REACTIONS OF CARBON DIOXIDE lOI 



distribution in organisms and is significantly present in 

 green plants. As the pyruvate oxidation system transfers 

 acetyl to coenzyme A and hydrogen to coenzyme I, it is 

 possible that the lipoic acid is concerned in this process by 

 virtue of the chemical properties of the disulphide group. 

 The oxidative decarboxylation of pyruvate has been found 

 to depend upon the presence of four coenzymes. Three of 

 these are sulphur compounds, namely CoA, cocarboxylase, 

 and lipoic acid. 



A reversal of the reactions of the Krebs cycle and the 

 pyruvic system could theoretically lead to the fixation of 

 carbon dioxide in phosphoglyceric acid. In the plant, how- 

 ever, such a process is considered improbable. The tracer 

 carbon, in the experiments of Calvin, was found to be only 

 slowly incorporated in the components of the tricarboxylic 

 acid cycle. The significance of the oxidation of pyruvate is 

 in connexion with the synthesis of cellular constituents from 

 carbohydrate. The synthetic reactions are due to the group- 

 transferring property of coenzyme A. For example, the oxi- 

 dative decarboxylation of alpha-ketoglutarate can lead to 

 succinyl CoA, which seems likely to be important in the 

 synthesis of the tetrapyrrolic pigments (p. 39 and Fig. 3.6). 



REDUCTION OF THE CARBOXYL GROUP 



The change in free energy in the reduction of a carboxylic 

 acid to an aldehyde is larger than in any of the other reduc- 

 tion steps that might be supposed to occur during the process 

 of reduction of carbon dioxide to sugar. For example, the 

 free energy of oxidation of acetaldehyde to acetic acid in 

 dilute aqueous solution is considerably greater than that 

 for the oxidation of formic acid to CO2 and water. Thus 

 hydrogen gas at atmospheric pressure even with a suitable 

 catalyst could not appreciably reduce acetate to aldehyde. 

 The aldehyde group is not ionized to any e^ctent in aqueous 

 solution while at a neutral pH the carboxyl group is regarded 

 as completely ionized. Thus in any process of direct reduc- 

 tion of the carboxyl group under physiological conditions 

 the energy of ionization is additional to the actual reducing 

 process. These two processes may be separated so that the 



