Evidence on the Pathways of Aromatic Amino Acid Biosynthesis

Isotopic Evidence on the Pathways of Aromatic Amino Acid Biosynthesis 

Head-to-tail union of 3-carbon units resulting in para labeling 

Two possibilities were considered for the origin of the side chain, (a) direct addition of a two-carbon unit to a cyclized seven-carbon unit, or (b) addition of a three-carbon unit to the seven-carbon unit with expulsion of the one-carbon side chain  

Ehrensvard and Reio favored alternative (a). Alternative (6) strikingly resembles the mechanism of phenylalanine biosynthesis revealed by mutant techniques (see p. 39) that this mechanism can apply to tyrosine was rendered possible by work of Thomas and co-workers and was rendered extremely probable by work of Sprinson and co-workers. 

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