BIOSYNTHESIS 



247 



methionine 



CHjOH 



I 

 CH, 



CHNHi 



COOH 



homoserine 



CH3 



I 

 HOCH 



^ CHNHa 



alanine 



a,/S-dihydroxy- 

 isovaleric acid 



COOH 



L-threonine 



pyruvic 

 acid 



CH3 CH3 



\ / 

 COH 



I 

 CHOH 



I 

 COOH 



CH3 CH3 



\ / 

 CH 



-CO, 



CO 



CH3 CH3 



\ / 

 CH 



CHa^- 



CO 



I COOH 



COOH 



a-ketoisocaproic a-ketoisovaleric 



+ C2 



acid 



CH3 CH3 



\ / 

 CH 



I 

 CH2 



i 

 CHNH2 



acid 



\ 



CH3 



I 

 CHa 



I 

 CO 



transaminase 



CH3 CH3 



\ / 

 CH 



CHNH2 



COOH 

 a-ketobutyric 

 acid 



CH, 



I 

 CH, 



I 

 :^ CHNHa 



I 

 COOH 



a-aminobutyric 



acid 



+ 



CH3 



■ I 

 CO pyruvic 



I acid 



COOH 



CH, CHo-COOH 



\ 



COH 



1 a,^-dihydroxy-^-methyl- 



CHOH glutaric acid 



I 

 COOH 



CH3 CH2 — CH3 



\ / 

 CH 



I 

 CO 



COOH 

 a-keto-j3-methylvaleric acid 



CH3 CHj — CH3 



\ / 

 CH 



CHNH2 



COOH 



leucine 



COOH COOH 



valine isoleucine 



Fig. 68 — Biosynthesis of leucine, isoleucine and valine. 



