METABOLIC FUNCTIONS OF B VITAMINS 



223 



metabolism of pyruvic acid presumably always involves the thiamine- 

 requiring oxidative decarboxylation. The hydrogen atoms donated to 

 riboflavin 10 in this reaction, as well as those donated to the diphospho- 

 pyridine nucleotide previously during the glycolytic process, are "trans- 

 ported" by the elaborate dehydrogenase-cytochrome systems described in 

 Chapter II B and are finally oxidized in reactions which reduce molecular 

 oxygen. The active phosphorylated acetyl molecule which is formed by 

 the oxidative decarboxylation of pyruvate can be directly used for energy 

 by employing it in reactions in which it acts as a phosphorylating agent. 11 

 Acetic acid is then the end product and accumulates. A more common 

 aerobic mechanism, however, is one in which the activated acetate is 

 completely oxidized to carbon dioxide and water by a series of reactions 

 designated as the tricarboxylic acid cycle. 2, 4 ' 12 In this cyclic process, 



-2 [2H] DPN 



-2 [2H] ? 



-2 [2H]tpn 



hexose unit 

 | DPN 



-2 [2H] F lavi n ? 



2C0 2 



2 pyruvic acid 



| THIAMINE PYROPHOSPHATE 



2 phosphorylated acetate 



-2 H 2 



COENZYME A 



2 ds-aconitic acid 

 +2 H 2 



2 isocitric acid 

 TPN 



2 oxalacetic acid 



2 oxalsuccinic acid 



2C0 2 



-2 [2H] F lavin? 2 C0 2 



i (BIOTIN COENZYME?) 

 2 ketoglutaric acid 



THIAMINE PYROPHOSPHATE 



2 succinic acid 



(FLAVOPROTEIN?) 



2 fumaric acid 

 +2H 2 



2 malic acid 

 DPN 



-2 [2H] DPN 



dehydrogenase- 

 cytochrome systems 



