142 



4. SUBSTRATE INHIBITION AND PRODUCT INHIBITION 



product inhibition can occur even in reactions where the equihbrium is 

 markedly in favor of the product. This is due to binding of product to the 

 sites involved in the reaction, even though it does not react. The inhibition 

 produced by the product in the general case is: 



. (P') [1 + (vjvj {1 + i/(s:)}] 



(P') + 1 + (S') 



(4-40) 



which is the same as the competitive inhibition expression 3-28 except for 

 the factor by which (P') is multiplied. The ratio VplV„i may be replaced 

 by KqK^IK^i. If there is only substrate at the beginning of the reaction 

 and the product is formed so that (S) + (P) is constant, the inhibition will 

 increase and the rate will fall as shown in Fig. 4-16. If the product is present 



20 



30 



40 



50 



60 



70 



90 lOOmM 



CS)- 



FiG. 4-17. Product inhibition by the reverse reaction. In the 

 upper graph A the rate is i^lotted against the substrate concen- 

 tration and in the lower graph B the fractional inhibition is 

 plotted against the substrate concentration. A'^ = 5 \\\M and 

 Kj, = 10 mil/. The concentration of product is constant at 10 

 ml/. Curves I: VJV„ = 1; curves II: VJV^ = OO. 



