POSSIBLE SITES AND MECHANISMS OF INHIBITION 247 



site subsequent to binding. Inasmuch as substrates occasionally form a 

 temporary chemical bond with the enzyme, an analog may do likewise but 

 fail to complete the reaction, remaining chemically attached to the site. 



(C) Binding of the analog to an enzyme site other than that with which 

 the normal reactant interacts. Such binding may be simply fortuitous or 

 the site may be specifically for the purpose of allowing feedback inhibition 

 by a product formed in the sequence in which the enzyme participates. 

 Regions outside the catalytic areas with which inhibitors can react are of- 

 ten called allosteric sites. 



(D) The analog may be a substrate of the enzyme and will inhibit the 

 reaction of the normal substrate to a degree dependent on the relative 

 binding affinities and reaction rates. 



{E) Binding of the analog to a complex of the enzyme with the normal 

 substrate, coenzyme, or activator. 



(F) The formation of a molecular complex of the analog with the normal 

 reactant as a result of their structural complementarity. Although such 

 complexes are probably uncommon and have seldom been considered in 

 work with analogs, we shall see that examples of this mechanism are known. 



(G) Inhibition by a mechanism only indirectly related, or completely 

 unrelated, to the structural similarity of the analog to the normal substra,te. 

 An analog may, for example, possess chelating properties not exhibited 

 by the substrate, or it may react with SH or carbonyl groups. 



When one is investigating more complex systems, particularly cellular 

 preparations, a number of other mechanisms for analog inhibition may be 

 proposed, and these should be added to the above list. 



(H) The analog may interfere with the transport mechanism by which the 

 normal substance is taken through the cell membrane, since the two sub- 

 stances may both combine with some membrane carrier or enzyme system 

 required for efficient transport or accumulation. 



(/) The analog may not be the actual inhibitor, but may be transformed 

 through a metabolic sequence into a substance which blocks a later reaction, 

 a process frequently termed lethal synthesis. In certain instances the analog 

 may complete a long and complex metabolic journey to terminate as a 

 component in some important cellular product. The incorporation of pyrim- 

 idine and purine analogs (e.g., the 5-halouracils, 2-thiouracil, 8-azagua- 

 nine, and 8-azathymine into RNA and DNA) and amino acid analogs 

 (e.g., tryptazan, 7-azatryptophan, ethionine, p-fluorophenylalanine, and 

 /5-2-thienylalanine into proteins) has been frequently demonstrated. The 

 products containing the analogs may be so abnormal as to fail to function 

 properly in the cells, thereby producing far-reaching and complex dis- 

 turbances. 



