SEVERO OCHOA 



has been estimated to be —9400 cal., and that of reaction (2) —8000 

 cal. (8): 



a-ketoglutarate + 2 H2O ^ 



succinate-- + HCO3- + 3 H+ + 2 e (2) 



Reactions of this type are often referred to as oxidative decarboxylations 

 and the reverse as reductive carboxylations. 



From the relationship between free energy change and equilib- 

 rium constant discussed above, it is clear that shifts of equilibrium 

 toward carboxylation, i. e., carbon dioxide fixation, can only be ac- 

 complished by an input of energy into the system. We shall consider 

 in some detail the enzymic mechanisms used by the cell for this 

 purpose. 



The fundamental pattern of biological carbon dioxide fixation 

 can be visualized in terms of the following steps: 



(7) Primary Fixation Reaction. Carboxylation. Since the equi- 

 librium of the reversible reaction involved is very unfavorable, only 

 small amounts of keto acid are formed at one time. 



(2) Reduction. Step 1 is followed by enzymic reduction of the 

 keto acid to the corresponding hydroxy acid. This shifts the equilib- 

 rium and more keto acid can be formed by step 7. 



(5) Reduction of the Pyridine Nucleotide Oxidized in Step 2. The 

 second and third steps will be discussed below. 



(4) Dehydration, Hydration, Isomerization. Further equilibrium 

 shifts can be brought about by secondary enzymic transformations of 

 the hydroxy acid formed by step 2. Thus, the hydroxy acid may be 

 dehydrated to the corresponding unsaturated fatty acid. The latter, 

 in turn, may be hydrated in a different position of the molecule to form 

 a new hydroxy acid isomeric with the one first formed. 



(5) Further Reduction. The unsaturated fatty acid formed in 

 step 3 may undergo further reduction to the corresponding saturated 

 acid. 



All the reactions concerned in the various steps just outlined are 

 reversible. 



The reduction of the keto acid in step 2 is catalyzed by a specific 

 pyridine nucleotide dehydrogenase. These dehydrogenases (27) con- 

 sist of a protein and a prosthetic group or coenzyme that combine with 



168 



