Enzyme X exhibits maximum activity at pH = 6.9. X shows a fairly sharp decrease in its activity when the pH goes much lower than 6.4. One likely interpretation of this pH activity is that:
A) a Glu residue on the enzyme is involved in the reaction.
B) a His residue on the enzyme is involved in the reaction.
C) the enzyme has a metallic cofactor.
D) the enzyme is found in gastric secretions.
E) the reaction relies on specific acid-base catalysis.

Question

Quizplus · Accepted Answer

The sharp decrease in activity below pH 6.4 suggests involvement of a histidine (His) residue, as its side chain has a pKa around 6, making it sensitive to changes in pH around this value. Histidine can act as a proton donor or acceptor near its pKa, playing a crucial role in enzyme catalysis at this pH range.

Enzyme X Exhibits Maximum Activity at PH = 6