CARBON DIOXIDE 



A possible series of reactions, modified from Lipmann, is suggested in 

 Table IV. The absolute value of the free energy change of step 6 



Table IV 

 Reductive Carboxylation 



has been considered to be the same as that of the step 7, on the assump- 

 tion that the bond energy of the anhydride linkage in 1-phosphoglycer- 

 ate is the same as that of the acetyl phosphate group. The reaction 

 series postulated in Table IV would lead from acetate to lactate and 

 the over-all reaction would be exergonic. A characteristic feature of 

 reductive carboxylation is that it must be started by input of a large 

 amount of energy from energy-rich phosphate bonds. 



Pyruvic acid formed by reductive carboxylation of acetate 

 might also be converted to carbohydrate after reduction to triose in- 

 stead of being reduced to lactic acid. Such a conversion would re- 

 quire additional phosphate-bond energy which might be amply avail- 

 able in chemosynthesis or photosynthesis. 



It is not unlikely that reactions Ilia' and Illb might also be 

 reversible. The main difference between these reactions and reactions 

 Ilia" and Ilia'" is one of mechanism, i. e., the hydrogen from the 

 keto acid is first transferred to a hydrogen acceptor instead of being 

 released either as molecular hydrogen or in combination with carbon 

 dioxide as formic acid. If such were the case, all of the intermediate 

 reactions involved in the oxidation of foodstuffs (i. e., in respiration) 

 would be reversible. 



i8i 



