SPECIFIC TYPES OF INHIBITION 165 



plotting procedures give curves markedly different from any type of usual 

 inhibition (Fig. 5-7). The inhibition constant K- , representing the disso- 

 ciation constant of the EIS complex into ES and I, may be readily cal- 

 culated by most of the graphical methods. It may be noticed that this is 

 the only type of inhibition considered which gives a hyperbolic-type 

 curve in type F plotting (Fig. 5-7 F). 



Inhibition in Systems Involving an Activator 



When the inhibition is noncompetitive with respect to both substrate 

 and activator, the curves obtained by all the above-described methods are 

 identical in behavior with those for noncompetitive inhibition in systems 

 without an activator (compare Fig. 5-3 and 5-8); however the values of 

 the slopes and intercepts are different. A more interesting situation arises 

 in inhibition that is competitive with the activator but not with substrate. 

 It is evident that plotting by the methods outlined will lead to curves of 

 a noncompetitive type because they have been plotted with respect to 

 the substrate. However, if the activator is essential for the binding of the 

 substrate, the curves will be of a competitive type (compare Fig. 5-9 and 

 5-10) because in such a system the inhibitor is essentially competing with 

 both activator and substrate, although with the latter indirectly. The 

 finding of competitive type curves can then mean that the inhibitor is 

 primarily reacting with the activator site and not with the substrate site. 

 If this is not realized, the calculations of constants will be in error. 



The true situation and accurate evaluation of constants can be attained 

 by plotting with respect to activator as in Fig. 5-11 to 5-13. The most 

 straightforward method is perhaps the single-curve plotting of type F 

 because the constants are directly obtainable but it has been rarely, if 

 ever, utilized. The method of plotting Ijv^ against 1/(S)(A) has been il- 

 lustrated for it has been used occasionally but it presents no obvious ad- 

 vantages. As pointed out in Chapter 3, the situation involving coenzymes 

 or donor-acceptor reactions follows activator kinetics and the plotting pro- 

 cedures are similar. In any case, where there is a second component in the 

 enzyme system, it is well to investigate the effects upon the inhibition of 

 varying its concentration, plotting the results with respect to this compo- 

 nent as well as substrate. 



Reaction of Inhibitor with Substrate 



The usual methods of plotting in this case lead to deviations from lin- 

 earity (Fig. 5-14). It is not difficult to detect such inhibition but the con- 

 stant K^j^ for the dissociation of the SI complex is not readily obtainable 

 from the curves. A procedure suggested in Chapter 3 is perhaps more prac- 

 tical (see Eqs. 3-73 and 3-74). 



