52 3. KINETICS OF ENZYME INHIBITION 



the various possibilities and in understanding the mechanisms and kinetics 

 of the inhibitions. Some of the numerous types of interference are repre- 

 sented schematically in Fig. 3-1 for one enzyme system. In the classification 

 above, reaction with the substrate site was put under a single heading; 

 it is obvious that there are various ways in which a substance can prevent 

 the substrate molecule from reaching this site. It may combine with the 

 entire site, much as the substrate does (A); or it may react with only part 

 of the site (D); or it may react not with the site itself but combine at an 

 adjacent region, sterically or electrostatically repelling the substrate (F). 

 Reactions of other types (G, H, J, and K) may also interfere with the forma- 

 tion of the ES complex. In fact, it is possible that varying degrees of inter- 

 ference may be produced and that inactivation of an enzyme may be partial 

 or complete and not necessarily an all-or-none phenomenon. Chemical reac- 

 tion with specific protein groups has been diagrammed only for the substrate 

 site but it could occur at any or all sites. It must also be understood that 

 the enzyme surface illustrated is only one of numerous possibilities; e.g., 

 the activator may be involved in binding the coenzyme rather than the 

 substrate, or the activator might link the substrate and coenzyme without 

 reacting itself with the apoenzyme, or the spatial relations between the 

 substrate and the coenzyme may be more intimate than indicated, the 

 mutual configurations they assume in the activated complex being obliga- 

 tory for reaction. 



REVERSIBLE AND IRREVERSIBLE TYPES OF INHIBITION 



It is of primary importance for the kinetics of inhibition to distinguish 

 between inhibitors that react reversibly and irreversibly. In the equilibrium: 



E + I ;^ EI (3-1) 



with K^ = k_Jki = (E)(I)/(EI), irreversibility may be rigorously defined 

 by equating it with the situation in which k_i = 0. Since such complete 

 irreversibility seldom occurs and since the ^,'s of the so-called irreversible 

 inhibitors are generally finite although small, it is evident that k_i is not 

 zero in most cases and a more practical definition of irreversibility must be 

 made. If the inhibited enzyme is placed in a solution not containing the 

 inhibitor, or dialyzed against inhibitor-free solution, irreversibility may be 

 characterized by the state in which restoration of enzyme activity is not 

 significant over an interval commensurate with the periods involved in the 

 kinetic experiments. Slow reversibility can be demonstrated with stable 

 enzymes, and kinetic experiments over intervals of hours may be performed 

 on this basis, but with very unstable enzymes many inhibitors must be con- 

 sidered to be irreversible with regard to the kinetics because insignificant 



