604 2. ANALOGS OF ENZYME REACTION COMPONENTS 



" Morphine = 1 mM. 



Some generalizations may be made: (1) increase in chain length increases 

 the binding energy, (2) unsaturation increases the binding by 1.0-1.2 kcal/ 

 mole, (3) each additional methylene group augments binding by approxi- 

 mately 0.3 kcal/mole, and (4) the inhibition by nalorphine is noncompetitive 

 (it was stated that this may be a slow pseudoirreversible inhibition but in- 

 cubation with morphine and inhibitors was for 2 hr. 



Dehydroshikimate Reductase 



The inhibition of this enzyme by various phenolic compounds points to 

 the manner in which the substrate is bound and the configuration of the 

 active site. Relative binding energies are given in Table 2-39. These inhibi- 

 tions are all strictly competitive. It is seen that all effective inhibitors have 

 a p-OH group, and Balinsky and Davies (1961 b) postulated from the pos- 

 sible ring configurations of shikimate that this group must lie approximately 

 in the equatorial plane. Additional OH groups increase the binding slightly 

 or not at all, so that m-OH groups seem to participate little in the binding. 

 One might expect the carboxylate group to be bound to an enzyme cationic 

 group, but this does not appear likely; e.g., the addition of a C00~ group 

 to catechol increases the binding very little, and the benzoates without a 

 p-OH are bound very poorly. The stronger reaction of the aldehyde group 

 in vanillin also indicates that the forces here are not merely electrostatic. 

 Substitution of benzoate in the o-position is detrimental to the binding and 

 this may be due to steric hindrance, as shown in the diagram of the active 

 site presented by Balinsky and Davies (Fig. 2-21). The energy of binding 

 of the p-OH group is greater than 2.3 kcal/mole and thus hydrogen bond- 

 ing may be involved. It is worth noting that the experiments were run at 



