ENZYME INHIBITION AND FEEDBACK CONTROL 79 



function in the usual peptone medium which would be rich in iso- 

 leucine. 



However, E. coli meets this lack of versatility by being able to 

 form two distinct L-threonine-L-serine deaminases (Umbarger and 

 Brown, 1957 ) . As shown in Table 3-4, the different properties and 

 the different conditions affecting their formation make the two 

 enzymes clearly distinguishable. The enzyme having primarily a 

 catabolic function is an adaptive enzyme studied in E. coli most 

 intensively by Wood and Gunsalus ( 1949 ) . It is formed only anae- 



TABLE 3-4 

 The Threonine-Serine Deaminases of E. coli 



Catabolic Enzyme Biosynthetic Enzyme 



Cofactors demonstrated Cofactor demonstrated 



Pyridoxal phosphate Pyridoxal phosphate 



Ghitathione 



Adenosine-5-phosphate 



First order kinetics Second order kinetics 



No effect of isoleucine Isoleucine represses formation and in- 



hibits action 

 Formation prevented by aeration and Formed in minimal media 



fermentable sugar 



No effect of aeration 



robically in the absence of a fermentable energy source. For it to 

 have a biosynthetic role would require the very special circum- 

 stances of anaerobic cultivation in an isoleucine-free mixture of 

 amino acids. Therefore, loss of the biosynthetic enzyme results in 

 isoleucine a-ketobutyrate auxotrophy even though the genetic ap- 

 paratus for forming the second enzyme is present. 



An exactly analogous situation has been described for the first 

 step in the pathway leading to valine, acetolactate formation (Hal- 

 pern and Umbarger, 1959 ) . It is well known to diagnostic bacteri- 

 ologists that one of the properties that differentiates E. coli from A. 

 aerogenes is the appearance of acetylmethylcarbinol in the culture 

 fields of the latter. As the studies of Juni (1952) have shown, the 

 formation of acetvlmethvlcarbinol proceeds by the way of acetolac- 

 tate. It is interesting that the system forming acetolactate and the 

 decarboxylase that accompanies it are formed and function at /;H 

 values of 6 or less. The fact that acetoin, a neutral product, would 



