330 



ANIMAL METABOLISM 



from a definite part of the original glucose molecule (see Fig. 13-3, espe- 

 cially reactions 5 and 6). This may be pictured as follows: 



(1) 



CHO 

 I 

 HCOH 



I 

 (3) HOCH 



(2) 



(3) 



(4) HCOH 



(5) HCOH 



I 



(6) H2COH 



Glucose 



CHj 



CO 



I 



COOH 



+ 

 COOH 



I 

 CO 



I 



CH3 (3) 

 Pyruvic acid 



(2) 



(1) 

 (1) 

 (2) 



The carbon atoms for the methyl groups of the two pyruvic acid molecules 

 (carbon 3 of the pyruvic acid) come from carbons 1 and 6 of the glucose, 

 those for the CO groups from 2 and 5, and those for the COOH groups 

 from 3 and 4. The correctness of these relationships has been well estab- 

 lished by studies with compounds containing isotopic carbon atoms in 

 known positions. 



Oxidation of pyruvic acid 



The Citric Acid Cycle. Pyruvic acid is metabolized by the reactions 

 shown in Fig. 13-4.^ Although oxygen does not appear in this scheme, 

 the process is an aerobic one because the hydrogen atoms produced at 

 several points are constantly being combined with oxygen by the cyto- 

 chrome system. The outstanding feature of the process is its cyclic 

 nature. Oxalacetic acid and acetic acid combine to form citric acid, 

 which then goes back to oxalacetic again (reactions 18-25). This is 

 called the citric acid cycle.^ The main sequence of reactions, normally 

 followed by the bulk of the pyruvic acid metabolized, is indicated in 

 Fig. 13-4 by heavy arrows. Reverse reactions are shown with light 

 arrows, and various associated processes by broken lines. 



The result of the operation of the citric acid cycle is that the original 

 molecule of pyruvic acid is completely broken down into carbon dioxide 

 and hydrogen, which later becomes water (see below). This may be 

 seen by reading clockwise around the cycle and noting what is added 

 or subtracted in each step. Starting with pyruvic acid, four moles of 

 water are added (reactions 16, 19, 22, 24) and one removed (reaction 



^ In this figure, and throughout this chapter, the two-carbon substance arising from 

 the metabolism of pyruvic acid and from fats is shown for simplicity as free acetic acid. 

 It is almost certain, however, that this intermediate is actually an acetyl group 

 (CH3CO— ), which is taken up by a coenzyme {Co A, p. 274) as fast as it is formed 

 and later transferred to some other substance {e.g., oxalacetic acid). 



''Also called the tricarboxylic acid cycle, or Krebs cycle. 



