The following pH dependence was found for the activity of a certain enzyme-catalyzed reaction.
If it is known that the only two ionizable residues in the active site are both glutamates,which conclusion can be drawn?
A)The glutamates have different microenvironments which cause their pKa's to differ.
B)One of the glutamates must be amidated.
C)Both glutamates have a pKa equal to 5.0.
D)Both glutamates are deprotonated during the reaction.

Question

The following pH dependence was found for the activity of a certain enzyme-catalyzed reaction. 
If it is known that the only two ionizable residues in the active site are both glutamates,which conclusion can be drawn?
A)The glutamates have different microenvironments which cause their pKa's to differ. 
B)One of the glutamates must be amidated. 
C)Both glutamates have a pKa equal to 5.0. 
D)Both glutamates are deprotonated during the reaction.

Quizplus · Accepted Answer

The pH dependence indicates that the enzyme activity is affected by the protonation state of the glutamate residues. If both glutamates had the same pKa, the activity would be expected to be affected by changes in pH around that value. However, the fact that the pH dependence curve is broad suggests that the glutamates have different pKa values, indicating different microenvironments. Therefore, choice A is the best answer. Choices B, C, and D are not supported by the information given.

The Following PH Dependence Was Found for the Activity of a Certain