AEROBIC GLYCOLYSIS 



89 



pyruvic acid is formed. The subsequent stages have been studied largely in pigeon 

 breast muscle by Szent-Gyorgyi and much of the modern development is due to Krebs. 



The cycle suggested is somewhat as follows : — 



(Aspartic acid) 



\r ^ (Alcjair\«) 



Pyruvic acia 



Succinic acid 



Oxaloacetic acid 



^ i.r-nk\r- ryr\A Malic acid 



Acetic acia 



I t 



Aconitic acid (^Citric ocid) Fumaric acd ^^ 



\> 



Isocitric acid 



i t 



Oxalosuccinic acid ^ > a-Ketoglutaric acid 



CO2 (Glutamic acid) 



Fig. 17 

 Citric acid cycle 



It will be seen that a complete cycle of reactions is visualised : at each turn of 

 the cycle a molecule of pyruvic acid (from glycolysis, etc.), enters the cycle and three 

 molecules of carbon dioxide are ejected. So the net result is : — 



CH3CO.COOH + 5 ^ 3 CO2 + 2H2O 

 Oxaloacetic acid (enol) starts in the cycle reacting with acetic acid from the 

 pyruvic acid introduced and a molecule of oxaloacetic acid is left at the end of the 

 cycle. 



Coenzymes I and II are concerned in reactions in the cycle, but this is not the 

 place for a discussion of the evidence for the various reactions and the effect of 

 inhibitors. 



There are important interrelations between the tricarboxyhc acid cycle and other 

 metabolic processes. For example by transaminase mechanisms, glutamic acid can 

 give rise to or be formed from a-ketoglutaric acid, aspartic acid from oxaloacetic 

 acid and alanine from pyruvic acid. 



RiCO.COOH + R2.CHNH2.COOH ^ R1.CHNH2.COOH + R2.CO.COOH 



The free energy changes involved in aerobic breakdown of carbohydrate are of 

 some interest and are dealt with in the next section. 



