An enzyme speeds up a reaction but is not used up by it. It lowers the reaction's activation energy — the "push" needed to start — so the reaction runs fast at the cell's gentle temperature. One enzyme molecule handles substrate after substrate.
Each enzyme has a pocket — the active site — whose shape is complementary to its substrate. The substrate slots in like a key into a lock. Because only the right shape fits, each enzyme is specific. ▶ Play to watch the substrate dock.
Once bound, an enzyme–substrate complex forms; the reaction occurs; the products — now a different shape — no longer fit and are released; the active site is free for the next substrate.
Warming speeds molecules up, so the rate rises to an optimum (about 37 °C in humans). Past it, heat shakes the protein apart, the active site loses shape, and the rate collapses. ▶ Play to sweep the temperature.
Beyond the optimum the enzyme is denatured: the bonds holding its fold break, the active site's shape is destroyed, and it cannot recover on cooling. (Cold only slows an enzyme — it is not denatured.)
The active site only holds its shape over a narrow pH range. Pepsin works best in the acidic stomach (pH ≈ 2); salivary amylase in the neutral mouth (pH ≈ 7). Far from the optimum, activity falls away. ▶ Play to sweep the pH.