PROPIONIC ACID FORMATION 



139 



described for Esch. coli. Werkman and his colleagues have 

 shown that if CO2 containing C^^ is used then the heavy- 

 carbon is fixed by the organism and appears in the — -COOH 

 group of succinic acid and also of propionic acid. This 

 suggests that propionic acid arises from the same initial 

 reaction, involving CO2 fixation, as succinic acid. It is now 

 known that the organism possesses a succinic decarboxylase 

 which removes COg from succinic acid, to form propionic acid. 



CO2 



+ 



CH, 



CO 



COOH COOH 

 I 



CH, 



CH. 



COOH 

 I 



CH 



COOH 



I 

 CH„ 



COOH 



CH, 



COOH 



(11) 



Some strains of these organisms carry out a further reduction 

 of propionic acid to w-propyl alcohol, 



CH3.CH2.COOH + 2H2 — >CH3.CH2.CH20H + H20 ...(12) 



and when C^^02 is used in such a fermentation, C^^ appears 

 in the — COOH groups of succinic and propionic acids and 

 also in the — CH2OH of propyl alcohol. 



CH3.CO.CO.CH3 



CH3. CHOH. COOH 



CH3.CO.CHOH.CH3 



CH3.CHOH.CHOH.CH3 



HOOC.CH2.CO.COOH 



6 n® 



H00C.CH=CH.COOH 



n® 



H00c.cH2.cH2.cooH 



/© 

 H00c.cH2.cH3 



/© 

 HOH2C.cH2.cH3 



The fermentation of pyruvic acid by Esch. coli, Aerobact. aerogenes, 

 and Propionibacteria. The numbers in rings in the above scheme 

 refer to the reactions similarly numbered in this chapter. 



