Question

The activity of an enzyme can be altered by changing the pH of its surroundings slightly. This change in pH works by

Answer 1

By changing the proton concentration (H+), pH alters the ionization (protonation state) of ionizable groups on the enzyme and on the substrate. That does several things:

- Changes charges on active‑site amino acids (e.g., His, Asp, Glu, Lys, Cys, Tyr), so catalytic residues involved in acid‑base catalysis can lose or gain protons and stop functioning.
- Alters substrate binding because ionic and hydrogen bonds between enzyme and substrate are weakened or strengthened.
- Modifies tertiary structure by disrupting salt bridges and hydrogen bonds, changing the active‑site geometry (small shifts are usually reversible; extreme shifts can denature the protein irreversibly).
- Changes the ionization state of the substrate itself, affecting how well it fits or reacts in the active site.

Net result: enzyme activity shows a pH optimum; moving pH away from that optimum reduces activity by impairing binding and catalysis (and at extremes causes denaturation). Examples: pepsin works best at low pH, trypsin at neutral–slightly basic pH.