IV 



BIOSYNTHESIS OF AMINO ACIDS 

 CH2 — NHg 



,(CH2)2 



CH NH2 



79 



CH2 CH2 TPNHp CH2 CH; 



CH CH— COOH 



Pyrroline 

 carboxylate 



CH2 CH— COOH 



N 



H 



Proline 



COOH 



Glutamic 

 semialdehyde 



Fig. 36. Conversion of glutamic acid to proline. 



take place to a significant degree. Hydroxyproline-^^N is a poor precursor of the 

 hydroxyproline of rat proteins. More isotope is found in protein hydroxyproline 

 after feeding ^^N-proline than '^N-hydroxyproline, likewise, the nitrogen of 

 ^^N-hydroxyproline is converted to a greater extent to protein glutamate than to 

 protein hydroxyproline. It is possible that proline is oxidized to hydroxyproline 

 while present in peptide linkage. Hydroxyproline occurs in appreciable amounts 

 in gelatin, collagen, and elastin but is present at very low concentrations in other 

 proteins. 



Ornithine synthesis. Isotope competition experiments suggest the following pathway 

 of ornithine synthesis from glutamic acid in E. coli cells : 



Glutamic— >N-acetyl glutamate— »N-acetyl glutamic semialdehyde 



H2O 



N-acetyl ornithine > ornithine 



glutamic 

 a-ketoglutaric 



An enzyme exists in E. coli which catalyzes the acetylation of glutamate. The latter 

 substance may be converted to acetyl ornithine by reduction and transamination. Acetyl 

 ornithinase has been partially purified from E. coli extracts (Vogel and Bonner, 1956) and 

 acetyl ornithine and free ornithine occur in certain plants (Virtanen and Linko, 1955). 

 It is significant that N-acetyl glutamic semialdehyde rather than glutamic semialdehyde 

 is an intermediate in ornithine synthesis, in that the acetylated compound, unlike free 

 glutamic semialdehyde, cannot cyclize to pyrroline carboxylate. Proline-' -'N is converted 

 to protein arginine in the mammal (Stetten, 195 1). The postulated intermediate, glutamic 

 semialdehyde, is however a relatively poor ornithine precursor, suggesting that the acetyl 

 glutamate pathway also exists in animal tissues. On the other hand, in Neurospora which 

 lacks an acetyl ornithinase, glutamic semialdehyde is a better ornithine precursor. Here, 

 glutamic semialdehyde inhibits the conversion of glucose- '■♦C to Neurospora arginine 

 (Vogel and Bonner, 1956). 



Literature p. 124 



