186 



5. DETERMINATION OF MECHANISMS AND CONSTANTS 



from which the plotting equations may be derived. The rate equations for 

 inhibition in zone B^ are too complex to be useful. We have already given 

 equations from which K^ may be determined for noncompetitive systems 

 (Eqs. 3-42 and 3-44) and it is also possible, but more laborious, to do so for 

 competitive cases. If at constant substrate concentration one determines 



lOmM 



0.1 -y 



(I) 



01 



I 

 (S) 



lOOmM 



Fig. 5-23. Isobologram for the situation in which the inhibitor 

 reacts with the substrate. 



accurately the degree of inhibition at two inhibitor concentrations (as 

 different as is practically possible), the value of K^ may be found from: 



J^i 



*2(I<)l - h(I<)2 



i.i. [1/(1 



1/(1 -^■,)] [1 +(S)/if,] 



(5-37) 



A potent inhibitor may either form a simple complex with the enzyme 

 or give rise to an EI complex by a chemical reaction associated with the 

 splitting off of a part of the inhibitor molecule. Krupka and Laidler (1959) 

 treated these situations kinetically and showed how they might be distin- 

 guished. The equations may be written as follows: 



E + I 



EI: 



(I< 



*(E,) = 



1 + 



(S) 



K, 



(5-38) 



