7b CONTROL MECHANISMS IN CELLULAR PROCESSES 



(Leavitt and Umbarger, 1960) has observed that vaHne competes 

 with pyruvate l)iit not with diphosphotliiamine or a-ketobutyrate. 

 He has therefore conckided that, in exerting control over vaHne 

 biosvnthesis, the end product competes with pyruvate in the gener- 

 ation of an active acetaldehyde-dipliosphotliiamine complex. 



Vv 



250 500 750 



l/S 



1000 



Fig. 3-4. Double reciprocal plot showing the competitive nature of the 

 valine inhibition of acetoloctate formation. l/S = reciprocal of pyruvate con- 

 centration in moles per liter. 1 V = reciprocal of Klett reading. For other 

 conditions, see Umbarger and Brov^n (1958b). (Reprinted from the Journal of 

 Biological Chemistry [Umbarger and Brown, 1958b] by permission.) 



By now tliere is an impressive list of reactions which appear to 

 be involved in feedback control mechanisms by virtue of the fact 

 tliat the enzymes catalyzing them are inhibited by the end products 

 of the respective biosynthetic chains. There would be an even more 

 impressive list if one were to catalog examples of indirect evidence 

 for the operation of feedback controls. Those that have been stud- 

 ied in vitro and have come to the author's attention are listed in 

 Table 3-2. One that is of interest is the inhibition by serine of phos- 

 phoserine phosphatase, the last enzyme in the sequence leading from 

 glucose to serine in animals. However, the two preceding enzymes, 

 3-phosplioglycerate dehydrogenase and the glutamate-phosphoserine 

 transaminase connect the serine pathway to glycolytic intermediates 



