48 BACTERIAL FERMENTATIONS 



which had been purified 350-fold from pig liver extracts. 



The available evidence indicates that butyrate synthesis 

 in CI. kluyveri involves reactions similar to or identical 

 with those shown in Fig. 2. However, the precise identities 

 of the postulated unsaturated and ^-hydroxy compounds, 

 here referred to as crotonyl-SCoA and /?-hydroxybutyryl- 

 SCoA, have not been established. The unsaturated com- 

 pound might be crotonyl-, isocrotonyl-, or vinylacetyl-SCoA; 

 and the hydroxy compound might be l(+)- or d( — )- 

 /3-hydroxybutyryl-SCoA. 



Despite the considerable information that is available 

 concerning the chemistry of the CI. kluyveri fermentation, 

 at least one major problem remains unsolved, namely the 

 nature of the mechanism by which the organism obtains 

 useful energy from the conversion of ethanol and acetate 

 to butyrate and caproate. The oxidation of ethanol to 

 acetyl-SCoA provides an energy-rich compound which might 

 be used to generate ATP by means of the phosphotrans- 

 acetylase and acetokinase reactions. However, most of the 

 acetyl-SCoA is converted to butyrate and caproate, and 

 therefore is not directly available as a source of energy for 

 ATP synthesis. Indeed, acetyl-SCoA must be used in this 

 way in order to produce the electron acceptors, acetoacetyl- 

 SCoA, crotonyl-SCoA, and their six carbon analogues, with- 

 out which the oxidation of ethanol and acetaldehyde would 

 be impossible. 



Since acetyl-SCoA seems to be excluded as a primary 

 energy source for synthetic reactions, some other source of 

 energy must be available to the organism. One possible 

 method of providing ATP is by the coupling of an electron 

 transport reaction with a phosphorylation as occurs in the 

 so-called oxidative phosphorylation in aerobic organisms. 

 In the CI. kluyveri system the most suitable electron trans- 

 port system for this purpose is that between ethanol or 



