BIOSYNTHESIS 



245 



The use of mutants of E. coli has enabled Vogel to identify the three 

 acetylated compounds shown in the scheme. 



Study of the pathway from ornithine to citrulUne, using mutants of 

 Neurospora, has shown that there are two stages. The first requires ATP; 

 CO2 and NH3 are fixed to form an as yet unidentified compound, which is 

 then transformed into citruUine. 



GOGH 



I 



CH, 

 H.— 



I 



COOH 



CH2 



I 

 CHj — 



CHO 



CH, 



I 

 CH, - 



CHNHs 



COOH 



glutamic acid 



CH,NH, 



I 



CH, 



I 

 CH, 



CHNH, 



COOH 



"cat 



I 



L 



ornithine 



CHNHCOCHj 



COOH 



N-acet}'lglutamic acid 



NH, 

 \ 



CHNHCOCH, 



COOH 



y-semialdehyde of 

 N-acetylglutamic 

 acid 



CH.NH, 



I 

 CH, 



I 

 CH, V 



CHNHCOCH, 



COOH 

 N-2-acetyl ornithine 



NH 



COOH 



NH, 



CNHCH 



compound 

 X 



t 



I ATP 

 CO, 



NHa 



COOH 



NH 



I 

 CH, 



c=o 



/ 



NH 



CH, 



CH, 



I 

 CH, 



CHNH. 



I 



COOH 



citrulline aspartic arginosuccinic 

 acid acid 



CH, 



I 

 COOH 



NH 



I 

 CH, 



C=NH 



COOH 



I 

 + CH 



CHNH, CH, 



I 

 CH, 



COOH 



1 

 CH, 



CHNH, 



I 

 COOH 



CH, 



I 

 CH, 



CHNH, 



I 



COOH 



arginine 



CH 



I 

 COOH 



fumaric 

 acid 



Fig. 66 (Davis) — Biosynthesis of arginine from glutamic acid. 



The path from citrulHne to arginine is made up of several stages. 

 Citrulline is condensed with aspartate to form argino -succinate. A specific 

 enzyme cleaves the argino-succinate to arginine and fumaric acid. 



(0) Biosynthesis 0/ C4 Amino Acids 



[Aspartic Acid, Methionine and Threonine) 



These molecules are derived from aspartic acid, itself formed from 

 members of the Krebs cycle. In plants and micro-organisms fumaric acid 

 is combined with ammonia in the presence of aspartase, whilst in mammals 

 which do not possess aspartase, aspartic acid is formed by reductive de- 

 amination of oxaloacetate in a reaction of unknown mechanism. 



Chemical genetic experiments on Neurospora have shown that threonine 

 and methionine have a common precursor. In fact, a mutant requiring 

 both amino acids at once can satisfy this double requirement when 

 L-homoserine is supplied. On the other hand a mutant whose only block is 

 in the synthesis of methionine accumulates threonine and L-homoserine in 

 ts mycelium. The idea of a single precursor has been confirmed with 



