Living cells run thousands of chemical reactions every second — at body temperature, in water, gently. They manage it because of enzymes: protein catalysts that make reactions fast enough for life.
An enzyme is a biological catalyst — a substance that speeds up a chemical reaction without being used up or permanently changed by it. Because the enzyme is released unchanged at the end, a single molecule can be used over and over, so cells need only tiny amounts.
Almost all enzymes are globular proteins: long chains of amino acids folded into a precise three-dimensional shape. That exact shape is the whole secret of how they work — and, as we'll see, the reason heat and pH can wreck them.
An enzyme has a small pocket on its surface called the active site. The molecule the enzyme acts on — the substrate — fits into this pocket. The shape of the active site is complementary to the shape of the substrate, like a key fitting a lock. This is the lock-and-key model.
Because the fit is so specific, each enzyme works on only one substrate (or one type of reaction) — this is enzyme specificity. (A refinement, the induced-fit model, says the active site moulds slightly around the substrate as it binds — a glove shaping to a hand.)
Once the substrate is held in the active site, an enzyme–substrate complex forms, the reaction happens, the products are released, and the active site is free to bind the next substrate:
Most enzymes are named after their substrate or reaction, with the suffix –ase:
Hydrogen peroxide (H₂O₂) forms in cells but is poisonous, so it must be removed fast. Catalase — packed into liver and potato cells — breaks it down almost instantly:
The oxygen given off makes the froth you see when liver is dropped into peroxide. You can measure exactly how this rate changes in the practical.
Three conditions matter most for Class XI: temperature, pH, and the amounts of substrate / enzyme.
As temperature rises, molecules move faster and collide more often, so the rate increases — roughly doubling for each 10 °C — up to the enzyme's optimum temperature (about 37 °C for human enzymes). Beyond the optimum the rate falls sharply, because heat shakes the protein apart and the active site loses its shape — the enzyme is denatured. Denaturation is permanent: cooling does not bring activity back.
Each enzyme has an optimum pH at which its active site holds the right shape. Move far from it — more acidic or more alkaline — and the rate drops as the active site is disrupted. Optima differ by where the enzyme works: pepsin ≈ pH 2 (the acidic stomach), salivary amylase ≈ pH 7 (neutral mouth).
With plenty of enzyme, adding more substrate raises the rate — until every active site is busy; then the rate levels off (saturation). Likewise, adding more enzyme raises the rate as long as substrate is available.
| Factor | Effect as it increases | Why |
|---|---|---|
| Temperature | Rises to an optimum (~37 °C), then crashes | Faster collisions, then denaturation of the active site |
| pH | Peaks at the optimum pH, falls either side | Wrong pH distorts the active site's shape |
| Substrate conc. | Rises, then plateaus | Active sites become saturated |
Denaturation is the loss of an enzyme's specific 3-D shape — and therefore its function — caused by high temperature or an extreme pH. The bonds holding the fold together break, the active site changes shape, and the substrate no longer fits. Because the precise fold cannot reassemble on its own, denaturation is essentially irreversible. (Cold, by contrast, only slows an enzyme — it is not denatured and works again on warming.)
Enzymes explain why we digest food, why a fever above ~40 °C is dangerous (our own enzymes start to denature), why washing powders contain proteases and lipases, and why fruit browns when cut. Master the optimum-and-denaturation idea here and the rest of biology — digestion, photosynthesis, respiration — gets much easier.