METABOLIC FUNCTIONS OF B VITAMINS 231 



simple amide bonds are created have been partially characterized and 

 shown to require adenosine triphosphate or else a coupling with an aerobic 

 system in which this phosphorylating agent is presumably generated: 



glutamic acid + NH 3 >■ glutamine+H 2 31 



benzoic acid+glycine > hippuric acid+H 2 32 



p-aminobenzoic acid+glycine > /j-aminohippuric acid+H 2 33 



The individual steps of these processes and the phosphorylated intermedi- 

 ates which may be formed have not yet been established. The occurrence 

 of acyl phosphates derived from the amino acids has never been demon- 

 strated. Hence, the formation of the peptide bond cannot be explained 

 on the basis of the utilization of acid anhydrides as was the case in the 

 formation of the ester and acetal linkages of fats and carbohydrates. No 

 B vitamin has as yet been directly implicated in the synthetic processes. 



Synthesis of Amino Acids. Many of the enzymatic reactions which 

 are utilized by organisms for the synthesis of their "nonessential" amino 

 acid requirements have yet to be clearly defined. The disclosure of certain 

 types of enzymatic reactions of general occurrence has indicated certain 

 steps which probably take place during most of these syntheses; but the 

 gaps which still exist in any outline of the total processes indicate how 

 much remains to be learned before a scheme for amino acid biosynthesis 

 can be drawn which will in any measure deserve the designation "com- 

 plete." An effective synthesis of amino acids usually takes place in one 

 of two ways: (1) by the direct animation of the corresponding keto acid, 

 or (2) by a reaction in which one a-amino acid is transformed into 

 another amino acid by a chemical alteration of the molecule which leaves 

 the original amino and carboxyl groups intact. 



The synthesis by amination of a keto acid either (1) utilizes an amino 

 (or amide) group of some other organic compound, a transamination, 

 or (2) introduces a molecule of inorganic ammonia into the organic struc- 

 ture by a reductive amination. In the former instance pyridoxal has been 

 shown to be required in all cases adequately characterized; in the latter 

 instance, nicotinic acid. Riboflavin enzymes catalyze most of the recog- 

 nized oxidative deaminations of amino acids (p. 147) and it may be 

 assumed that these function in amino acid synthesis by the reversal of 

 such reactions. The equilibrium established by the flavoprotein enzyme, 

 however, is so much in favor of deamination that this system has not yet 

 been shown to be a method by which amino acid synthesis can effectively 

 occur. However, the glutamic acid dehydrogenases, which are activated 

 by nicotinic acid coenzymes, catalyze a reaction in which concentrations 

 can exist which will favor the reverse reaction, amino acid formation. 34 

 It is significant that, of all the amino acids, only glutamic acid has been 



