CARBON METABOLISM 



247 



H 



CHO 



-. I 

 H:COH 



ii 

 0:CH 



M 



H;COH 



■--: I 



H;COH 



CHO 



I 

 C.OH 



II 

 CHo 



CHO 



+ H2O 



HCOOH 

 + 

 > CH3CHO 

 (acetaldehyde) 



> H2 + CO2 



oxidation 



> CH3COOH 



(acetic acid) 



CH,:OH 



C.OH +H2O 



CH, 



(glucose) 



COOH 



I 

 HCOH 



I 



CH3 

 (lactic acid) 



(methylglyoxal) 



This is an over simplified expression of the mechanism 

 of lactic acid fermentation by such organisms as Esch. 

 coli, but obviously does not account for the fermentation 

 by the homofermentative lactic acid bacteria which 

 give almost 100 per cent, lactic acid. Almost certainly 

 phosphorylation reactions are involved in lactic acid 

 fermentation by all bacteria (see p. 249). 



In alcoholic fermentation by yeast, pyruvic acid is 

 decarboxylated to give acetaldehyde and carbon dioxide 

 (see Chapter XV), but this mechanism cannot apply 

 to the bacterial fermentations since it does not account 

 for the formic acid and hydrogen found in many such 

 fermentations. Possibly with some bacteria the pyruvic 

 acid is broken down in another way with formation of 

 formic and acetic acids : — 



COOH H 



CO + OH 



HCOOH 



CH3 



(pyruvic acid) 



CH3COOH 



(acetic acid) 



There is evidence that the reaction occurs through 

 phosphopyruvic acid and is reversible, for when pyruvic 

 acid is dissimilat^d by Esch. coli in presence of formic 

 acid containing " heavy " carbon, C^^, the residual 

 pyruvic acid contains C^^ in the carboxyl group and the 

 rate of transfer is accelerated l)y the addition of in- 

 organic phosphate. Heavy carbon is also found in the 



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