PRIMING REACTIONS 221 



is) Tryptophan 



The ketonic acid corresponding to tryptophan is a-keto-^-indolyl- 

 pyruvic acid, which can be obtained by transamination. The metabohsm 

 of this substance is unknown but it is not the main metaboUc pathway for 

 tryptophan which follows a series of different paths, as shown in Fig. 53. 

 The most notable of these is the one leading to nicotinic acid. 



The main pathway of tryptophan breakdown is still unknown. 



(h) Glycine and Serine 



Glycine can follow any one of a number of paths and during metabolism 

 it may be transformed into a variety of substances : formate, acetate, 

 ethanolamine, serine, aspartic acid, fatty acids, purines, pyrimidines, 

 ribose or protoporphyrin. Its complete degradation, like that of serine or 

 ethanolamine into which it is readily transformed, may be brought about by 

 conversion to pyruvic acid from whence it can enter the glycolysis chain 

 or the tricarboxylic acid cycle. 



CHoOH CH2OH CH2 CH, CH3 



I " I II I I 



CH0NH2 CHNH, - H.O C-NH2 C=NH + H2O CO + NH, 



^1 - I -I - I 



H-COj COOH COOH COOH COOH 



serine pyruvic acid 



But this is not the only entry of glycine into the priming reactions. 

 Glycine can be converted into COg and water by means of the Shemin 

 cycle, where the catalyst is not oxaloacetate as in the Krebs cycle, but 

 succinate, whose active form, succinyl-CoA, condenses with the glycine. 

 The Shemin cycle can therefore be worked into that of Krebs to form a 

 shunt (Fig. 54). 



The succinyl-CoA condenses with glycine, at the a-carbon atom, to 

 form a-amino-^-ketoadipic acid which is then decarboxylated to 8-amino- 

 levulinic acid. The latter is deaminated to ketoglutaraldehyde which is 

 oxidized to ketoglutaric acid. This compound enters the tricarboxylic acid 

 cycle or is decarboxylated to succinic acid. In one revolution of the cycle 

 one molecule of glycine is completely oxidized to CO2 and water. 



(/) Proline 



Proline is first oxidized to glutamic acid whose metabolic path it then 

 follows. Hydroxyproline gives rise to ;8-hydroxyglutamate-semialdehyde. 



