FERMENTATIONS OF NITROGENOUS COMPOUNDS 71 



hydration of the double bond. The product of the reaction 

 has been identified by chemical and enzymatic methods as 

 (+) -citramalate. Cysteine and Fe++ are cofactors for the 

 reaction, and orthophenanthroline and other reagents that 

 combine strongly with Fe++ are powerful inhibitors. As 

 in the aconitase and fumarase reactions, the equilibrium 

 favors the formation of the hydroxy acid. 



Citramalate is cleaved by a reversible aldolase-type reac- 

 tion to acetate and pyruvate. The enzyme citramalase 

 which catalyzes this reaction shows a high degree of speci- 

 ficity since it attacks (+) -citramalate but neither the levo- 

 rotatory isomer nor several structurally related compounds 

 including citrate, isocitrate, and malate. The equilibrium 

 of the reaction favors the cleavage of citramalate. 



The analysis of the CI. tetanomorphum system has estab- 

 lished the existence of a new pathway of glutamate metab- 

 olism which may be regarded as a substitute for a part of 

 the tricarboxylic acid cycle in the sense that it permits a 

 conversion of glutamate to pyruvate, acetate, and carbon 

 dioxide. All of the reactions between glutamate and pyru- 

 vate are readily reversible and involve only small free- 

 energy changes. Therefore this sequence differs from the 

 oxidative part of the tricarboxylic acid cycle in not provid- 

 ing energy for the synthesis of adenosine triphosphate. 



The occurrence of the mesaconate pathway in other 

 organisms has not yet been investigated. However, indirect 

 evidence for this pathway in several anaerobic bacteria, in- 

 cluding CI. botulinum, CI. tetani, CI. saccharobutyricum, 

 and Fusobacterium nucleatum, is provided by the observa- 

 tion that these species, like CI. tetanomorphum, form 1 mole 

 of carbon dioxide per mole of glutamate fermented. The 

 occurrence of mesaconate in sugar cane and cabbage, and of 

 (— ) -citramalate in apples, indicates that these compounds 

 also have a role in the metabolism of higher plants. 



