'^tf' Aromatic nuclei 



The shikimic acid pathway for the biosynthesis of aro- 

 matic compounds, including amino acids, from carbohydrates 

 has been well established by the work of Davis (38) and his 

 collaborators, who used biochemical mutants of E. coli. 

 Without going into the details of this pathway, we may point 

 out that the starting materials are phosphoenolpyruvate, 

 which is readily available as a photosynthetic intermediate, 

 and D-erythrose-4-phosphate, which is also an intermediate of 

 the carbon reduction cycle. Presumably, therefore, the syn- 

 thesis of aromatic amino acids in photosynthesizing plants 

 would follow a pathway similar to the shikimic acid path- 

 way. The first step in that pathway is the condensation of 

 phosphoenolpyruvate with erythrose-4-phosphate to give a 

 seven-carbon compound which has been identified as 2-keto- 

 3-deoxy-D-araboheptonic acid-7-phosphate. This intermediate 

 subsequently undergoes ring closure to give dehydroquinic 

 acid. Rearrangements via a number of additional steps gives, 

 eventually, phenylalanine and tyrosine. Higuchi (121) has 

 summarized some of the reasons for believing that the shi- 



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