CARBON METABOLISM 255 



The triose phosphate, derived via phosphorylation of 

 glucose, gives a non-reducing suljstance of unknown 

 structure which may give phosphoglyceric acid by 

 oxidation, or which may give methylglyoxal by loss 

 of the phosphate group. Phosphoglyceric acid is conveited 

 to pyruvic acid which is reduced via lactic acid to pro- 

 pionic acid. Methylglyoxal may be converted to lactic 

 acid directly (presumably by the action of glyoxalase) 

 or it may be oxidised to pyruvic acid ; propionic acid is 

 then formed as in the alternative scheme. These workers 

 postulated that part of the pyruvic acid might undergo 

 another series of reactions to give succinic and propionic 

 acids, pyruvic acid giving the hydrate which would split 

 to give acetic acid and carbon dioxide ; two molecules 

 of acetic acid then condeixse to give succinic acid which 

 in its turn is decarboxylated to form propionic acid and 

 carbon dioxide : — 



/OH — 2H 

 > CHaCZ—COOH > CH3COOH + CO2 



^OH 



-2H 



CHX'Ho.COOH -r CO, 



In view of the recent work on the fixation of carbon 

 dioxide it seems more probable that the succinic acid 

 arises by way of condensation of carbon dioxide with 

 pyruvic acid (see p. 258). 



Carbon Dioxide Fixation. — Recently work on the 

 fixation of carbon dioxide by bacteria has thrown a new 

 light on the mechanism of acid production by bacteria. 

 It has been known for a long time that autotrophic 

 bacteria utilise carbon dioxide, either by photosynthetic 

 or by chemosynthetic processes, as their sole source of 

 carbon. It has also been known for many years that 

 carbon dioxide plays an important role in the metabolism 

 of some heterotrophic bacteria : thus Brucella aboi'tus, 

 when first isolated from cattle, will not grow unless the 



