Biosynthesis of Branched-Chain Compounds 



29 



unsaturated acid, which in turn might fix carbon dioxide. This mecha- 

 nism is preferred because on purely chemical grounds fixation of C0 2 

 by a hydroxy acid is improbable, whereas both chemical and bio- 



CH a 



CHa 



±H 2 



CH 3 



CH 3 



CH— CH 2 C*OOH 



0) 



C=CH— COOH ~ 



C— CH 2 O0()H 



CH, 



(2) / | 



ch 3 on 



IV 



HIV 



\ 



t ±H 2 

 j(3) 



CH 3 



(6) 



H 2 C 



S 



C— CH 2 C-OOH 



MVA 



T ±co 2 



1(4) 



CH 3 



CH a 



-co* 



C— CH 3 <- 



CH DMA 



COOH 



CH 3 



CO +CH ;! OOOH 



/ 

 CH 2 



I 

 COOH 



Fig. 3. 



(5) 



C— CH 2 C«OOH 



(8) 



CH MCA 



COOH 



T ±H 2 



1(7) 



H 3 C OH 



\l 



C— CHoC'OOH 



/ 

 CH 2 HMG 



I 

 COOH 



IV: isovaleric acid. 

 DMA: /3-dimethylacrylic acid. 

 HIV: /3-hvdroxyisovaleric acid. 



MVA: /3-methylvinylacetic acid. 

 MGA: /3-methylglutaconic acid. 

 HMG: /3-hvdroxy-/3-metliylglutaric acid. 



chemical analogies exist for the carboxylation of ethylenic compounds. 

 Moreover, it appears that crotonase catalyzes reversible reactions 

 which would allow an equilibrium to be established between the x, /?, 

 and /3,y unsaturated acids, and the /3-hydroxy acid. If the fi,y acid 

 (methylvinylacetic acid) were indeed the acceptor for carbon dioxide, 

 one of two geometric isomers of methylglutaconic acid would be the 

 product. Their structure makes these acids attractive on several 



