Biological Assimilation and Dissimilation of Nitrogen 



531 



1952, 1953). However, Snell and his coworkers have stressed the role of chelation 

 of the azomethines with some metal or other, considering this as the factor of 

 major importance in intensifying the decrease of electron density of the a-carbon 

 atom and in determining the prevalence of definite transformations of the azo- 

 methines. This is certainly correct with respect to the non-enzymic model 

 reactions. But it is evident that, in most instances of reactions catalysed by 

 pyridoxal enzymes, the same functions are exercized by the specific enzyme 

 proteins, or apoenzymes [7, 8, 9], instead of chelating metals. 



"Protein 



OH 



Fig. 2. Activation (weakening) of chemical bonds in amino acid molecules by 



pyridoxal enzymes 



Ruptured 

 bonds: 



I. a-C—H 



2. a-C— N 



3. a-C— COOH 



4. a-C — H and 



^-C— Y 



5. ß 



7- 



-C— H and 

 y-C— X 



a-C—ß-C and 

 ß-C—K 



ß-C—y-C 



Reaction types: 



Dissociation of a-H 

 Condensations of glycine 



Transamination j deamination 



a-Decarboxylation 

 Removal (or replacement) 

 of a-H and ^-substituent 



Removal (or replacement) 



of ß-H and y-substituent 



Aldol decondensation of 



a-hydroxy amino acids 



Cleavage of y-keto-a-amino acids 



jS-Decarboxylation 



Enzymes: 



Aminopherases, racemases etc. 



Serine sjTithease; S-aminole- 

 vulinic acid synthease 



Aminopherases ; Diamine oxi- 

 dase 



Amino acid a-decarboxylases 



Tryptophanase; serine- and 

 threonine-dehydrases ; ß- 

 thionases; alliinase; trypto- 

 phan synthease; cystathio- 

 nine synthease 



Homoserine deaminase; y-thio- 

 nases; threonine-sjnnthease 



Serine- and threonine-aldolases 



Kynureninase 



Aspartic- ^-decarboxylase 



With a view to the problem of abiogenic synthesis it is particularly important 

 that, as shown by Snell's group, many transformations of amino acids in model 

 systems, including condensations, can also be effected if pyridoxal is replaced by 

 simpler aromatic aldehydes of similar electronic structure and even by a carbonyl 

 compound as simple as glyoxylic acid — the oxidation product of glycine. It was 

 shown, for instance [12], that glyoxylic acid readily catalyses, especially in the 

 presence of aluminium or copper salts, the cleavage of hydroxy amino acids to 



