IV 



BIOSYNTHESIS OF AMINO ACIDS Sj 



indicated that the carboxyl carbon and carbons one and two of shikimic acid were derived 

 from a three carbon intermediate of glycolysis while the remaining carbon atoms probably 

 originated from a tetrose which in turn was formed by the action of transketolase and 

 transaldolase on fructose-6-phosphate (Srinivasan et al., 1956). In the presence of non- 

 labelled glucose, labelled bicarbonate, acetate, formate, or pyruvate were poorly in- 

 corporated into shikimic acid. 



Cell free extracts of an E. coli mutant were next obtained which converted fructose-6- 

 phosphate or fructose diphosphate to dehydroshikimic or shikimic acid in 5% yield but 

 converted sedoheptulose diphosphate to shikimic acid almost quantitatively (Srinivasan 

 el al., 1955). Dehydroshikimic was also formed from erythrose-4-phosphate and phospho- 

 enol pyruvate by this enzyme system. Phosphoenolpyruvate and erythrose phosphate were 

 shown to be the more immediate precursors, since fluoride and iodoacetate, which block 

 the formation of phosphoenolpyruvate, inhibited the formation of shikimic from sedoheptu- 

 lose diphosphate but not from phosphoenol pyruvate and erythrose phosphate, and since 

 DPN* was required when fructose diphosphate was used as substrate but not when 

 phosphoenolpyruvate and erythrose phosphate were the substrates. 



The experiments with the Neurospora mutants which accumulated protocatechuic acid 

 were also consistent with the above mechanism (Gross et al., 1956; Tatum and Gross, 1956). 



The next step in the synthesis of phenylalanine and tyrosine from compound Zj 

 involves the formation of prephenic acid. This substance accumulates in the 

 medium of a "phenylalanineless"' E. coli mutant. It may be converted non enzyma- 

 tically in acid solution to phenylpyruvic acid and enzymatically by the wild type 

 oi E. coli to phenylalanine (Weiss et al., 1954). It is possible that the terminal steps 

 in the conversion of prephenic acid to phenylalanine and to tyrosine are different. 

 Thus, enzymes from an E. coli mutant (83-5) which is blocked in the conversion of 

 prephenic to phenylpyruvic catalyzed the formation of j&-hydroxyphenyl lactic 

 acid from prephenic acid. However mutants blocked in the synthesis of both 

 phenylalanine and tyrosine did not form /?-hydroxyphenyllactic acid from pre- 

 phenic acid (Ghosh, 1956). 



The pathway for phenylalanine and tyrosine synthesis (Fig. 27) is also consistent 

 with isotope experiments carried out with yeast cells (Gilvarg and Bloch, 1950; 

 Thomas et al., 1953). Glucose- 1-^^*0 and pyruvate-2-^'^C, were shown to be the 

 precursors, respectively, of the [3- and a-carbons of the tyrosine and phenylalanine 

 side chains (Gilvarg and Bloch, 1952). 



Animal tissues, although incapable of synthesizing phenylalanine, are able to 

 form tyrosine from dietary phenylalanine. Tyrosine can spare the dietary require- 

 ments of phenylalanine. Direct evidence for the conversion of phenylalanine to 

 tyrosine was obtained by Udenfriend and Mitoma (1955). These investigators fed 

 dogs phenylalanine-3-^'*C and isolated labelled tyrosine from the serum albumin. 

 The synthesis of tyrosine by a soluble rat liver enzyme has also been demonstrated 

 (Mitoma, 1956). Two protein fractions are required. The first protein fraction 

 occurs only in liver whereas the second fraction could also be obtained from kidney 

 or heart. Reduced TPN"^, oxygen, and ferrous iron were required for maximum 

 tyrosine synthesis. 



Terminal steps in tryptophane synthesis. It is probable that anthranilic acid and 

 indole are terminal intermediates in the synthesis of tryptophane. "Trypto- 

 phaneless" E. coli and Neurospora mutants have been obtained which accumulate 



Literature p. 124 



