CARBON DIOXIDE 



It is thus dear that the biological reduction of the keto acid 

 formed in step 7 of carbon dioxide assimilation requires the presence 

 of the specific dehydrogenase and of the reduced form of its prosthetic 

 group. Step 2 can then be represented as follows: 



keto acid + P • C0H2 t± hydroxy acid + P • Co (3) 



Since the complex formed by the coenzyme with the protein 

 component of the dehydrogenases is dissociable to a relatively large 

 degree, there are always small amounts of free coenzymes present in the 

 cell. When equilibrium has been reached in a reaction of the type 

 represented by reaction (3), it will stop unless provision is made for a 

 reduction of the oxidized coenzyme formed so as to displace the 

 equilibrium to the right. Step 3 occurs here. It is carried out through 

 the action of another dehydrogenase which functions with the same 

 prosthetic group as that acting in step 2. Such a reaction may be, for 

 instance: 



hydroxy acid a + PrCo ?:± keto acid a + PrCoHa (4) 



and is possible because of the dissociable nature of the complex formed 

 by the coenzyme with the protein components of the dehydrogenases, 

 so that the coenzyme can alternatively be bound by either protein. 

 Thus, some of the oxidized coenzyme dissociating from P • Co can be 

 bound by the second protein, Pi, to form Pi -Co, as in reaction (4). 

 Since the coenzyme oxidized in reaction (3) is reduced in reaction (4), 

 the net result of these two reactions can be expressed by: 



keto acid + hydroxy acid a <=± hydroxy acid + keto acid a (5) 



This type of reaction is known as a coenzyme-linked dismutation, and 

 is, in general, reversible. The extent to which it will proceed in a given 

 direction depends on the equilibrium constants of the two dehydrogen- 

 ase systems involved and on the concentration of reactants. Thus, in 

 our case, a high concentration of hydroxy acid a will favor carbon 

 dioxide fixation. 



We shall now consider the individual carbon dioxide fixation 



systems. 



^-Cavhoxylaiion 



Carbon dioxide fixation by a-ketoglutaric acid. Although 

 this type of carbon dioxide fixation is the most recently discovered 



171 



