PRIMING REACTIONS 



219 



and acetyl-CoA. Isoleucine undergoes a similar fate, the fixation of COg 

 being replaced by a dehydrogenation. Finally, acetyl-CoA and propionyl- 

 CoA (Fig. 50) are obtained. 



The path of valine joins that of leucine at the level of a-ketoisocaproic 

 acid. 



NH, 

 1 

 CHjCH-COOH 



hydroxyl«»e ipK- 



f 

 CHjCH-COOH 



O 



phenylalanine 



Fe++ 

 O 



CHjC-COOH 



OH 



O 



II 



V 



OH 



tj'rosine 



stcorbste 



V 



+ 



— COa 

 + 



OH OH 



p-hydroxyl-phenylpyruvic acid 2,5-dihydroxy phenylpyruvic acid 



(1^^=^ CHjC-COOH 



u 



OH 



COOH 





CH5COOH H 



+ o» 



/ 



Fe-H- 

 ~SH H 



H, 



Ha 



H H H , „^ 

 HOOC ^C ,C ,C +"»" 

 GSH V^ \^<^ \^^ \ 



OH 



homogentisic acid 



^ /C ^<- 



^C ^C XOOH 



II 



o 



II 

 o 



maleylacetoacetic acid 



H 



C 

 OH 



C 

 OH 



COOH 



fumarylflcetoacetic acid 



HOOC 



I 

 HC 

 II 

 CH 



I 



COOH 

 fumaric acid 



O 



II 

 + CHj-C-CHjCOOH 



acetoacetic acid 



Fig. 52 (after Knox) — Degradation of phenylalanine and tyrosine. 



(e) Lysine 



Lysine does not participate in the general deamination reactions of the 

 amino acids. It is degraded by way of pipecolic acid both in animal tissues 

 and in plants. The steps leading from lysine to a-ketoglutaric acid shown 

 in Fig. 51, have been demonstrated both in the rat and in the mould 

 Neurospora. 



As the scheme shows, the various steps of lysine catabolism are not all 

 reversible and though they have all been demonstrated to occur in the rat 

 this is not the case for Neurospora. 



(/) Phenylalanine and Tyrosine 



Phenylalanine and tyrosine are degraded as far as fumaric acid and 

 acetoacetic acid as shown in Fig. 52. 



