Contents
1. Enzyme Structure and Action
A biological catalyst — a protein that speeds up a chemical reaction without being used up. Each enzyme has a specific active site that fits only one substrate (or group of related substrates).
Lock and key model
The substrate (key) fits exactly into the active site (lock) of the enzyme. This forms an enzyme-substrate complex. The reaction occurs and the products are released. The enzyme is unchanged and can be reused.
- The active site has a specific shape complementary to the substrate.
- Each enzyme is specific — it only catalyses one type of reaction.
- Enzymes are not consumed — they can catalyse many reactions in succession.
2. Effect of Temperature
As temperature increases from 0°C towards the optimum: rate increases because particles have more kinetic energy → more frequent and more energetic collisions between enzyme and substrate.
Above the optimum temperature: the enzyme denatures. The active site changes shape permanently — the enzyme-substrate complex can no longer form. Rate falls sharply to zero.
| Temperature | Effect on enzyme | Rate of reaction |
|---|---|---|
| 0°C (very cold) | Active — but low kinetic energy | Very slow (not denatured) |
| Optimum (~37°C for body) | Maximum activity | Maximum rate |
| Above optimum (~60°C+) | Denatured — active site changed permanently | Zero (irreversible) |
"If you cool the enzyme back down it will work again" — FALSE once denatured. Denaturation permanently changes the shape of the active site. Cooling only slows an enzyme down — it does not denature it.
3. Effect of pH
Each enzyme has an optimum pH at which it works fastest. Moving away from the optimum (too acidic or too alkaline) changes the charges on the active site, altering its shape — the substrate no longer fits. Extreme pH causes denaturation.
| Enzyme | Optimum pH | Location |
|---|---|---|
| Pepsin (protease) | pH 2 | Stomach |
| Salivary amylase | pH 7 | Mouth |
| Pancreatic amylase | pH 7–8 | Small intestine |
| Lipase | pH 7–8 | Small intestine |
Stomach acid (HCl, pH ~2) creates the optimum pH for pepsin to digest proteins. It also kills many pathogens swallowed with food.
4. Enzyme Inhibition
| Competitive inhibitor | Non-competitive inhibitor | |
|---|---|---|
| Where it binds | Active site | Different site on enzyme (allosteric site) |
| Effect on active site | Blocks substrate from binding | Changes shape of active site |
| Can be overcome? | Yes — by increasing substrate concentration | No — increasing substrate has no effect |
| Example | Malonate inhibits succinate dehydrogenase | Heavy metal ions (Hg²⁺, Pb²⁺) |
- Enzymes are NOT consumed - they are catalysts that can be reused many times
- Low temperature: enzyme inactive but NOT denatured (reversible - warm up restores activity)
- High temperature OR extreme pH: enzyme DENATURED (irreversible - active site permanently altered)
- Pepsin optimum pH ~2 (stomach); salivary amylase ~7 (mouth); pancreatic enzymes ~7-8 (small intestine)
- Competitive inhibitor: overcome by more substrate. Non-competitive inhibitor: cannot be overcome
5. Common Exam Traps
"The enzyme is consumed in the reaction" is always wrong. Enzymes are catalysts — they are chemically unchanged and can be reused. Only denaturation permanently stops an enzyme from working.
When explaining pH effects, say: "pH change alters the charges on amino acids in the active site, changing its shape so the substrate no longer fits." Simply saying "the enzyme changes shape" loses marks.
At 0°C, enzymes are inactive but NOT denatured. Warming them up restores activity. At high temperatures, denaturation is irreversible. These are different processes and must not be confused.
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Original study notes for Singapore students. Not affiliated with MOE, SEAB or Cambridge.