At pH 9, pepsin activity is

• A. High activity
• B. Moderate activity
• C. Low activity
• D. Zero activity

Answer: (D)

Enzyme activity is dictated by pH because it changes the charge states of amino acids in the active site and can alter the protein’s shape. Pepsin is a protease that works best in very acidic conditions, with an optimum around pH 1.5–2.0. At that acidity, the active site residues are properly protonated to catalyze peptide bond cleavage and the enzyme maintains the conformation needed to bind substrates.

When the environment becomes highly basic, such as pH 9, those key residues lose the right protonation states for catalysis, and the overall structure of pepsin can become destabilized. This combination almost completely shuts down its catalytic function, so pepsin activity drops to essentially zero at that pH.

In short, the correct interpretation is that pepsin is inactive at pH 9 because its acidic optimum is far from that pH and the enzyme can be denatured or lose catalytic ability in alkaline conditions.