SPECIFIC TYPES OF INHIBITION 157 



The substrate constant that is calculated would thus be: 



K. ,' = K. , 



PK, ^ {A) 



(5-20) 



Determination of iiT, would normally involve dividing the slope of the inhi- 

 bited curve bj^ the slope of the uninhibited curve: 



slope. , = E slope . , 



inh uninh 



and E. would be set equal to 1 -f {1)!K,. However, when an activator occurs: 



so that the calculated K/ will be less than the true K^ by a factor that may 

 be appreciable. Of course, if the occurrence of the activator is recognized 

 and its effects are determined, it is possible to calculate the true K^. It 

 may be observed that if the inhibitor or enzj^me active site has groups 

 that dissociate within the experimental pH range, the calculated K/ will 

 include a factor in (H+) and thus depend on the pH. 



SPECIFIC TYPES OF INHIBITION 



The basic types of inhibition on simple systems — completely and par- 

 tially competitive and noncompetitive, and mixed — give the curves 

 shown in Fig. 5-1 and 5-3 to 5-6. The marked differences between the 

 curves in completely competitive and noncompetitive systems make these 

 two types of inhibition easy to distinguish if the data and plotting are 

 accurate, whatever system of plotting is used. However, a problem arises 

 if the inhibition is partially competitive or noncompetitive, because in 

 general the curves will be of the same basic nature but with modified 

 slopes or intercepts. If this is not realized it will lead to errors in the cal- 

 culation of the constants. As an example we shall take a case of partial 

 competitive inhibition where the inhibitor does not prevent binding of 

 substrate [a = oo) but only increases the dissociation constant five-fold 

 {a = 5). The ratio R between the slopes of the inhibited and uninhibited 

 curves in a type A plot would normally be expected to be equal to 1 + 

 {l)jK^ but when a 7^ oo its value is given by: 



aKi + a(I) 

 aKi + (I) 



Thus the calculated K- is not the true ^, but: 



E 



K/ = K, 



(5-23) 



