VARIATION OF ENZYME INHIBITION WITH 1)H 695 



the substrate or the inhibitor to the active center, especially if the binding 

 involves ionic interactions. One may divide such electrostatic effects into 

 those originating from vicinal groups near the active center and to those 

 resulting from changes in the total over-ail charge of the enzyme protein. 

 The value of K^ is determined by the energy of interaction of the inhi- 

 bitor with the enzyme. The effect of a vicinal group on K^ would then be 

 calculated from the additional interaction energy term derived from the 

 electric field of this group and the position of the inhibitor in this field. 

 A simple system is diagramed in Fig. 14-12. where a positively-charged 

 inhibitor is repelled by a positively-charged vicinal group at a distance 

 d from the bound inhibitor. The affinity of the enzyme for the inhibitor 

 will be less and the p/i^, lower than when the vicinal group is not protonated. 



Fig. 14-12. Schematic illustration of the 

 effect of a vicinal ionizing group on inhi- 

 bitor binding at the active center (cross- 

 hatched area). The positively charged 

 inhibitor is repelled by the positively 

 charged vicinal group. 



If we start with the pH above the piiC^ of the vicinal group, this group will 

 not be charged and the interaction of the enzyme with the inhibitor will be 

 restricted to the active center: when the pH is lowered, the vicinal group 

 will eventually become protonated so that in the region of pH around the 

 Y>K,^ there will be a change in K^. If pA"/ is plotted against pH, there will 

 be a jog in the curve, but not an inflection to a new slope. Some types of 

 expected effects are shown in Fig. 14-13. When the pA'/ increases due to 

 this mechanism, the charges on the inhibitor and the vicinal group are 

 opposite (curve A) and when it decreases, they are of the same sign (curve 

 B). A similar jog may occur in those sections of the curves with slope — 1, 

 + 1, etc. (curve C). The degree of interaction with the vicinal group is 

 equated with the vertical deviation, J/jA,. 



For the state in which the vicinal group is uncharged and does not con- 

 tribute to the interaction, the binding free energy can be written as zJ-Fj = 

 — 2.3RT pA, , and for the state in which the vicinal group is protonated, 

 zli^.3 = — 2.2,RT pA,^. The change in pA, is then given by: 



