REACTIONS OF CARBON DIOXIDE 89 



oxidation of carbohydrate or of acetate to carbon dioxide 

 and water occurs via the Krebs cycle the fixation of COg 

 in pyruvic acid to form oxalacetic acid is a definite step in 

 respiration. If no alternative path is available then CO2 be- 

 comes essential once the supply of four carbon acids becomes 

 depleted. The complete oxidation of pyruvate and acetate 

 depends on the formation of acetyl coenzyme A which then 

 reacts with oxalacetate to give citrate. Acetate itself will only 

 give acetyl CoA at the expense of an active phosphate trans- 

 ferring system involving, for example, ATP. Pyruvate gives 

 acetyl CoA directly by oxidative carboxylation, involving 

 the simultaneous reduction of DPN, independently of an 

 additional energy source. 



The fixation of COg by a heterotrophic organism was 

 discovered by Wood and Werkman in the propionic 

 bacteria. They later found that during the fermentation of 

 glycerol CO2 was utilized for the formation of succinic acid 

 (see, for example, the review by Wood (1946)). Elsden 

 showed independently that the rate of formation of succinate 

 by B. colt in presence of pyruvate depended on the pressure 

 of CO2; the CO2 presumably appearing in the succinate by 

 the following reaction sequence: 



The first step, the incorporation of carbon dioxide into 

 pyruvic acid to form oxalacetic acid, is often referred to as 

 the Wood and Werkman reaction. 



The CO2 fixed in a cellular metabolite in this way might 

 well be expected to find its way by synthetic side reactions 

 into a variety of cellular constituents. This has been shown 

 in animal tissues using tracer CO2: the glycogen in liver 

 contains carbon derived from COg. When such experiments 



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