86 BACTERIAL FERMENTATIONS 



The most likely pathway for the formation of acetate is 

 via serine and pyruvate as follows: 



CH2NH2COOH + "HCHO" — *- CH2OHCHNH2COOH 



j-NH 3 



CH3COOH + C0 2 ^ CH3COCOOH 



H2O 



This scheme postulates an hydroxymethylation of glycine 

 by an active formaldehyde group which could be derived 

 from the formimino group of formiminoglycine via formyl- 

 THFA. The formation of serine has not yet been experi- 

 mentally demonstrated in this system. The conversion of 

 L-serine to pyruvate and the oxidation of pyruvate to acetate 

 and carbon dioxide are catalyzed by both intact cells and 

 extracts. 69 Presumably the oxidation of pyruvate can be 

 coupled with the formation of ATP although this has not 

 yet been shown. 



The above scheme is further supported by a variety of 

 tracer experiments. 69,70 The carboxyl groups of both glycine 

 and formiminoglycine are converted to carbon dioxide; 

 glycine-2-C 14 yields pyruvate-2-C 14 ; both glycine-2-C 14 and 

 formiminoglycine-2-C 14 yield acetate-1-C 14 ; and formimino- 

 glycine-C 14 labeled in the formimino group is converted 

 to acetate-2-C 14 . All of these results and others which will 

 not be mentioned are consistent with the above scheme. 



We have seen that the formimino carbon of formimino- 

 glycine can undergo two types of reactions. (1) It can be 

 converted to formate via formyl-THFA with the simulta- 

 neous formation of ATP, or (2) it can be reduced and 

 transferred, presumably via the THFA derivatives, to 

 glycine to give serine or an analogous compound. The 

 relative importance of these two processes obviously de- 



