BIOSYNTHESIS 231 



Reaction (3) can be caused by proceeding from right to left by coupling 

 the system to an oxido-reduction system capable of reducing TPN, for 

 example the glucose-6-phosphate dehydrogenase system, and we obtain 



G— 6— P + TPN+ ^ 6-phosphogluconic acid + TPNH (4) 



a-ketoglutaric acid + COj -f TPNH ^ isocitric acid + TPN+ (5) 



The sum of (4) and (5) gives 



G — 6 — P + a-ketoglutaric acid + COj ^ 6-phosphogluconic acid 



+ isocitric acid (6) 



If aconitase is added, the equilibrium is shifted still more in favour of 

 COj fixation since 90% of the isocitric acid is continuously removed. 

 Since the enzyme systems here described are universally distributed in 

 cells, both in animals and in plants and in micro-organisms, this means of 

 fixation is an important one. 



(b) Fixation of CO 2 by pyruvic acid. — The entry of pyruvic acid into 

 the tricarboxylic acid cycle by way of acetyl-CoA, then by formation of 

 citric acid, depends upon there being a sufficiently high concentration of 

 oxaloacetic acid. This concentration is maintained by the fixation of CO, 

 by pyruvate with formation of oxaloacetate. 



From the moment that the C4 acids are used up, COg becomes essential 

 for the continuation of respiration by means of the cycle. 



The discovery of the fixation of COg by heterotrophes was first made in 

 the propionic acid bacteria (Wood and Werkmann). In these bacteria when 

 fermenting glycerol the CO2 is utilized for the formation of succinic acid. 

 Elsden has shown that for E. coli the rate of formation of succinate in 

 the presence of pyruvate depends upon the CO2 pressure. 



The CO2 is incorporated into succinate according to the following 

 reactions : 



CHs +CO2 COOH +H2 GOGH -H2O COOH -fHa GOGH 



► I > I > I > I 



CO CH2 GH2 GH CH, 



I I t II I 



GOOH CO GHOH GH CH2 



1 i I I 



COOH COOH COOH COOH 



pyruvic acid oxaloacetic acid malic acid fumaric acid succinic acid 



y 



Wood-Werkmann Reaction 



By use of labelled carbon it has been shown that COg fixed by pyruvate 

 contributes to carbohydrate synthesis in heterotrophes. For example, the 



