282 



6. INTERACTIONS OF INHIBITORS WITH ENZYMES 



the experimental values, from which it is probable that the reduced inter- 

 action energy is indeed due to the increasing separation between the charge 

 centers. 



Effect of Group Position on Interaction Energy 



The antibody binding site for the antigenic group must be patterned 

 quite closely to the van der Waals' surface of this group because replace- 

 ment of the small hydrogen atom on a benzene ring by a large group reduces 

 the interaction energy. The energy decrease attending the substitution of 

 a chlorine atom is pictured in Fig. 6-18 for a typical hapten-antibody 

 interaction. The fact that different energy decreases are observed for the 



Fig. 6-18. The steric repulsion energies for cliloro-siibstituted haptens reacting with 

 antibodies to o-, 7n-, and ^j-azophenylazobcnzoate. The numbers show the decrease in 

 over-all binding energy when chlorine atoms are substituted in the positions indi- 

 cated; the values are in cal/mole and have been recalculated from the combining con- 

 stants given by Pressman et al. (1954). The outlines give approximately the van der 

 Waals' contovu's of the antigenic groups. 



various ring positions means that the closeness of fit of the antibody around 

 the hapten is not the same everywhere. It is generally true that ortho substi- 

 tution produces the greatest energy change, indicating that the fit is closer 

 here than in the meta position, as is seen in Table 6-21 where the results 

 with substituted benzoates are given. Para substitution results in increased 

 binding, of course, because the group then fits in the region formed around 

 the — N=N— group. An increase in size from the hydrogen atom to the 



