Effect of pH on Enzyme Stability and Activity

If Proinebrium is toxic, then we can reduce its blood concentration 19-fold (to [I] == 2 X 10 M) and still achieve 50% inhibition. 

Consider a system in which the substrate is a weak acid, HA, but only the ionized form, A, binds to the enzyme. The true substrate then is A. At a fixed concentration of total weak acid, the proportion that is in the proper ionic form can be calculated from the Henderson-Hasselbalch equation. Thus, at pH s= pKa, half of the total concentration is present as A. When the pH is one unit above the p-K , 10/11 of the total is present as A (i.e., the ratio of [A ]/[HA] = 10). When pH = pK +2, 100/101 of the total is present as A. When the pH is one unit less than pK, 1/11 of the total is present as A,  

A complete analysis of pH effects can be quite complicated, especially when we consider that many biological compounds possess multiple ionizable groups and that the active site of an enzyme may also possess two or more ionizable groups that must be in the proper ionic form before substrate binding or catalysis occurs. 

The active site of an enzyme contains a single ionizable group that must be in the negative form before the substrate can bind and catalysis occur. The pK of this group is 5.0. The substrate is a positively charged compound and remains completely ionized over the pH range studied, (a) Write the reactions showing the effect of pH on the distribution of enzyme species, (b) Derive a velocity equation for this system, (c) What would a plot of v versus pH look like  

for this simple monoprotic system, H acts as a competitive inhibitor. As [H ] increases (i.e., pH decreases), the enzyme is driven to the inactive, dead-end EH form. 

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