Explain why an enzyme stops working when the pH moves far from its optimum, and describe how this differs from the effect of temporarily lowering the temperature on the same enzyme.

Each enzyme has an optimum pH at which it works best. When the pH moves far from this optimum, the ionisation state of amino‑acid side chains in the active site changes, breaking the hydrogen bonds and ionic interactions that maintain its shape. The active site is altered or denatured, so the substrate can no longer bind and the enzyme becomes inactive. This denaturation is permanent – the enzyme cannot regain its original structure.

In contrast, lowering the temperature reduces the kinetic energy of molecules, slowing the rate of the enzyme‑catalysed reaction. The enzyme’s structure is unchanged and the effect is reversible; activity returns when the temperature rises again.