315 



Amino Acids and Related Compounds 



O OH 



CH3— C— COOH > CH3— C— C— COOH 



(3) ® ® CH3CHO® ® I®® 



Pyruvic Acid ® ® CH3 



"active acef- ® 



aldehyde" a-Acetolactic Acid 



thiamine 

 O' pyrophosphate 



II 

 CH3— C— COOH 



rearrangement 



O OH OH 



II H2O I I TPNH 

 CH3— CH— C— COOH <- CH3— C— CH— COOH < 



CH3 

 a-Ketoisovaleric 

 Acid 

 transamination 



reduction 



CH3 

 a,/3-Dihydroxy- 

 isovaleric Acid 



OHO 



CH3— C— C— COOH 



® ® I @ ® 



CH3 

 ® 

 a-Keto-/3-hydroxy- 

 isovaleric Acid 



CH3 



CH— CH— COOH 



CH3 



NH2 

 Valine 



The intermediates, a-acetolactic acid and a,^-dihydroxyiso- 

 valeric acid, have been isolated from a variety of microorgan- 

 isms and are well characterized. a-Keto-^-hydroxyisovaleric 

 acid has not been reported yet, although when it is mixed with 

 enzyme preparations from molds and yeasts together with 

 TPNH, it is reduced to a,^-dihydroxyisovaleric acid. a-Ketoiso- 

 valeric acid is aminated by numerous microorganisms. 



The scheme for isoleucine is believed to be analogous, but 

 with a-ketobutyric acid replacing pyruvic acid as the initial sub- 

 stance. This four-carbon acid is, in turn, derived from homo- 

 serine or threonine, and ultimately from aspartic acid. Some 

 of the steps of the valine and isoleucine syntheses are known to 

 share common enzymes. 



Leucine biosynthesis is apparently the same as that of valine 

 up to the final amination step. Leucine, however, requires 3 

 moles of pyruvate for its 6-carbon atom chain rather than the 



