272 



NITROGEN NUTRITION AND METABOLISM 

 COOH 



CH 2 — CH— COOH 



NH 2 



p-Aminobenzoic acid 



i . 



NH, 



Tryptophan 



COOH 



-* "X" 



HO 



~OH 



OH 



Shikimic acid 



COOH 

 I 

 CH— NH 2 



I 

 CH, 



COOH 

 I 

 CO 



I 



HOOC CH 2 



^\^COOH 



^^NH 2 



Anthranilic acid 



COOH 



i 

 CO 



I 



CH 2 



Indole 



COOH 



I 



CH— NH 2 

 I 

 CH, 



OH 



Tyrosine 



Prephenic 

 acid 



Phenylpyruvic 

 acid 



'^ 



Phenylalanine 



Figure 10. The biosynthesis of tryptophan, phenylalanine, tyrosine, and p-amino- 

 benzoic acid. This is a general scheme based on data from several organisms but 

 consistent with knowledge of Neurospora crassa. Many of the reactions shown are 

 multiple. 



Neurospora crassa (241). However, an antigenically related protein, 

 possibly an enzyme precursor or possibly a complex of altered enzyme 

 with an inhibitor, has been isolated from tryptophan-requiring mu- 

 tants (514a). Cultivation of N. crassa in a medium deficient in zinc 

 reduces the concentration of the synthesizing enzyme (386, 512). 



This summary of amino acid synthesis may give a false impression 

 of finality. There are, in fact, many complexities and uncertainties 

 which cannot be considered here, and it is to be expected that our pic- 

 ture of these reactions will change as time goes on and more informa- 

 tion becomes available. 



