250 2. ANALOGS OF ENZYME KE ACTION COMPONENTS 



(E) 111 cellular systems, or possibly in subcellular preparations of some 

 structural complexity, the failure of the analog to penetrate to the site of 

 inhibition as readily as the substrate would obviously distort the kinetics, 

 although the inhibition itself is truly competitive. 



{F) If the substrate possesses several groups through which it is bound 

 to the enzyme, an analog which has only one of these groups (perhaps an 

 analog comprising only a part of the normal substrate molecule) may block 

 off one enzyme attachment point but allow the substrate to bind through 

 the remaining groups. In many cases this will prevent catalysis, but in 

 others it could only reduce the rate at which the substrate is reacted. 

 Simultaneously there will be a reduction in the affinity of the enzyme for 

 the substrate. The plots will give evidence of a mixed inhibition, which is 

 the actual situation, but nevertheless competition of a type is occurring. 



Analogs of coenzymes often present a special problem in this connection 

 since the coenzyme may be bound quite tightly to the apoenzyme. The 

 addition of analog to the complete enzyme may not result in significant 

 displacement of the coenzyme and little inhibition will be observed. How- 

 ever, if the enzyme is resolved into its components by dissociating the coen- 

 zyme in some manner, competition between the coenzyme and analog may 

 be demonstrated in recombination experiments in which the analog reduces 

 the ability of added coenzyme to reactivate the enzyme. 



The type of inhibition may depend on the concentration of the analog. 

 It has been noted several times that an inhibition may be competitive at 

 low analog concentrations and partially or completely noncompetitive at 

 higher concentrations. The noncompetitive elements of the inhibition 

 probably reflect the increasing unselectivity of action that is a common 

 property of all inhibitors when the concentration is raised beyond a cer- 

 tain level. 



An analog of a substrate will occasionally undergo reaction in the presence 

 of the enzyme and, since both substrate and analog bind at the same site 

 on the enzyme, competition will occur. The behavior in such a situation 

 was discussed in Chapter 1-3 (page 96). The inhibition observed will depend 

 on what is determined. If we designate the substrate by S and its analog 

 by S': 



E + S ^ES— >E + P (2-1) 



E + S' ;?t ES' ^ E + P' (2-2) 



and the individual rate expressions may be written as: 



F„.(S) 

 {S)+K, 1 + Vv 



A, 



