Discussion 309 



J. hiol. Chem., 233, 415). Isoleucine inhibits, by feedback, the reaction 

 of threonine deaminase which is on its pathway. Therefore, isoleucine 

 could block its own synthesis without interfering with the synthesis of 

 threonine or methionine. 



Magasanik : The enzyme that converts threonine into ketobutjnrate is 

 quite irreversible, and so it is, therefore, quite advantageous for the 

 cell to have the block at this point. A block after that would still be 

 effective in preventing isoleucine from being formed, but it would 

 permit a drain on the threonine, which would be converted to keto- 

 butyrate and would thus not be available for protein synthesis. 



Potter : There is a clear generalization emerging from the answers to 

 Prof. Lehninger's question, i.e. that those mechanisms tend to survive 

 in the course of evolution when the feedback block is effected at a 

 branch point such that the inhibition of the synthesis of the product is 

 specific, and interference with the synthesis of other products does not 

 occur. 



Lipmann: I should like to draw attention to the complication of 

 building an enzyme of the type where there is not only a site for the 

 substrate, but also one for the inhibitor, which seems to be specific and 

 does not necessarily seem to be related to the substrate at all. 



Magasanik: This touches on interesting observations made by Dr. 

 Umbarger (Umbarger, H. E. (1957). J. BacL, 73, 105). He found that 

 Esch. coli possesses two enzymes catalysing the same reaction: the 

 conversion of threonine to ketobutyrate. One of these enzymes is 

 produced by cells growing in a minimal glucose-containing medium. 

 It plays an essential role in the biosynthesis of isoleucine, as shown by 

 the fact that its loss through mutation results in a nutritional require- 

 ment for isoleucine. The action of the enzyme is inhibited by isoleucine. 

 The other enzyme is produced in cells growing anaerobicaliy in a medium 

 rich in amino acids and free of glucose. It deaminates not only threonine, 

 but also serine, and its metabolic role is presumably a catabolic one : it 

 converts the amino acids threonine and serine to the keto acids keto- 

 butyrate and pyruvate, which presumably are further metabolized to 

 yield energy and building blocks for the formation of cell material. The 

 action of this enzyme is not inhibited by isoleucine. Dr. Umbarger 

 discovered a similar situation in Aerobacter aerogenes (1957, J. Amer. 

 chem. Soc, 79, 2980). This organism contains two enzymes capable 

 of converting pyruvate to acetolactate, a reaction essential for the 

 biosynthesis of valine. One of these enzymes is formed by cells growing 

 in a minimal medium at neutral or slightly alkaline pH, and is inhibited 

 by valine. The other enzyme is only formed when the pH of the medium 

 is low, and apparently is responsible for the formation of acetoin, a 

 neutral product of glucose fermentation in place of acetic and formic 

 acid ; the physiological role of this enzyme seems to be the prevention of 

 further production of acidity in the medium. The action of this enzyme 

 is not inhibited by valine. 



These two examples show that enzymes attacking the same substrates 

 can differ strikingly in their affinity for inhibitors. Furthermore, it 

 would appear that control of enzyme action by negative feedback is 



