UDP-galacturonic acid

The substrate for HGA synthesis is UDP-D-galacturonic acid (UDP-GalA) (81). UDP-GalA has been isolated from plants (59) and several pathways for its synthesis in planta have been reported (31). In vitro studies on pectin biosynthesis have been hampered since radiolabeled UDP-galacturonic acid is not commercially available. Previous researchers have used several... 

Scheller H.V., Doong R.L., Rodley B.L., and Mohnen D. 1999. Pectin biosynthesis a solubilized a-l,4-galctouronosyl transferase from tobacco catalyzes the transfer of galacturonic acid from UDP-galacturonic acid onto the reducing end of homogalacturonan. Planta 207 512-517. 

Fig. 42. Oxidative degradation of 1C atom Formation of UDP xylose, UDP glucuronic acid and UDP galacturonic acid.

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. 

This paper begins with a brief description of pectin structure and an overview of the general mechanism of cell wall polysaccharide biosynthesis. This is followed by a summary of previous research on PGA-GalAT and a description of a facile method to synthesize UDP-[ Cj-galacturonic acid. Finally, the paper ends with a summary of our work on the identificadon, partial characterization, and initial solubilization of the homogalacturonan biosynthetic enzyme PGA-GalAT. 

When particulate enzyme preparations from Lemna minor were incubated with UDP-[U-14C]galacturonic acid (UDP-GalA), radioactive label was incorporated into pectic acid and into material soluble in organic solvents. This radioactive material had the solubility properties, and the chromatographic behavior on paper and DEAE-cellulose, of a lipid derivative of GalA however, confirmation awaits further characterization. Its formation is enhanced by UMP, and by addition of a polar, lipid fraction prepared from Lemna minor.98 Transfer of radioactivity from this material to pectic acid has been obtained.177... 

It has been shown that UDP-D-galacturonic acid is present in higher plants (32) and that they contain enzymes which lead to the formation of this uronic acid nucleotide, starting with UDP-D-glucose by the following pathway (33, 34) ... 

UDP-D-galacturonic acid has also been isolated from mung beans. 

In comparison with mammals, plants contain considerably more GTs because, in addition to the reactions carried out by mammalian GTs, they are required to convert the products of photosynthesis into diverse cell carbohydrates. For example, these GTs synthesize cell wall polysaccharides as well as secondary metabolites and xenobiotics. Plant GTs differ from mammalian GTs even more by their diversity of nucleotide donors. They use not only eight of the nine mammalian nucleotide donors, but numerous others such as UDP-L-rhamnose, GDP-L-glucose, GDP-L-galactose, UDP-L-arabinose, UDP-D-galacturonic acid, UDP-D-apiose, and so on. (29). 

A nucleoside pyrophosphoric ester of this uronic acid, UDP-d-galacturonic acid, was shown to be present in mung beans. " In this plant were also found enzymes that led to the formation of this uronic acid derivative of a nucleoside pyrophosphate by the following pathway ... 

The sequence of these enzymic reactions led to the postulate that UDP-D-glucose is a precursor of UDP-D-galacturonic acid, from which the pectin (D-galacturonan) is synthesized. The results of experiments performed in vitro proved to be in accord with this hypothesis. 

Experiments with a number of plants showed that the enzyme that utilizes UDP-D-galacturonic acid as the donor of the D-galactosyl-uronic acid group for the formation of D-galacturonan is fairly specific however, some incorporation of this uronic acid into D-galacturonan was also observed with a particulate enzyme preparation from tomatoes when dTDP-D-galacturonic acid and, to a lesser extent, CDP-D-galacturonic acid were used as donors. ... 

The D-galacturonic acid transferase has been found to have an apparent Michaelis constant of 1.7 fiM for UDP-D-galacturonic acid ... 

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