50 3. KINETICS OF ENZYME INHIBITION 



The mechanisms by which enzymes can be inhibited may be classified 

 kinetically in terms of the deviations produced in the normal kinetics as 

 discussed in the previous chapter, or according to the exact sites and manner 

 of action relative to the molecular events of the catalysis; the former is the 

 most common method and the latter is the most difficult but preferable. The 

 kinetic analysis, along with other pertinent data from various approaches, 

 can often be a valuable aid to understanding the details of an inhibitory 

 mechanism but usually should not be an end in itself. The most important 

 thing to know about an inhibition is the exact site of action of the inhibi- 

 tor, with what substance or enzyme group it interacts. Only then can one 

 interpret the kinetic data meaningfully and derive quantitative parameters 

 that characterize the inhibition. It is often stated that an inhibitor reacts 

 with the active center of an enzyme. This is sometimes vague and ambiguous 

 since on many enzymes there are two or more regions necessary for the 

 catalysis of the entire reaction sequence, some pertaining to the substrates, 

 some to coenzymes, and some to activators. It is better, when possible, to 

 avoid the general term active center, unless the entire surface involved is 

 referred to, and use a terminology indicative of more precise localization. 



It is useful to designate the regions on the enzyme involved in the 

 binding of the different components as substrate sites, coenzyme sites, and 

 activator sites, where such distinctions may be made. These would to- 

 gether make up the active center, which would embrace all the areas on 

 the enzyme involved in the catalysis. The substrate site may, of course, in- 

 clude the activator if the substrate is bound wholly or in part through the 

 activator. The term apoenzyme will be used to designate the protein part 

 of an enzyme and these various sites may be considered as regions in the 

 apoenzyme, or occasionally in prosthetic groups, under which are included 

 nonprotein components, such as hemes, that are for practical purposes un- 

 dissociable from the apoenzyme and function as a permanent part of the 

 enzyme. 



All enzyme inhibitions are the result of an interaction of the inhibitor with 

 some component of the enzyme system. A classification of the sites of inhi- 

 bition can be based on the component primarily involved in the inhibition. 



(I) Reaction of inhibitor with apoenzyme 



A. Chemical reaction ivith specific protein groups: these groups, such as 

 sulfhydryl, amino, or phenolic groups, may react irrespective of 

 their position relative to the active center. 



B. Specific reaction with sites on the apoenzyme: the specificity of inter- 

 action here resides in the spatial pattern of matter and charge over 

 the site rather than in a simple chemical group. 



(1) Substrate site 



(2) Coenzyme site 



(3) Activator site 



