SEVERO]]OCHOA 



muscle adenylic acid), with release of inorganic phosphate and of the 

 energy of the bond. By enzymic transphosphorylation, ATP trans- 

 fers phosphate to carboxyl, enol, and guanidine groups in a reversible 

 manner. Since the energy content of the various types of energy-rich 

 phosphate bonds is nearly the same, in the neighborhood of 12,000 cal., 

 these transphosphorylations involve relatively small changes of free 

 energy (16). 



The general type of oxidative decarboxylations can be repre- 

 sented by reaction Ilia (17), where X stands for a hydrogen acceptor. 



GH3COOPO3- 



+ CO2 + H2X 



(Ilia') 



cHaCO : cooH + H : opo,- 



Pyruvic acid 



± CH3COOPO3— + CO2 + 



H2 



(Ilia") 



CH3COOPO,-- + HCOOH 



Acetyl phosphate (Ilia ) 



The oxidative decarboxylation of a-ketoglutaric acid generates phos- 

 phate bonds (19) and may be represented by reaction Illb. All these 

 reactions occur in bacteria; reactions Ilia' and Illb also occur in ani- 

 mal tissues. 



C00HCH2CH2C0 : COOH + H : OPO,— + x 



a-Ketoglutaric acid 



COOHGH2CH2COOPO,-- + CO2 + H2X (Illb) 



Succinyl phosphate 



Two reactions are known at present through whose reversibility 

 reductive carboxylation can take place: (a) the splitting of formic acid 

 to carbon dioxide and hydrogen; and (b) the splitting of pyruvic acid 

 to acetyl phosphate and formic acid. Since acetyl phosphate can be 

 formed enzymically by a reversible reaction between acetic acid and 

 ATP, we have a biological system of reductive carboxylation of acetate 

 to pyruvate. 



The pattern of carbon dioxide fixation established for /3-car- 

 boxylations must be modified here to include two preliminary steps, 

 viz., those of phosphorylation and of carbon dioxide reduction, respec- 

 tively, and a final step for regenerating the ATP used at the beginning. 



180 



