditions. Furthermore, no trace of pentoses was noted in any of the reaction mixtures, which eliminates previous suggestions of their role in the mechanism of 2-furaldehyde for-... 

The kinetics of the decarboxylation reaction have been studied by a number of workers98,99 D. M. W. Anderson and Garbutt100 showed it to be an acid-catalyzed reaction, and they101 found that C-6 of the hexuronic acid is the source of the carbon dioxide. A quantitative yield of carbon dioxide is obtained from D-galacturonic acid when it is treated with 3.29 M hydrochloric acid under reflux for 6 hours.90 Table I indicates the relative rates of decarboxylation of a number... 

Compound 74 undergoes decarboxylation to give 2-furaldehyde, as evidenced by the results of chromatographic experiments.110 However, the fact that 74 is decarboxylated102,110,118 at less than 2% of the rate for D-galacturonic acid under comparable conditions indicates that it is essentially an end product in the reaction, and not an intermediate, either in the production of 2-furaldehyde or in the decarboxylation reaction. Although mechanistic studies have not been reported for this conversion, the obvious structural relationships between 74 and its precursors suggest that 74 is formed by mech-... 

The hemicelluloses also include the polyuronides, or polyuronic acids, for instance a polymer of a hexuronic acid such as galacturonic acid. There is a possible generic link between the polyuronides and the pentosans since the latter might be produced as the result of the decarboxylation of hexuronic acids. The possibility of transforming hexuronic acids into pentosans by the removal of carbon dioxide... 

Zweifel and Deuel demonstrated that glycuronic acids are decarboxylated under relatively mild conditions in the presence of heavy metals thus, L-arabinose is produced from D-galacturonic acid.234 This decarboxylation, which appears to follow a different mechanism from the one already... 

We may return now to the polysaccharides present in the peanut for a brief consideration of the relationship of the other components present in the pectic materials to the araban constituent. All the evidence indicates that the pectic acid portion of the peanut is identical with normal pectic acid and, as was indicated in the previous section, this material, which is very stable to acid hydrolysis and possesses a high positive rotation contains a main chain which is built up of D-galacturonic acid residues of the pyranose type. If, therefore, the araban associated with the pectic acid had been derived directly from the pectic acid by decarboxylation without intermediate hydrolysis of the poly-galacturonide, the sugar residues in the araban should also be in the pyranose form. The experimental evidence shows clearly, however, that the arabinose residues in araban are furanose in type and it follows that any hypothesis concerning the direct conversion of pectic acid into the araban by decarboxylation is untenable. 

The mechanism of the decarboxylation is not well known. The above equation is not entirely correct, for the maximal yield of furfural (C6H4O2) is only about 40%. It is unlikely that the reaction proceeds through the formation of a pentose. Pentoses have never been isolated from such a reaction, when the decarboxylation is conducted under mild conditions such that any added pentose could be recovered 108). Also, in the case of arabinose, the action of boiling 12% hydrochloric acid causes a 70 to 80% conversion to furfural, but in the case of galacturonic acid only 42 % furfural is obtained. 

UDP-Glucuronate carboxylyase is quite specific for the glycosyl moiety of the nucleotide sugar and does not, e.g., decarboxylate UDP-galacturonic acid. However, TDP-ghicuronate is a substrate for the enzyme, although the reaction velocity is only 11% of that observed with UDP-glucuronate under identical reaction conditions. 

M. A. Madson and M. S. Feather, The acid-catalyzed decarboxylation of D-xyluronic, D-galacturonic, and D-glycero-D-gulo-hepturonic acid, Carbohydr. Res., 70 (1979) 307-311. 

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