204 6. INTERACTIONS OF INHIBITORS WITH ENZYMES 



(F) The fact that most active sites comprise amino acid side-chains with 

 acidic or basic groups makes it necessary to consider the effects of pH 

 on the constants of inhibition and the interaction energy. 



(6r) Since the active site may bind water or ions, the effects of these on 

 the topography and electrical fields of the site may be important in the 

 reaction with an inhibitor. Also the energy of displacement of these sub- 

 stances must be included in the total interaction energy. 



(H) The possibility that some active sites may be within holes or crev- 

 ices introduces an additional factor into the question of the specificity 

 of inhibition because steric or electrostatic repulsion may prevent the inhi- 

 bitor from entering such spaces, even though the inhibitor could react 

 readily with the groups within the cavity. 



(/) The region surrounding the active site, although not catalytically in- 

 volved, can be of importance in inhibition inasmuch as an inhibitor might 

 react outside the site and yet produce steric or electrostatic influence on 

 the binding and reaction of the substrate. The inhibitor does not have to 

 react at the active site and groups that might be indicated by this inhibi- 

 tion do not have to participate in the catalysis. 



(J) The fact that the active site might include sections of a folded poly- 

 peptide chain or two or three turns in a helix or might involve two adjacent 

 helices makes it possible for substances that can alter the tertiary protein 

 structure, the configuration of the polypeptide chain in space, to modify 

 enzyme activity. Reactions of inhibitors with hydrogen bonds or the various 

 types of cross-linkages may occur and distort the active site, so that again 

 inhibition does not necessarily imply reaction at the active site. 



A variety of interaction forces, both attractive and repulsive, are usually 

 involved in the binding of an inhibitor to an enzyme and to evaluate these 

 it is necessary to know the chemical nature and configuration of the enzyme 

 surface. The knowledge of inhibition mechanisms and the accuracy in the 

 calculation of the interaction energies involved will progress with the ad- 

 vance in active site delineation. Indeed, the use of inhibitors is one of the 

 most powerful tools in the elucidation of the nature of the active site. 



INTERMOLECULAR FORCES AND INTERACTION ENERGY 



The forces binding inhibitor molecules to enzymes are those recognized 

 by physicists as occurring generally between various types of molecules. 

 That is, all inhibitor interactions can be adequately explained on the basic 

 of a few fundamental well-known forces and there is no necessity to pos- 

 tulate any unique effects inherent only in protein or enzyme reactions. 

 The only difference between enzyme-inhibitor interactions and interactions 



