Aspartate and lysine are in the active site of an enzyme.They are both known to participate directly in catalysis.The pKa's of the residues are found to be 3.2 and 9.6,respectively for aspartate and lysine.The optimum pH for the enzyme is 6.4.Which forms of these two residues will predominate when the enzyme is most active?
A)Aspartate is protonated;lysine is deprotonated.
B)Both residues are protonated.
C)Aspartate is deprotonated;lysine is protonated.
D)Both residues are deprotonated.

Question

Quizplus · Accepted Answer

At pH 6.4, the aspartate residue will exist in its deprotonated form (pH>pKa) and the lysine residue will exist in its protonated form (pH<pKa). Since both residues participate in catalysis, the deprotonated aspartate and the protonated lysine forms will predominate when the enzyme is most active. Therefore, choice C (Aspartate is deprotonated; lysine is protonated) is the best choice.

Aspartate and Lysine Are in the Active Site of an Enzyme.They