466 



ANIMAL BIOCHEMISTRY 



Figure 20-2 suniniari/es the metabolic patterns of the three aromatic 

 amino acids, plienylalanine, tyrosine, and tryptophan, commonly 

 occurring in proteins. Since many steps are involved in the metab- 

 olism of these amino acids and since synthesis and degradation go by 

 different routes, a number of known reactions have been omitted for 

 simplicity. Both phenylalanine and tryptophan are nutritionally 

 essential in certain mammals, perhaps in all. Therefore some of the 

 synthetic steps leading to these two compoiuids may not occur in 

 animals. However, it is not entirely clear whether mammals are 

 devoid of all ability to make the so-called essential amino acids. 

 Rather it is possible that a small synthetic supply exists but does 

 not meet the demand. This point is considered again on page 478. 

 The scheme outlined does occur in bacteria and might therefore be 

 used indirectly by ruminants. The pathway shown probably accounts 

 for the phenylalanine and tryptophan of plants. 



The amino acids concerned in Figure 20-2 are deaminated and 

 metabolized by routes connecting with the tricarboxylic cycle and 

 leading ultimately to carbon dioxide and water. Certain important 

 side products also appear. Of these, p-aminobenzoic acid has been 



Glucose -e ^--QsPOCHaCHOHCOO 



3 - phosphoglycerate 



— transam- - 



HOCH2COCOO < i„ation ^ HOCH2CHCOO 



nh; 



hydroxypyruvate 



I 

 I 



H3NCH2COO + OOCCH2CH2COCOO -^^►OHCCOO'+'OOCCHgCHgCHCOO 



glycine 



a-ketoglutarate 



HoN 



glyoxalate 



(CH2)2COO' 

 CHCOO 

 CONH 



tetrahydrofolic acid 



HOCH2CHCOO- 



NH 



NH3+ 



glutamate 



(CH2)2COO 



CONHCH 



I 

 COO" 



N ^"^^ -methylenetetrahydrofolic acid 



H3NCH2COO" 

 glycine 



