58 3. KINETICS OF ENZYME INHIBITION 



(3) Completely noncompetitive inhibition {a = \ and /? = 0): the inhibitor 

 does not alter the binding of substrate but prevents its transformation into 

 products. 



- ^' ^^' (3-16) 



(I) + K, (S) + K, 



= (^) 

 (I) + K, 



(3-17) 



(4) Partially noncompetitive inhibition {a = \ and < /3 < 1): the binding 

 of substrate is not changed but the rate of breakdown of the EIS complex 

 is slower than the ES complex. 



WD + A-, (S) 



' = » - « (TTT^. '^-''' 



(5) Mixed inhibition (oo > a > 1 and fi = 0): the affinity of the enzyme for 

 the substrate is reduced by the inhibitor and the breakdown of the EIS com- 

 plex is prevented. 



alii (S) 



(I) +aK, (S)-f7ir,{[«(I)+aAM/[(I) + aK,]] 



. ^ (I) 



' {\)+K,{[oc(^)+aK,]i[{^)+aK,]) 



(3-20) 



(3-21) 



The inhibitor in competitive inhibition is said to change the Michaelis 

 constant (which we have assumed for convenience here to be K^) without 

 affecting the maximal rate, as indicated by Eqs. 3-12 and 3-14; in non- 

 competitive inhibition it is the maximal rate that is changed and the K^ 

 is unaltered, as in Eqs. 3-16 and 3-18. The fifth type of inhibition is called 

 mixed because both F„; and K^ are modified by the inhibitor and thus the 

 inhibition partakes somewhat of the characteristics of both competitive 

 and noncompetitive inhibition. Of course, mixed inhibition occurs if /? is 

 less than 1 and a is greater than 1 (but not very large or infinity) and the 

 relevant Eqs. are 3-9 and 3-11; one might designate it completely mixed 

 inhibition when /? = and partially mixed inhibition when < /? < 1. 

 It is necessary to understand that the term partially when applied to com- 

 petitive and noncompetitive inhibition does not indicate mixture of inhi- 

 bition types or impurity; it means that the inhibition is still purely com- 

 petitive or noncompetitive, but the degree of inhibition is less than in the 

 completely competitive or noncompetitive types, due to the fact that in 

 the former the binding of substrate is not completely prevented or in the 

 latter the breakdown of the EIS complex is not completely reduced to zero. 



