At which pH is pepsin most active?

• A. pH 2.0
• B. pH 7.0
• C. pH 9.0
• D. pH 5.5

Answer: (A)

Pepsin is a gastric protease whose ability to catalyze peptide bond cleavage depends on the acidity of its environment. It has a strongly acidic optimum because the active site relies on protonation to maintain its shape and charge for cutting proteins. In the stomach, the very low pH created by hydrochloric acid—around 1.5 to 2.0—provides this optimal condition, so pepsin works at its highest rate there. When the pH moves toward neutral or basic, pepsin loses activity because the enzyme’s structure and the ionization state of key residues are no longer suited for catalysis. At a moderately acidic pH like 5.5, some activity remains, but it’s far lower than the peak reached at pH 2.0. So the pH 2.0 environment is where pepsin is most active.