74 BACTERIAL FERMENTATIONS 



CH3CHNH2COOH . NHj 



p CH 2 =CHCOOH -ini 



CH3CHOHCOOH ^ H2 ° 



CH3CH2COOH (7) 



The occurrence of such a sequence in the reverse direction 

 was investigated by Stadtman 49 by studying the oxidation 

 of propionate by cell-free extracts of CI. propionicum. He 

 found that propionate was oxidized only after being con- 

 verted to propionyl CoA. The supposed product of the 

 oxidation, acrylyl CoA, could not be directly identified. 

 However, indirect evidence for this compound was provided 

 by the finding that acrylyl thioesters react enzymatically 

 with ammonium ion to form /?-alanyl thioesters and ulti- 

 mately /^-alanine itself in this system (equation 8) , and 



+nh 3 H2O 



CH 2 =CHCOSR --> CH2NH2CH2COSR -4- 



CH 2 NH 2 CH 2 COOH + HSR (8) 



that /^-alanine is also formed in the oxidation of propionyl 

 thioesters. Presumably the acrylyl thioester is so reactive 

 that it cannot accumulate to an appreciable extent under 

 the experimental conditions used. These experiments sug- 

 gest that acrylyl CoA may be a precursor of propionate in 

 CI. propionicum. However, as yet a conversion of alanine 

 or lactate to acrylyl CoA has not been demonstrated nor 

 is there any indication of the role of /^-alanine in these 

 fermentations. 



Threonine is exceptional among the substrates fermented 

 by CI. propionicum in that it gives rise to butyrate and 

 propionate (equation 9) instead of propionate and ace- 

 tate. 50 This difference is obviously dependent on the fact 

 that threonine is the only four-carbon compound attacked. 

 Evidently the carbon chain of 1 mole of threonine is reduced 



