BIOSYNTHESIS OF AMINO ACIDS 



269 



L-Ornithine 



L-Citrulline 



L-Aspartic acid 



L-Argininosuccinic 

 acid 



Urea 



L-Arginine 



If 



Fumaric acid 



Protein 

 Figure 8. The ornithine cycle (429). 



the ornithine cycle in net formation of urea has not yet appeared. 

 Ornithine appears in the medium during breakdown of arginine by 

 Streptomyces venezuelae (212). 



Canavanine, a hydroxyguanidine derivative structurally related to 

 arginine, inhibits the growth of some normal strains of Neurospora 

 crassa (251). The inhibition has been traced (576) to the formation of 

 a canavinino-succinate which blocks the splitting of arginino-succinate. 



Of the short-chain amino acids — glycine, alanine, serine, and cysteine 

 — only the first named has received much attention in fungi. A glycine- 

 requiring mutant of Neurospora crassa can grow normally if given 

 either glycolic or glyoxylic acids (613). This is in agreement with 

 animal studies (356, 582), and suggests the following sequence: 



HOOC— CH 2 OH 



Glycolic acid 



HOOC— CHO 



Glyoxylic acid 



HOOC— CH 2 NH 2 (13) 



Glycine 



Drawing on experience with other organisms, we may suggest that the 

 glycolate arises from pentose phosphate (583), and that the amino 

 group is introduced by a transamination from glutamate or other 

 amino donor. 



Glycine and serine are closely related in Penicillium notatum (26) 

 and Eremothecium ashbyii (206, 207). Both participate in a variety of 

 biochemical reactions and are clearly related in their biosynthesis; it 

 cannot yet be said which is formed first. 



