CH, . CO . COOH 



THE VITAMIN B COMPLEX 



+ C02 



HOOC . CHa . CO . COOH 



+ Ha 



HOOC . CH2 . CHOH . COOH 



+ H2O 

 HOOC . CH : CH . COOH 



]|-H.O 



The second route whereby pyruvic acid is dehydrogenated was 

 discovered by H. A. Krebs and was originally called the " citric acid 

 cycle ", because the formation of citric acid was believed to be one of 

 the stages involved. It was subsequently shown, however, that iso- 

 citric acid is actually the intermediate product, and that citric acid is 

 only a by-product. Consequently, the name, " citric acid cycle ", 

 has been abandoned in favour of the term, " tricarboxylic acid cycle ". 

 In its present form this can be represented as follows : 



CH3 . CO . COOH 



pyruvic acid 

 + COa 



CH3 . CO . COOH HOOC . CH2 . CO . COOH 



pyruvic acid oxaloacetic acid 



HOOC . CH2 . CHOH . COOH 



malic acid 



H2O 



HOOC . CH : CH . COOH 



fumaric'acid 



CH . COOH 



J 



C . COOH 



I 

 CH2 . COOH 



cw-aconitic acid 



+ H2O 



CH2 . COOH CHOH . COOH 



I ! 



C(OH) . COOH CH . COOH 



CH2 . COOH 



citric acid 



CH2 . COOH 



i»ocitric acid 

 Ha 



CO . COOH 

 I 



CH . COOH 



I 

 CH2 . COOH 



oxalosuccinic acid 



■COj 



626 



HOOC . CH2 . CH2 . COOH 



succinic acid 

 CO2 



CO . COOH 



I 

 CH2 



I 



CH2 . COOH 



a-ketoglutaric acid 



