216 



UNITY AND DIVERSITY IN BIOCHEMISTRY 



mole of formamide and one mole of ammonia are produced. The separate 

 steps of this degradation have been worked out from a study of a number of 

 systems and are shown in Fig. 49 (still hypothetical steps are between 

 brackets). 



H 



N NH 



I NH, 



HC=C — CH,— CH— GOGH 



1 histidase, histidine deaminase 

 H 



N NH 



II + NH, + H,0 

 HC=C — CH = CH — COOH^- --?"- 

 urocanic acid 



I 



H| 



^\ 



N NH 



I 



I 



o=c 



CH— CH,— CHj— COOH J 



H 



^\ 



N NH 

 .HO — CH— C= CHI— CH, — COOH. 



Hj 



/% 



N NH 



HO— C == C— CH,— CH,— COOH . 



imidazole propionic acid 



I 



I 

 NH 



II 

 CH 



NH 



CHO 



NH 



+ NH, 



I + H,0 I 

 HOOC— CH— CH,— CH,— COOH > HOOC— CH— CH,— CH,— COOH 



formamidinoglutaric acid formylglutamic acid 



I + H,0 I + H.O 



i I 



O NH, 



HC + HOOC— CH— CH,— CH,- COOH 



NH, 



formamide 



NH, 



HOOC— CH—CH,—CH,— COOH + HCOOH 

 glutamic acid formic acid 



glutamic acid 



Fig. 49 (after Tabor) — Degradation of histidine. 



The glutamic acid can enter the tricarboxylic acid cycle through a-keto- 

 glutaric acid, so histidine is thus degraded to COg and water. 



(d) Leucine, Isoleucine, Valine 



The degradation of leucine, isoleucine and valine operates by oxidative 

 deamination and then decarboxylation of the corresponding keto-acids. 



Leucine is first transformed into a-ketoisocaproic acid. This latter 

 combines with CoA and is oxidized to senecioyl-CoA, this is followed by a 

 hydration with formation of j8-hydroxyisovaleryl-CoA. Then, in the course 

 of a reaction requiring ATP, COg is attached to the end of the chain and the 

 ^-hydroxy-^-methylglutaryl-CoA formed is split into acetoacetic acid 



