246 



UNITY AND DIVERSITY IN BIOCHEMISTRY 



E. colt and with yeast in isotopic experiments, and in enzymic experiments. 

 The synthesis of methionine is the reverse of the synthesis of cysteine 

 described previously (p. 243), via homocysteine and with cystathionine as 



coon 

 I 



CH. 



I 

 CHNH. 



I 



coon 



aspartic acid 



CONH2 

 I 

 CHa 



I 



CHNH2 

 I 

 COOH 



a>paragine 



COOPO3H2 CHO 



I I 



CH2 TPN CH, 



I I 



CHNHj CHNH2 



COOH COOH 



j5-aspartylphosphate aspartic ^-semialdehyde 



HO 

 I 

 CH3CH ■ 



NH, 



CH-COOrt 



CH20H 



I 



TPN or DPN CHj 



CHNHj 

 I 

 COOH 



homoserioe 



ChreoDinc 



(?) 



NHj 

 I 

 CH3-CH2-CH-COOH 



a — arainobutyric acid 

 CHaSH 



CHNHj 



COOH 



cysteine 



CHjSCH, 



I 

 CHi 



I 



CHNH, 



I 

 COOH 



methionine 



C, 



CH2SH 



I 

 CHj 



I 

 CHNHj, 



I 

 COOH 



CH — S — CHi 



CH2 



I 

 CHNH2 



I 

 COOH 



CHNH2 

 I 

 COOH 



homocysteine cystathionine 



Fig. 67 (Davis) — Biosynthesis of C4 amino acids. 



an intermediate (Fig. 67). (In micro-organisms cystathionine can be split 

 on either side of the sulphur atom. By contrast, in mammals it can only be 

 cleaved between the sulphur atom and the seryl residue. Hence mammals 

 cannot synthesize homocysteine.) 



(p) Biosynthesis of Isoleucine, Valine and Leucine 

 The starting point of our knowledge of the mechanism of the biosyn- 

 thesis of these substances is the discovery of a Neurospora mutant requiring 

 both isoleucine and valine. This mutant accumulates the corresponding 

 dihydroxy-derivatives — dihydroxy-methylvaleric and dihydroxy-isovaleric 

 acids. Furthermore, a mutant of E. coli accumulates the corresponding 

 ketonic acids, a-ketomethylvaleric and a-ketoisovaleric acids. The mutants 

 accumulating the ketonic acids respond only to isoleucine and valine whilst 

 the accumulators of dihydroxy-acids respond not only to the amino acids 

 but also the the corresponding keto-acids. In an auxotrophe for isoleucine 

 alone, dihydroxy-methylvaleric acid and dihydroxy-isovaleric acid are 

 active, whilst in an auxotrophe for valine alone, dihydroxy-isovaleric acid 

 and a-ketoisovaleric acid are active. 



