THE ROLE OF ENZYME REGULATION IN METABOLISM 



87 



% 



cc 



LlI 



> 



o 



LJ 

 CO 

 < 

 UJ 



a: 







20 30 



MINUTES DF GROWTH 



50 



Figure 5. Production of orotic acid by a pyrimidineless mutant upon depletion of uracil. 



receive the substrates. Presumably the advantages for growth provided 

 by feedback inhibitions caused a selection of organisms which made 

 enzymes with the built-in feedback feature. 



How frequently does one find feedback inhibition in biosynthetic 

 pathways? There are about a dozen well-substantiated cases of feed- 

 back inhibition. Also there is a general sort of experiment which sug- 

 gests that feedback inhibitions are to be found in most major biosyn- 

 thetic pathways. If bacteria are grown on a radioactive carbon com- 

 pound (glucose) plus a non-radioactive amino acid, virtually all of that 



TABLE I 

 Inhibition of Aspartate Transcarbamylase 



Inhibitor 



Molar Concentration 



Inhibition 

 (per cent) 



25 

 40 

 70 

 100 

 25 

 10 

 



