Enzymes are biological catalysts — proteins that speed up chemical reactions in living organisms without being used up in the reaction.
Without enzymes, many reactions in the body would happen too slowly to sustain life.
Every enzyme is a protein with a specific 3D shape. Part of this shape forms the ACTIVE SITE — a region with a very specific shape that fits only ONE type of molecule (the SUBSTRATE).
This specificity (one enzyme = one substrate) is explained by the LOCK AND KEY MODEL.
The Lock and Key Model
The lock and key model explains enzyme specificity.
The ENZYME is the lock — its active site has a unique shape.
The SUBSTRATE is the key — only the correctly shaped substrate fits into the active site.
What happens:
1. The substrate collides with the enzyme's active site.
2. The substrate binds to form an ENZYME-SUBSTRATE COMPLEX.
3. The enzyme catalyses the reaction — substrate is converted into PRODUCTS.
4. Products are released from the active site.
5. The enzyme is UNCHANGED and ready to bind another substrate molecule.
This is why enzymes are NOT used up — they can be used over and over again.
Real-life example: amylase (enzyme) has an active site that ONLY fits starch molecules (substrate). It will not break down proteins or fats — their shape is different.
Effect of Temperature on Enzyme Activity
Temperature has a huge effect on how fast enzymes work.
LOW TEMPERATURE (e.g. 10°C):
Particles have less kinetic energy.
Fewer enzyme-substrate collisions per second.
Reaction rate is slow.
RISING TEMPERATURE:
More kinetic energy → more collisions → more enzyme-substrate complexes formed → faster reaction rate.
OPTIMUM TEMPERATURE:
The temperature at which the enzyme works fastest.
For most human enzymes: approximately 37°C (body temperature).
ABOVE OPTIMUM TEMPERATURE:
Vibrations in the enzyme become too violent.
The shape of the active site is permanently changed — DENATURATION.
Substrate can no longer fit into the denatured active site.
Reaction rate falls rapidly to zero.
IMPORTANT: denaturation is PERMANENT. Cooling the enzyme back down does NOT restore its activity.
Effect of pH on Enzyme Activity
Each enzyme also has an OPTIMUM pH — the pH at which it works best.
PH AWAY FROM THE OPTIMUM:
Changes in pH alter the charges on the amino acids that form the active site.
Hydrogen bonds in the enzyme are disrupted.
The shape of the active site changes.
The substrate no longer fits — enzyme activity decreases.
At extreme pH values, the enzyme DENATURES permanently.
Different enzymes have different optimum pH values depending on where they work:
Salivary amylase — optimum pH ~7 (neutral — the mouth).
Pepsin — optimum pH ~2 (acidic — the stomach, where HCl is present).
Pancreatic enzymes — optimum pH ~7–8 (neutral to slightly alkaline — small intestine, where bile has neutralised the acid).
This is WHY the body produces acid in the stomach and bile to neutralise it — creating the right pH for each enzyme in each location.
Factors That Affect Enzyme Rate — Summary
Three key factors affect the rate of enzyme-catalysed reactions:
1. TEMPERATURE — increases rate up to optimum (~37°C for human enzymes), then denaturation causes rate to fall sharply.
2. pH — each enzyme has an optimum pH. Too acidic or too alkaline = active site changes shape = slower reaction or denaturation.
3. SUBSTRATE CONCENTRATION — more substrate molecules = more collisions with enzyme = faster rate (up to a point). Once all enzyme active sites are occupied, adding more substrate has no effect — the enzyme is saturated.
⚠️ Common Mistake
Enzymes are DENATURED at high temperatures — NOT killed. They are proteins, not living things. You cannot kill a protein. Denaturation means the active site shape is permanently changed. Cooling the enzyme down afterwards will NOT bring it back to life — the damage is permanent.
📌 Key Note
Lock and key: enzyme active site + specific substrate → enzyme-substrate complex → products released → enzyme reused. Denaturation is permanent — caused by high temperature or extreme pH.
🎯 Matching Activity — Match the Enzyme Concept
Match each term to its correct description. — drag the symbols on the right to match the component names on the left.
Active site
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Substrate
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Enzyme-substrate complex
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Denaturation
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Optimum temperature
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Lock and key model
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The specific region of an enzyme where the substrate binds
The specific molecule that fits into an enzyme's active site
Permanent change in the shape of an enzyme's active site — caused by high temperature or extreme pH
Explains why each enzyme only works with one specific substrate
The temperature at which an enzyme works at its maximum rate
Formed when the substrate binds to the enzyme's active site
🧪 Required Practical
🔬 RP3 — Investigate the effect of pH on the rate of amylase activity. Use iodine solution to test for starch at regular intervals. Compare time taken to digest starch at different pH values.
Know the method, variables, equipment and how to analyse results.
🎯 Test Yourself
Question 1 of 5
1. An enzyme is heated to 80°C. What happens to it?
2. Why does pepsin work best at pH 2?
3. According to the lock and key model, why can amylase only digest starch — not proteins?
4. What happens to the rate of an enzyme-catalysed reaction when substrate concentration increases?
5. After an enzyme-catalysed reaction, what happens to the enzyme?
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