VARIATION OF ENZYME INHIBITION WITH pH 



679 



of plotting may be readily obtained in a similar way. We shall write the 

 appropriate equations for each of the six types of inhibition. 



(a) Competitive inhibition on the active form. HE (scheme 14-52) 



1 



+ 



K. 



VJS) 



Ki ^ (H) 



1+^(1 +A/) 



+ 



Ksil) 



(1)(1 - i)li - (S): 



(I)(l - t) 



fke'Ki + 



VraKA^) 



K, 



(14-81) 

 (14-82) 

 (14-83) 



Comparing these equations with those obtained for the nonionizing case 

 (see Eqs. 5-1, 5-5, and 5-7), it is seen that a type A plot will have the same 

 intercept on the l/i', axis but the slope will be greater due to the term K^j 

 (H). If K^ is calculated from the slope (s), the following expressions are ob- 

 tained: 



Nonionizing: 



Ionizing: 



K, 



Ki 



K.{1) 



^ ' m Jhe -"-a 



(14-84) 



(14-85) 



The calculation of K^ will thus be in error at pH's near or above the piiC^ 

 if the ionization is not considered. A plot of type D, on the other hand, gives 

 a different intercept but the same slope as the nonionizing case. The calcu- 

 lation of K^ from the slope will not be in error since the slope doesn't de- 

 pend on the pH. A type F plot will give an intercept of f/^/K^ and a slope 

 of KJK, so that a calculation of K, from the intercept, which is usual, 

 must take the ionization into account. 



(6) Competitive inhibition on the active fortn E {scheme 14-56) 



What has been said above api^lies equally here and the only difference is 

 the replacement of f/^/ by //. 



