SEVERO OCHOA 



The TPN formed in reaction Ic can be reduced through a 

 coenzyme-linked dismutation as discussed above. This results in the 

 shifting of the equilibrium toward the side of carbon dioxide fixation. 

 Such a shifting has been accomplished with the glucose-6-phosphate 

 dehydrogenase system (see Table I) which catalyzes reaction Id. The 

 combined result of reactions Ic and Id, when a mixture of glucose-6- 



GLUCOSE-6-PHOSPHATE DEHYDROGENASE 



glucose-6-phosphate + TPN ^ 



6-phosphogluconate + TPNH2 (Id) 



phosphate, a-ketoglutarate, and carbon dioxide is incubated with 

 glucosephosphate dehydrogenase, oxalosuccinic carboxylase, isocitric 

 dehydrogenase, manganese ions, and TPN, is reaction le: 



a-ketoglutarate + CO2 + glucose-6-phosphate <=^ 



/-isocitrate + 6- phosphogluconate (le) 



The equilibrium constant of reaction le has not yet been de- 

 termined experimentally, but it can be calculated from free energy data. 

 Thus, the free energy change of reaction Id can be estimated from the 

 equation, —AF = nFAE, relating free energy change to the potential 

 difference between two reacting oxidation-reduction systems (15). 

 The reacting systems in reaction Id are the glucose-6-phosphate ^ 

 phosphogluconate {Eq — —0.43 v. at pH 7.0) and the TPN ;=± 

 TPNH2 system, the potential of which is unknown but can be con- 

 sidered to be near that of the DPN «=± DPNH2 system (Eo = —0.28 v. 

 at pH 7.0). For a potential difference of 0.43-0.28 = 0.15 v., the 

 AF of reaction Id would be —6890 cal., corresponding to an equilib- 

 rium constant K^ = (6-phosphogluconate) (TPNH2)/ (glucose-6-phos- 

 phate)(TPN) of the order of 10^. Hence, the equilibrium constant of 

 reaction le: K^. = (/-isocitrate) (6-phosphogluconate)/(glucose-6- 

 phosphate) (a-ketoglutarate) (CO2) = K^X K^= \.'iX 10"" X 10^ = 

 13. 



A further shift of the equilibrium of reaction le toward carbon 

 dioxide fixation occurs in the presence of aconitase. This enzyme is 

 widely distributed in animal and plant cells and catalyzes the inter- 

 conversion between /-isocitric, czV-aconitic, and citric acids according to 

 reaction If, where the figures in parentheses give the percentage of the 



174 



