346 2. ANALOGS OF ENZYME REACTION COMPONENTS 



(5) The different potencies of the substituted benzoates would be explained 

 on the basis of the effects, such groups would have on resonance and the 

 magnitude of the positive charge, but in addition other factors must play 

 a role, for example the dipole moments of the ring X bonds and the possible 

 interactions of the substituent groups themselves. One might expect ortho 

 groups to decrease the binding and it is true that all are less inhibitory than 

 benzoate. Bartlett pointed out that the inhibition increases markedly with 

 the electronegativity of the halogens. It is very interesting to attempt to 

 interpret the results in Table 2-15 but without more knowledge about the 

 nature of the binding any hypotheses must be vague for the time being. 



(6) It is unlikely that the degree of ionization of the carboxyl group is 

 important here because all of this work was done between pH 7.6 and 8.3 

 where a negligible fraction is undissociated. The p/iT^'s of all the substituted 

 benzoates tested run from 2.85 to 4.65. However, the series of phenols 

 studied by Frisell et al. (1956) may have to be considered in terms of the 

 ionization of the OH groups when relating structure to activity. 



(7) The K-^ for some selected inhibitors were determined by Frisell 

 et al. (1956) and from these the relative binding energies may be estimated 

 (see accompanying tabulation). The 1.45 kcal/mole difference between cin- 



Source of enzyme Inhibitor 



A", Relative — AF oi binding 

 {mM) (kcal/mole) 



namate and benzoate (it is 1.52 kcal/mole from Klein's data) may be 

 attributed to a more satisfactory location of the positive charge in benzo- 

 ate. The lesser affinity of the enzyme for nicotinate compared with benzoate 

 (0.51 kcal/mole) could be due to the reduction in resonance brought about 

 by the asymmetry of the former. If the 1.94 kcal/mole difference between 

 pyrrole-2-carboxylate and furan-2-carboxylate is related to the different 

 amounts of positive charge on the nitrogen and oxygen atoms, this would 

 not be surprising since pyrrole should resonate more effectively. Finally, 

 if the positive charge theory is valid, the 3.02 kcal/mole difference between 

 benzoate and phenylacetate might indicate the amount of binding contri- 

 bution from the positive charge. The positive charge may, of course, be 

 stabilized somewhat by an enzyme anionic site with which it interacts. 



