Galacturonic acid biosynthesis

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. 

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... 

By analogy with the biosynthesis of L-ascorbic acid and with other oxidation-reduction enzyme systems, it seems likely that D-galactose is oxidized to D-galacturonic acid (XXIII) on reduction at Cl, this yields L-galactonic acid (XXIV), the 1,5-lactone (XXV) of which, on reduction at Cl, would give L-galactose (XXII). 

Pathway (a) for example, exists in the biosynthesis of Types I, II, III, VIII, and XXXIII polysaccharides, and pathway (b), in the biosynthesis of Types I and XXXIII polysaccharides, which contain galacturonic acid as a constituent. 

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. 

Figure 10.6. Biosynthesis of various monosaccharide units (UDP-i>galactose, UDP-d-glucuronic acid, UDP-D-galacturonic, UDP-D-xylopyranose, and UDP-L-arabinose) from UDP-D-glucose.

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