Alkaline phosphatase (ALP) optimum pH is typically around:

• A. 9–10
• B. 7–8
• C. 4–5
• D. 5–6

Answer: (A)

Enzymes have a pH at which their catalytic activity is highest, because the ionization states of active-site residues and cofactors must be just right for chemistry to proceed efficiently. Alkaline phosphatase is categorized as an alkaline enzyme, so its active site functions best when the environment is basic. At higher pH, key residues stay in the deprotonated forms needed to attack the phosphate ester and facilitate bond breaking, and the metal cofactors (like zinc) work optimally. This makes the enzyme most active around pH 9–10. At neutral or acidic pH, protonation of critical groups interferes with catalysis, so activity drops. In clinical assays, reactions are typically carried out in alkaline conditions to maximize rate and color development, reinforcing that the optimum is in the alkaline range around 9–10.