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), p-coumaric acid (3.30), caffeic acid (3.32), and 3,4, 5-trihydroxycinnamic acid (3.44), or (5) 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.
Werner et al. (1997) used 13C-labeled glucose in feeding experiments with the fungus Phycomyces blakesleeanus and in leaves of sumac (Rhus typhina). After incubation with the labeled glucose, gallic acid and aromatic amino acids were isolated and subjected to NMR analyses. The NMR data indicated that in both the fungus and the plant the carbon atoms in gallic acid were biosynthetically similar to those in shikimate (3.21), and different from those in phenylalanine (3.27) and tyrosine (3.28). Based on these data 5-dehydroshikimate (3.46) was proposed as the most likely precursor of gallic acid. Ossipov et al. (2003) indeed isolated an enzyme from birch (Betula pubescence) that was able to reduce 5-dehydroshikimate to gallic acid.
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