Enzymes
This lesson covers the structure and function of enzymes as required by the Edexcel GCSE Biology specification (1BI0), Topic 1: Key Concepts in Biology. You need to understand how enzymes work, the lock-and-key model, the concept of specificity, and what happens when enzymes are denatured.
What Are Enzymes?
Enzymes are biological catalysts. A catalyst is a substance that speeds up a chemical reaction without being used up or permanently changed in the process. This means an enzyme can be used over and over again.
Enzymes are proteins — they are made of long chains of amino acids folded into a precise 3D shape. This shape is critical because it determines which substrate the enzyme can act on.
Key Points
- Enzymes speed up reactions that would otherwise occur too slowly to sustain life.
- They work inside cells (intracellular enzymes, e.g. catalase in cells, DNA polymerase) and outside cells (extracellular enzymes, e.g. digestive enzymes in the gut).
- Enzymes are not living — they are molecules. But they are produced by living cells.
- Enzymes are highly specific — each enzyme catalyses only one type of reaction.
Exam Tip: The correct definition is: an enzyme is a biological catalyst that speeds up a chemical reaction without being used up. Memorise this — it appears regularly on exams.
The Lock-and-Key Model
The lock-and-key model explains how enzymes work:
- Each enzyme has a specially shaped region called the active site.
- The active site has a specific shape that is complementary to the shape of its substrate (the molecule the enzyme acts on).
- The substrate fits into the active site like a key fits into a lock.
- When the substrate binds to the active site, an enzyme-substrate complex is formed.
- The enzyme catalyses the reaction — the substrate is converted into product(s).
- The product(s) are released from the active site.
- The enzyme is unchanged and ready to bind with another substrate molecule.
Diagram Description
Imagine three stages:
- Stage 1: The substrate approaches the enzyme. The active site shape matches the substrate shape.
- Stage 2: The substrate binds to the active site → enzyme-substrate complex is formed.
- Stage 3: The reaction occurs. Products are released. The enzyme is free to catalyse another reaction.
Substrate + Enzyme → Enzyme-Substrate Complex → Enzyme + Product(s)
Enzyme Specificity
Because the active site has a specific shape, only one type of substrate can fit into it. This is called enzyme specificity.
- Amylase only breaks down starch into maltose (a sugar).
- Protease only breaks down proteins into amino acids.
- Lipase only breaks down lipids into glycerol and fatty acids.
If the substrate does not have the correct shape, it cannot bind to the active site and no reaction occurs. A different enzyme is needed for each substrate.
Exam Tip: When describing enzyme specificity, always say the active site has a complementary shape to the substrate. Do not say they have the "same shape" — they are complementary (they fit together), not identical.
Factors Affecting Enzyme Activity
Temperature
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As temperature increases, the rate of enzyme activity increases because:
- Molecules have more kinetic energy.
- Substrate and enzyme molecules move faster and collide more frequently.
- More successful collisions per unit time → more enzyme-substrate complexes form.
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Each enzyme has an optimum temperature — the temperature at which the rate of reaction is highest. For most human enzymes, this is approximately 37°C (body temperature).
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Above the optimum temperature, the enzyme begins to denature:
- The bonds holding the 3D shape of the enzyme break (especially hydrogen bonds).
- The active site changes shape permanently.
- The substrate can no longer fit into the active site.
- The rate of reaction drops rapidly to zero.
Denaturation is a permanent change — the enzyme cannot recover its shape.
pH
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Each enzyme has an optimum pH at which it works best.
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Most human enzymes work best at around pH 7 (neutral).
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Some enzymes have different optima:
- Pepsin (a protease in the stomach) works best at pH 2 (acidic).
- Pancreatic lipase works best at around pH 8–9 (slightly alkaline).
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Extreme pH values (very acidic or very alkaline) cause denaturation — the active site shape changes and the substrate can no longer bind.
Substrate Concentration
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As substrate concentration increases, the rate of reaction increases because:
- There are more substrate molecules to collide with enzyme active sites.
- More enzyme-substrate complexes form per unit time.
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Eventually, the rate plateaus (levels off) because:
- All enzyme active sites are occupied (saturated) — every enzyme molecule is working.
- Adding more substrate makes no difference because there are no free active sites available.
- The rate can only increase further if more enzyme is added.
Rate of Reaction Graphs
Temperature Graph
The graph of enzyme activity vs temperature is an asymmetric bell curve:
- Rate increases gradually from low temperatures.
- Rate reaches a peak at the optimum temperature (~37°C for human enzymes).
- Rate drops sharply above the optimum (denaturation).
- The curve is not symmetrical — the drop-off above the optimum is much steeper than the increase below it, because denaturation is rapid and irreversible.
pH Graph
The graph of enzyme activity vs pH is a narrower bell curve:
- Rate is highest at the optimum pH.
- Rate decreases on both sides of the optimum.
- At extreme pH values, the enzyme is denatured and the rate is zero.
- Different enzymes have different optimum pH values (pepsin ~pH 2, amylase ~pH 7, trypsin ~pH 8).
Substrate Concentration Graph
The graph of enzyme activity vs substrate concentration shows:
- An initial steep increase (more substrate = more collisions = faster rate).
- The curve gradually flattens.
- Eventually a plateau — all active sites are occupied.
Exam Tip: When interpreting graphs, always relate your answer to the underlying biology. For example, if asked why the rate drops above the optimum temperature, say: "The bonds holding the enzyme's 3D shape break, causing the active site to change shape (denature), so the substrate can no longer fit — fewer enzyme-substrate complexes form."
Denaturation — A Closer Look
Denaturation means the permanent alteration of an enzyme's 3D structure (particularly the active site) so that it can no longer bind to its substrate.
Denaturation can be caused by:
- High temperatures (above the optimum)
- Extreme pH (too acidic or too alkaline)
Important distinctions:
- Denaturation does not destroy the enzyme — the protein still exists, but its shape has changed.
- Denaturation is not the same as digestion — the enzyme is not broken down into amino acids.
- Denaturation is generally irreversible at GCSE level.