CARBON DIOXIDE 



The combined result of reactions Ila and lib is reaction IIc; 



OXALAGETIC CARBOXYLASE AND MALIC DEHYDROGENASE 



CH, COOH 



I I 



CO + CO2 + DPNH2 _1 CH2 + DPN 



(lie) 



COOH 



Pyruvic acid 



CHOH 



1 

 COOH 



/-Malic acid 



In step 3, the DPN formed in reaction IIc can be reduced by 

 lactic acid through a coenzyme-Hnked dismutation with the lactic de- 

 hydrogenase system. Lactic dehydrogenase catalyzes reaction lid. 



CH, 



I 

 CHOH + DPN ; 



LACTIC DEHYDROGENASE 

 CHs 



COOH 



/( + )-Lactic acid 



CO + DPNHj 



I 

 COOH 



Pyruvic acid 



(Hd) 



The combined result of reactions IIc and lid is reaction He. 

 pyruvate + CO2 + /( + )-lactate ^ /-malate + pyruvate (He) 

 The free energy change of this reaction (see Table III), is about +1500 



Table III 

 Carbon Dioxide Fixation by Pyruvic Acid 



177 



