Shikimate dehydrogenase, aromatic amino acid biosynthesis

Derivatives of the dehydroshikimate analogue 1,6-dihydroxy-2-oxoisonicotinic acid (Fig. 4) were fairly powerful inhibitors of shikimate dehydrogenase from pea, but they failed as herbicides in vivo. There is a certain irony in the fact that in the same year in which Baillie et al. announced the inconclusive outcome of their efforts, Jaworski proposed inhibition of aromatic amino acid biosynthesis as the mode of action of glyphosate, which was just emerging as a highly successful herbicide. 

A bifunctional dehydrogenase fromNeurospora which catalyzes the dehydrogenation of quinate and shikimate, functions in the inducible quinate catabolic pathway (Section IV). The catabolic form of the enzyme is distinguished from the form that occurs in the aggregate which is involved in the biosynthesis of the aromatic amino acids (Ahmed and Giles, 1969). 

L-Phenylalanine,which is derived via the shikimic acid pathway,is an important precursor for aromatic aroma components. This amino acid can be transformed into phe-nylpyruvate by transamination and by subsequent decarboxylation to 2-phenylacetyl-CoA in an analogous reaction as discussed for leucine and valine. 2-Phenylacetyl-CoA is converted into esters of a variety of alcohols or reduced to 2-phenylethanol and transformed into 2-phenyl-ethyl esters. The end products of phenylalanine catabolism are fumaric acid and acetoacetate which are further metabolized by the TCA-cycle. Phenylalanine ammonia lyase converts the amino acid into cinnamic acid, the key intermediate of phenylpropanoid metabolism. By a series of enzymes (cinnamate-4-hydroxylase, p-coumarate 3-hydroxylase, catechol O-methyltransferase and ferulate 5-hydroxylase) cinnamic acid is transformed into p-couma-ric-, caffeic-, ferulic-, 5-hydroxyferulic- and sinapic acids,which act as precursors for flavor components and are important intermediates in the biosynthesis of fla-vonoides, lignins, etc. Reduction of cinnamic acids to aldehydes and alcohols by cinnamoyl-CoA NADPH-oxido-reductase and cinnamoyl-alcohol-dehydrogenase form important flavor compounds such as cinnamic aldehyde, cin-namyl alcohol and esters. Further reduction of cinnamyl alcohols lead to propenyl- and allylphenols such as... 

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