ENZYME INHIBITION AND FEEDBACK CONTROL 81 



tlie two functions with one or a pair of enzymes. Experiments to 

 answer this question were performed bv Dr. Y. S. Halpern, visiting 

 tliis countr\' from the Hadassah Medical School in Israel. 



A summar\' of Dr. Halpern's results is given in Table 3-6. The 

 strain of A. aerogenes (1033) examined not only had the classic 

 acetolactate-forming svstem discovered by Juni but also had a system 

 with a higher /;H optimum that was almost identical in its properties 

 to that found in E. coli:' The one functioning at a higher pH was 

 concerned with supplying acetolactate for valine biosynthesis since 

 it was under feedback control b\' valine. It might therefore be called 

 a biosvnthetic enzyme. The second enzvme was not under feedback 

 control by valine— an observation that accounts for the formation of 



TABLE 3-6 

 The Acetolactate Forming Systems of A. aerogenes 



Catabolic Enzyme Biosynthetic Enzyme 



Low optimal pH High optimal pH 



High affinity for Mg++ and diphospho- Low affiinity for Mg++ and diphospho- 



thiamine tliiamine 



No effect of valine Valine represses formation and inhibits 



action 



Formed at pH 6 or less Formed above pH 6 



Absent in E. coli Present in E. coli 



acetvlmethylcarbinol in media containing valine supplied in the bac- 

 teriological peptone of nutrient broth. Its most striking role is 

 played during the fermentation of glucose. It therefore has a cata- 

 bolic function. Since growth occurred at pH values at which the 

 biosynthetic enzyme is absent, Dr. Halpern concluded that the usu- 



allv catabolic enzvme then secondarilv assumes the biosvnthetic 



J J J 



function of supplying acetolactate required for valine biosynthesis. 

 This could be fulfilled in spite of the presence of acetolactate de- 

 carboxvlase in the cell. 



i5 Radhakrishnan and Snell (1960) recently reported that £. coli also exhibited an 

 acetolactate-forming system which functioned at pH 6 in addition to the system func- 

 tioning at pH 8. This apparent discrepancy can perhaps be resolved by recalling the 

 observation made by Juni and Heym (1956) that most pyruvic oxidase systems (in- 

 cluding that from £. coli) will form acetolactate in the presence of high concentrations 

 of pyruvate. This acetolactate, however, is a racemic mixture and, therefore, imlike 

 the one formed at /jH 6 by A. aerogenes and at pH 8 by both A. aerogenes and E. coli, 

 only half of it could be con\ crted to valine or to acetoin by bacterial enzymes. Thus, 

 while there is a physiological role for the function of acetolactate at either pH by 

 A. aerogenes, it seems likelv that E. coli exhibits a functionallv significant acetolactate- 

 forming activity only at the higher pH. 



