534 



A. E. BRAUNSHTEIN 



Porphobilinogen ( PBg) 



COOH CH 



I I 



COOHCHj COOH CH2 



en 



CH2 CHj Ç^2 CH2 



=CH- 



NH 



(4)4 PBq -^'^^ Uroporphyrinogen HT —*- 



I . 

 Coproporphyrinogen III 



Protoporphyrin ^ 



Haenr» Chlorophyll 



CH, CH, 

 COOH CH, 



CH 



I 

 COOH 



II 



CHj COOH 

 COOH 



Uroporphyrin HE 



spontaneously. This is an instructive example of the pitfalls of superficial and 

 incompetent appreciation of the thermodynamic probability of the spontaneous 

 formation of orderly structures. 



Let us now consider the nature of the primary reactions leading to the assimi- 

 lation of ammonia in biological amino acid synthesis. 



The classical Embden-Knoop theory, assuming that the organisms synthesize 

 a-amino acids from their a-keto analogues and ammonia by reductive amination, 

 has imdergone substantial revision [3, 4, 5, 13, 15]. 



The role of a-keto acids as the direct natural precursors of a number of amino 

 acids (about ten, see Fig. i) is confirmed by the recent studies on biosynthesis 

 [5, 3]. However, there is a considerable body of evidence showing that in most 

 organisms a-oxoglutaric acid is the only a-keto acid that can act as the imme- 

 diate acceptor of the assimilated ammonia [14, 15]. The formation of glutamic 

 acid by reductive amination of a-oxoglutarate is a reversible reaction catalysed 

 by the specific glutamic dehydrogenase. This is the only widespread and highly 

 active enzyme hitherto known to effect reductive amination. 



Some plants and bacteria contain the enzyme, aspartase, which forms aspartic 

 acid by the reversible addition of ammonia across the double bond of fumaric 

 acid; in present-day organisms, with the possible exception of some types of 

 bacteria, this reaction seems to be of minor importance in nitrogen assimilation. 



During the last two decades, the pathways of assimilation of ammonia in 

 animals, plants and micro-organisms (and of oxidized forms of nitrogen in certain 

 types of organisms capable of utilizing them) have been thoroughly studied by 

 many authors (see [3, 15, 16]). Different approaches have been used for this 

 purpose, e.g. direct chemical analysis of the products of nitrogen assimilation in 

 yeast cells or pea seedlings (Virtanen and associates), investigation of the 

 metabolic fates of compounds labelled with ^^N (Schoenheimer, Vickery, 



