Ascorbic acid biosynthesis in plants

L-Ascorbic acid biosynthesis in plants and animals as well as the chemical synthesis starts from D-glucose. The vitamin and its main derivatives, sodium ascorbate, calcium ascorbate, and ascorbyl palmitate, are officially recognized by regulatory agencies and included in compendia such as the United S fates Pharmacopeia/National Formula (USP/NF) and the Food Chemicals Codex (FCC). 

Jain, A.K., and Nessler, C.L., 2000. Metabolic engineering of an alternative pathway for ascorbic acid biosynthesis in plants. Mol. Breeding 6 73-78. 

In all plants and most animals, L-ascorbic acid is produced from D-glucose (4) and D-galactose (26). Ascorbic acid biosynthesis in animals starts with D-glucose (4). In plants, where the biosynthesis is more compHcated, there are two postulated biosynthetic pathways for the conversion of D-glucose or D-galactose to ascorbic acid. 

Baig, M.M., Kelly, S., and Loewus, F., 1970, l-Ascorbic acid biosynthesis in higher plants from L-gulono- or L-galactono-1,4-lactone. Plant Physiol. 46 277-280. 

Saito, K., Nick, J. A., and Loewus, F. A., 1990, o-Glucosone and L-sorbosone, putative intermediates of L-ascorbic acid biosynthesis in detached bean and spinach leaves, Plant Physiol. 94 1496-1500. 

Much progress has been made in recent years in the chemistry of ascorbic acid. At least six new syntheses of L-ascorbic acid have been devised since 1971. One of those methods was used to prepare specifically labeled L-ascorbic acid to investigate its biosynthesis in plants. Proton magnetic resonance at 600.2 MHz has shown that the side chain of L-ascorbic acid and its sodium salt in aqueous solution adopt the same conformation as crystalline L-ascorbic acid. The conformation of crystalline sodium L-ascorbate, on the other hand, is different. 

Many fruits are important sources of vitamin C (Table 18.33). Its biosynthesis in plants starts from hexoses, e. g., glucose. It is postulated that following C-1 oxidation and cyclization to 1,4-lactone (II), the 5-keto compound (III) appears as an intermediary product which is oxidized to the 2,3-endiol (IV) then reduced stereospecifically to L-ascorbic acid (V) (cf. Formula 18.40). 

Figure 10.5 Typical L-ascorbic acid biosynthesis pathway in plants.

Ishikawa T, Dowdle J, Smirnoff N. 2006. Progress in Manipulating Ascorbic Acid Biosynthesis and Accumulation in Plants. Physiol Plant. 126 343-355. 

Fig. 10. Suggested pathway for the biosynthesis of L-ascorbic acid (with retention of configuration) in higher plants based on D-glucose-l- C...

As a number of excellent articles have been published that review various aspects of the biological roles of L-ascorbic acid,8 the biosynthesis of L-ascorbic acid in plants and animals,7-11 and radicals derived from L-ascorbic acid,12 these topics will not be treated. Likewise, the methods by which L-ascorbic acid is assayed13 and the uses of L-ascorbic acid and related molecules in a wide variety of assays and oxidation-reduction systems will not be discussed. 

Loewus FA (1999) Biosynthesis and metabolism of ascorbic acid in plants and of analogs of ascorbic acid in fungi. Phytochemistry 52, 193-210. 

Conklin PL, Norris SR, Wheeler GL et al (1999) Genetic evidence for the role of GDP-mannose in plant ascorbic acid (vitamin C) biosynthesis. Proc Natl Acad Sci USA 96 4198-4203... 

SMIRNOFF, N., WHEELER, G.L., Ascorbic acid in plants biosynthesis and function, Crit. Rev. Biochem. Mol. Biol., 2000,35,291-314. 

Conklin, PL., 2001, Recent advances in the role and biosynthesis of ascorbic acid in plants. Plant Cell Environ. 24 383-394. 

Smirnoff, N., Conklin, P.L., and Loewus, F.A., 2001, Biosynthesis of ascorbic acid in plants A renaissance. Anna. Rev. Plant Physiol. Plant Mol. Biol. 52 437 167. 

Side-chain oxidized derivatives of ascorbic acid are also implicated in the catabolism of ascorbic acid in plants. Loewus et al. (62) have established the intermediacy of ascorbic acid in the biosynthesis of tartaric acid in the grape. Labeling studies have established a metabolic pathway that must involve C5 and C6 oxidation of ascorbic acid. 

M7. Mapson, L. W., The biosynthesis of L-ascorbic acid in plants and animals. Proc. Intern, Congr. Biochem. 4th Congr. Vienna 11, 1-16 (1960). 

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