154 THE BIOCHEMISTRY OF B VITAMINS 



not. These two classes are also chemically distinct with respect to whether 

 they involve a- or /?-keto acids. 



a-Decarboxylation. Thiamine is essential only when the substrate is 

 an a-keto acid and only when the reaction results in the rupture of the 

 bond between the keto carbon atom and the adjacent carboxyl group. 



^-Decarboxylation. The other reactions in which carbon dioxide is 

 formed from a keto acid are those which have been termed ^-decarboxyl- 

 ations because they result in the direct decarboxylation of acids in which 

 the keto group is /? to the reacting carboxyl group. Biotin has been asso- 

 ciated with this type of reaction, although it is questionable if its role 

 in these reactions is a direct one. 



The Coenzyme Derived from Thiamine 



Carboxylase, an enzyme which converts pyruvic acid to carbon dioxide 

 and acetaldehyde, was one of the first components to be recognized in 

 zymase, the complex of enzymes used by yeast in fermenting sugars. 108 

 Some twenty years after its discovery it was shown (1932) that an 

 essential thermostable organic component could be removed from the 

 holoenzyme by washing with weakly alkaline solution. 109 This coenzyme 

 was designated as cocarboxylase. The isolation of the coenzyme in 

 crystalline form was achieved by using an enzymatic assay method (re- 

 activation of a carboxylase apoenzyme) to follow the concentration of the 

 active principle. The chemical structure was established by both degrada- 

 tion and synthesis and found to correspond to the pyrophosphoric acid 

 ester of thiamine. 110 



CH, 



O O 



=C— CH 2 — CH 2 0— P— 0— P— OH 



OH O- 



Thiamine pyrophosphate 



The compound has been most often referred to as cocarboxylase, but 

 other synonyms and abbreviations are frequently encountered — thiamine 

 pyrophosphate (TPP), diphosphothiamine (DPT), and aneurin pyro- 

 phosphate (APP) . This coenzyme should not be confused with code- 

 carboxylase, the name often used to denote pyridoxal phosphate. The 

 synonym, thiamine pyrophosphate, has been chosen as the one most 

 suitable for use in the discussions which follow, since it indicates the 

 vitamin component, describes the chemical nature of the coenzyme, and 

 eliminates confusion with other types of coenzymes catalyzing decar- 



