Enzymes
What Are Enzymes?
Enzymes are biological catalysts: proteins that speed up metabolic reactions without being used up. They lower the activation energy needed for a reaction to occur.
- Made of protein
- Highly specific: each enzyme catalyses only one type of reaction (or acts on one type of substrate)
- Unchanged at the end of the reaction
Lock and Key Model
Enzyme Substrate
┌──────────┐ ┌───┐
│ │ │ │
│ Active │ + ──────│ │ ──→ Enzyme-substrate complex ──→ Products released
│ site │ └───┘
└──────────┘
- The active site is the specific region on the enzyme where the substrate binds.
- The substrate fits into the active site like a key into a lock.
- This forms an enzyme-substrate complex.
- The reaction takes place, products are released, and the enzyme is unchanged (ready to be used again).
- Because the active site has a specific shape, each enzyme can only work with a substrate that fits: this is enzyme specificity.
Effect of Temperature
| Temperature | Effect | Reason |
|---|---|---|
| Below optimum | Slow rate | Particles have less kinetic energy; fewer successful collisions per second |
| At optimum (~37 °C in humans) | Maximum rate | Most frequent successful collisions |
| Above optimum | Rate falls sharply | High temperature changes the shape of the active site: the enzyme is denatured |
Denaturation Is Permanent
When an enzyme is denatured, the active site shape is permanently changed. The substrate can no longer fit. The enzyme cannot be “un-denatured” by cooling it down.
Graph shape: Rate increases as temperature rises, reaches a peak at the optimum, then drops sharply to zero as denaturation occurs.
Effect of pH
- Each enzyme has an optimum pH at which it works best.
- Changing pH alters the charges on the amino acids that form the active site, changing its shape.
- At extreme pH values, the enzyme is denatured.
Examples:
- Pepsin (stomach protease): optimum pH ≈ 2 (acidic)
- Salivary amylase (in mouth): optimum pH ≈ 7 (neutral)
- Trypsin (small intestine): optimum pH ≈ 8 (slightly alkaline)
Graph shape: Rate peaks at optimum pH, drops off steeply on both sides.
Practical: Effect of Temperature on Enzyme Activity (Spec 2.12)
Common example: Amylase breaking down starch, tested with iodine at intervals.
Method:
- Prepare water baths at different temperatures (e.g. 20, 30, 37, 50, 70 °C).
- Place amylase solution and starch solution in separate tubes at each temperature; allow to equilibrate.
- Mix amylase and starch; start timing.
- Every 30 seconds, remove a drop and test with iodine on a spotting tile.
- Record the time taken for the iodine to no longer turn blue-black (starch fully broken down).
- Repeat at each temperature and compare results.
Expected result: Fastest breakdown around 37–40 °C; no reaction (or very slow) at very low or very high temperatures (denaturation).
Exam Questions
Q: Explain what is meant by the term “active site.”
The active site is the specific region on an enzyme where the substrate binds. It has a complementary shape to the substrate, allowing them to form an enzyme-substrate complex.
Q: Explain why increasing temperature above the optimum destroys enzyme activity.
High temperatures provide too much kinetic energy, causing the bonds holding the enzyme’s shape to break. This changes the shape of the active site so the substrate can no longer bind. The enzyme is denatured. This is a permanent change.
Q: A student tests amylase activity at pH 2 and pH 7. Suggest which pH gives higher activity and explain why.
pH 7 gives higher activity. Salivary amylase has an optimum pH around 7. At pH 2, the acidic conditions alter the charges on the amino acids forming the active site, changing its shape so that starch cannot bind effectively. The enzyme may be denatured.
Q: Why are enzymes described as specific?
Each enzyme has an active site with a unique shape that is complementary to only one type of substrate. This means each enzyme can only catalyse one specific reaction.
Exam Tip
Always use the phrase “active site changes shape” (not “the enzyme breaks down”) when explaining denaturation. Examiners are looking for evidence that you understand the structural cause of denaturation, not just that it stops working.