Gallic acid biosynthesis

Theoretically, many of the above discrepancies could be settled by experiments with carboxyl-labeled shikimic acid because this functional group would be lost in the formation of phenylalanine, but retained in the case of a direct conversion to gallic acid. Only ambiguous evidence was obtained, however, from such efforts (10), and it was concluded that at least two pathways for gallic acid biosynthesis must exist (14), with the preferential route depending on leaf age and plant species investigated (15,16). 

Sauo, R., Pathway of gallic acid biosynthesis and its esterification with catechins in young tea shoots, Agric. Biol. Chem., 47, 455-460 (1983). 

The biosynthesis of gallic acid (3.47) has been under investigation for more than 50 years. Different biosynthetic routes have been proposed, as depicted in Figure 3-6 (/) direct biosynthesis from an intermediate of the shikimate pathway, (2) biosynthesis via phenylalanine (3.27), cinnamic acid (3.29), />coumaric acid (3.30), caffeic acid (3.32), and 3,4, 5-trihydroxycinnamic acid (3.44), or (3) biosynthesis via caffeic acid (3.32) and protocatechuic acid (3.45). The possibility that different pathways co-existed in different species or even within one species was also considered. 

Wemer, R. A., Rossmann, A., Schwarz, C., Bacher, A., Schmidt, H.-L., and Eisenreich, W., 2004, Biosynthesis of gallic acid in Rhus typhina discrimination between alternative pathways from natural oxygen isotope abundance, Phytochem. 65 2809-2813. 

Ellagic acid is the dilactone of the dimer of gallic acid (3,4,5-trihydroxybenzoic acid). The biosynthesis of ellagic acid (XVH) in plants occurs via an one-electron oxidation of gallic acid (XVHI) followed by dimerization of the gallic acid radicals, and then proceeds by stabilization... 

In contrast to walnuts and pistachios, almond pellicle contains no hydrolysable tannin and therefore no gallic acid was detectable in the cultivars Nonpareil and Mission. This is consistent with their propensity to accumulate aflatoxins [9] and their ability to support aflatoxin biosynthesis. However, there are considerable differences in aflatoxin production in vitro between cultivars and these must be due to compounds other than hydrolysable tannins. Almond pellicle has been shown to contain other phenolic constituents, primarily phenolic acids and flavonoids, which may suppress aflatoxin production although less effectively than the tannins [27-29],... 

For synthesis, see The biosynthesis proceeds through oxidative dimerization of gallic acid and lactone formation. 

C7H,o05, Mr 174.15. needles, D. 1.6, mp. 178-180°C, [a]g -157° (H2O), pKg4.15 (14.1 °C), soluble in water. S. is a widely distributed component of plants and occurs especially in fruits of the star anise (lllicium anisatum, syn. /. religiosum, Illiciaceae Japanese shi-kimi-no-ki). S. is a key intermediate of the so-called shikimic acid pathway which includes the biosynthesis of the aromatic amino acids phenylalanine, tyrosine, and tryptophan. These, in turn, are precursors of numerous alkaloids, flavonoids, and lignans, as well as 4-amino- and 4-hydroxybenzoic acid, gallic acid, tetrahydrofolic acid, ubiquinones, vitamin K, and nicotinic acid. The synthetic racemate melts at 191-192 °C. 

Conn, E. E. and T. Swain, Biosynthesis of gallic acid in higher plants, Chem. Ind., 592-593 (1961). 

Dewick, P. M. and E. Haslam, Phenol biosynthesis in higher plants. Gallic acid, Biochem. J., 113, 537-542 (1969). 

Against this backcloth it is perhaps not surprising to learn, that, despite its distinctive position in the overall patterns of plant phenol metabolism, ambiguity still surrounds the biosynthesis of gallic acid. Several pathways have been proposed. Zenk formulated a conventional pathway (Fig. 7, a) from L-phenylalanine to 3,4,5-trihydroxy-cinnamic acid followed by 6-oxidation to give gallic acid. 

Comparatively little is yet known concerning the biosynthesis of the various derivatives of gallic acid, such as the esters and depsides which occur in plants. 

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