COOH 



I 

 CHOH 



I 

 HC— COOH 



I 

 CH2 



I 

 COOH 



ACONITASE 

 COOH 



CH 



CARBON DIOXIDE 



COOH 



CH, 



■HoO 11 +H2O I 



^ C— COOH . HOC— COOH 



+ H2O 



(If) 



-H2O 



CH2 



I 

 COOH 



/-Isocitric acid (7.7%) m-Aconitic acid (3.1%) 



CH2 



COOH 



Citric acid (89.2%) 



individual components present at equilibrium at 37° C (18). Under 

 these conditions, over 90% of the /-isocitric acid formed in step 3 is 

 converted to <:2i--aconitic and citric acids. 



The free energy changes of the various steps of this system are 

 given in Table II. Combination of the four steps gives an over-all 

 balance of about —3000 cal., i. e., the complete system is exergonic 

 by a fairly ample margin. 



Table II 

 Carbon Dioxide Fixation by a-KETOGLUTARic Acid 



There is a possibility that the above reaction series might not 

 stop with the formation of citric acid. Some microorganisms have 

 been reported to split citrate to oxalacetate and acetate (2), a reaction 

 that would favor carbon dioxide fixation via the a-ketoglutaric carboxyl- 

 ation system by displacing the equilibrium still further. Since, as 

 we shall see later, acetate can be converted to pyruvate by reductive 

 carboxylation and pyruvate forms carbohydrate in cells, the biological 

 formation of acetate from citrate would be of considerable importance. 



Carbon dioxide fixation by pyruvic acid. The primary re- 



175 



