Acid phosphatase (ACP) often has optimal activity at pH approximately:

• A. 4–5
• B. 7–8
• C. 9–10
• D. 6–7

Answer: (A)

Enzyme activity is highly pH-dependent because the ionization states of amino acids in the active site and of the substrate must be just right for catalysis. Acid phosphatase is an acid hydrolase, so it operates best in an acidic environment. This means its catalytic efficiency peaks at a relatively low pH, around 4 to 5, where the active-site residues and substrate are in their optimal charged forms for hydrolysis of phosphate esters. In practice, assays for acid phosphatase use a buffer around pH 4.5–5 to maximize activity. At neutral or alkaline pH (near 7–8 or 9–10), the enzyme’s active site is not in its ideal state, so activity drops markedly. That’s why the acidic range around pH 4–5 is the correct choice.