456 



ANIMAL BIOCHEMISTRY 



CHo — CHr 



COOH 



propionvl CoA | 



-COSCoA + ATP + COc =r HoC — C— H + ADP + HPO; 



carboxylase, Mg 



+ + 



propionyl CoA 



COSCoA 



methylmalonyl CoA 



COOH 



I 

 CH2 



CH2 

 COSCoA 



succinyl CoA 



Besides being an intermediate in the Krebs cycle, succinyl CoA is 

 used in a variety of metabolic reactions such as in the generation of 

 an energy-rich pyrophosphate bond. Like acetyl CoA, succinyl CoA 

 can act as an acylating agent. Thus, it is possible to succinylate amino 

 groups in a fashion quite analogous to their acetylation by acetyl 

 CoA. Complete information with regard to the importance of suc- 

 cinylation in vivo is lacking at the present time. 



Ketone Bodies and Ketosis 



The degradation of fatty acids in the animal body results in the 

 production of some intermediates commonly called ketone bodies. 

 These intermediate compoiuids, including acetoacetic acid, /3-hydroxy- 

 butyric acid, and acetone, are all formed from acetoacetyl CoA 

 through a variety of chemical reactions. For example, acetoacetic acid 

 can be formed by a transacylation of acetoacetyl CoA with succinic 

 acid. 



Acetoacetyl CoA + succinic acid ~ 



thiophorase 



succinyl CoA + acetoacetic acid 



L-/?-Hydroxybutyric acid is formed from acetoacetic acid in the 

 presence of DPNH and y8-hydroxybutyric dehydrogenase. 



CH..CCH2COOH + DPNH + H3O"— 



II 

 O 



dehydrogenase 



CH3CHCH2COOH + DPN + H2O 

 OH 



L -^-hydroxybutyric acid 



Acetone formed in vivo from acetoacetic acid by decarboxylation 

 may undergo further cleavage to yield a two-carbon acetyl and one- 

 carbon formyl fragments. On the other hand acetone can be trans- 



