292 6. INTERACTIONS OF INHIBITORS WITH ENZYMES 



the 4— CH3 derivative is only 0.16 kcal/mole more stable than the unsub- 

 stituted ion, it may be concluded that 0.51 kcal/mole originates from the 

 steric effect the 4— CH3 group has on the 3 — OH group, making config- 

 uration (II) above statistically more probable and more favorable to 

 hydrogen bond formation. 



Separation of the benzene ring from the trimethylammonium ionic group 

 by one — CHg— group leads to a reduction in binding as it did in the 

 hapten-antibody interactions discussed previously. However, the fit here 

 on the enzyme is not as critical and only 0.68 kcal/mole is lost from the 

 binding energy by this separation. 



INTERACTION FORCES IN VARIOUS ENZYME SYSTEMS 



Four more enzyme systems will be briefly discussed to illustrate the 

 various types of interactions and the problems involved in the calculations 

 of the binding energies. 



Inhibitors of Carboxypeptidase 



Ion-ion type interaction is known to be important in the binding of 

 substrates and inhibitors to peptidases but Smith et al. (1951) have presented 

 evidence that van der Waals' forces are also involved. The inhibition con- 

 stants and calculated binding energies for a number of compounds with 

 crystalline pancreatic carboxypeptidase are shown in Table 6-26. Car- 

 bobenzoxyglycol amino acids are hydrolyzed by the enzyme as follows: 



O I R 



Jl II! I 



<p-CH2-0-C-NH-CH2-C-NH-CH-COO~ 



I 

 I 



Si)litting occurs at the dotted line and the nature of the K-chain is important 

 in determining the rate of the reaction. Interactions of the terminal —COO 

 and the peptide carbonyl group probably occur, but van der Waals' inter- 

 action of the R-chain provides a significant contribution to the total binding 

 energy. Some insight into these interactions may be obtained from a study 

 of the inhibitors in Table 6-26. 



The differences in the binding energies of these inhibitors may be attri- 

 buted mainly to van der Waals' forces, and particularly to dispersion forces. 

 A side-chain of sufficient size on the —COO" group is necessary for binding, 

 inasmuch as acetate is scarcely bound. In the aliphatic acid series, the 

 addition of a — CH.2— group results in about 1.5 kcal/mole more binding 

 energy from acetate to butyrate, but further additions have little effect. 

 One might conclude that there is a region on the enzyme, about 6-8 A from 



