Enzymes
What enzymes are
Enzymes are biological catalysts — proteins that speed up chemical reactions in living things without being used up. Almost every reaction in the body (digestion, respiration, protein synthesis) relies on enzymes.
How enzymes work — the lock and key model
Each enzyme has a specially shaped region called the active site. The reacting molecule (the substrate) fits into the active site like a key into a lock.
- The enzyme is specific: its active site only fits one particular substrate.
- The substrate binds, the reaction happens, and products are released. The enzyme is then free to work again.
Factors affecting enzyme activity
Temperature
- As temperature rises, molecules move faster and collide more, so the rate increases.
- Above the optimum (about 37 °C in humans), the enzyme's shape is permanently changed — it denatures. The active site no longer fits the substrate, so the reaction stops.
- Denaturing is permanent — cooling down does not fix it.
pH
- Each enzyme has an optimum pH. Most body enzymes work best around pH 7, but some don't:
- Pepsin (stomach protease) works best in acidic conditions (~pH 2).
- Enzymes in the small intestine prefer alkaline conditions.
- Too far from the optimum pH also denatures the enzyme.
Substrate concentration
- More substrate = faster rate, up to a point. Once all active sites are busy, adding more substrate makes no difference (the rate plateaus).
Digestive enzymes
| Enzyme | Breaks down | Into | Made in |
|---|---|---|---|
| Amylase (a carbohydrase) | Starch | Sugars (glucose) | Salivary glands, pancreas |
| Protease | Proteins | Amino acids | Stomach, pancreas |
| Lipase | Lipids (fats) | Fatty acids + glycerol | Pancreas, small intestine |
Bile (made in the liver, stored in the gall bladder) is not an enzyme but helps: it emulsifies fats (breaks them into small droplets, increasing surface area) and neutralises stomach acid so intestinal enzymes work.
Worked example
Explain why an enzyme stops working at 60 °C.
- The high temperature changes the shape of the enzyme's active site (it denatures). The substrate no longer fits, so the enzyme can't catalyse the reaction. This is permanent. ✓
Common mistakes
- Saying enzymes are "killed" — enzymes aren't alive; they denature.
- Thinking denaturing is reversible — it is permanent.
- Forgetting that bile is not an enzyme (it emulsifies and neutralises).
Exam tips
- Always link denaturing to the active site changing shape so the substrate no longer fits.
- Learn the three digestive enzymes, their substrates and products.
- For graph questions, describe the rise to the optimum, then the sharp fall as it denatures.
Key facts to remember
- Enzymes are protein catalysts; the substrate fits the active site (lock and key) — they are specific.
- Activity depends on temperature, pH and substrate concentration; too much heat or wrong pH denatures them (permanent).
- Amylase → sugars, protease → amino acids, lipase → fatty acids + glycerol; bile emulsifies fat and neutralises acid.