COENZYMES DERIVED FROM B VITAMINS 189 



an acetic acid derivative. The reaction has been pictured as occurring in 

 this fashion: 



O O 



H || O H || O 



HC— C— O— P— OH + HC— C— O— P— OH < — > 



A H A 



H H 



O O 



H || H || O 



HC— C— C— C— O— P— OH + H3PO4 

 H H ^ 



H 



Recent investigations have definitely disclosed that the phosphoric acid 

 derivative of acetic acid is not the simple acetyl phosphate pictured above 

 (p. 190) , but it probably reacts in a comparable manner. The biologically 

 active phosphoryl derivative of acetic acid is associated with a panto- 

 thenic acid coenzyme, and the numerous reactions in which it can be 

 utilized constitute essential steps in many fundamental processes. 



A third type of condensation utilized by living organisms is one which 

 involves a reactive single carbon unit related to formic acid but not to 

 carbon dioxide (or the carbonate ion). The only synthetic reaction of 

 this type now recognized occurs in the conversion of glycine to serine 

 (p. 201) and is dependent upon a p-aminobenzoic acid-containing enzyme 

 and possibly vitamin Bi 2 . The exact mechanism of this type of condensa- 

 tion is not known, nor is it yet possible to state how important reactions 

 of this type will ultimately prove to be in the synthetic activities of cells. 



A fourth type of condensation which has been observed is one catalyzed 

 by a thiamine system — the formation of acetoin from the intermediates 

 formed during the metabolism of pyruvic acid. This reaction has never 

 been shown to be of value to any cell from the standpoint of synthesis 

 of cellular components, and it is believed to be a mechanism the only 

 purpose of which is to dispose of end products of carbohydrate metab- 

 olism. 



Two reactions, catalyzed by pyridoxal phosphate, constitute a fifth 

 type of condensation: (1) the formation of tryptophan from indole and 

 serine, and (2) the cleavage of tryptophan into indole, pyruvic acid and 

 ammonia (p. 183). It should be noted that the a-amino group in the 

 presence of pyridoxal-containing enzymes is a "potential keto" group 

 (p. 186) ; hence these reactions of a-amino acids are comparable to those 

 involving the reactive methylene carbon and hydrogen atoms alpha to 

 carbonyl groups. 



Because the equilibria of the decarboxylation reactions discussed in the 



