Enzymes

Summary: Enzymes are biological catalysts — globular proteins that speed up metabolic reactions without being consumed. Each enzyme has a specific active site shape (lock-and-key model), making it specific to one substrate. Enzyme activity is affected by temperature, pH, substrate concentration, and enzyme concentration. Enzymes have many industrial and biological applications. Tags: igcse biology enzymes catalysts lock-and-key metabolism Created: 2026-07-16 Last Updated: 2026-07-16

1. What Are Enzymes?

Definition: Enzymes are biological catalysts — they are globular proteins that speed up the rate of metabolic (chemical) reactions in living organisms without being used up or chemically changed in the process.

Key properties of enzymes:

• They are proteins (made of amino acids folded into a specific 3D shape)
• They are catalysts — they lower the activation energy of a reaction, allowing it to proceed faster at lower temperatures
• They are not used up — after catalysing a reaction, the enzyme molecule is free to catalyse another reaction of the same substrate
• They are highly specific — each enzyme catalyses only one particular reaction (or small group of related reactions)
• They work at relatively low temperatures compared to inorganic catalysts (body temperature, ~37 degrees C)
• Enzyme-catalysed reactions are reversible

Why enzymes are important: Without enzymes, metabolic reactions would be far too slow to sustain life at body temperature. Enzymes allow reactions to proceed millions of times faster at 37 degrees C than they would without a catalyst.

2. The Lock-and-Key Model

The lock-and-key model explains how enzymes work:

1. Every enzyme has an active site — a specific region on the enzyme’s surface with a unique 3D shape
2. The active site is complementary in shape to the enzyme’s specific substrate (the molecule the enzyme acts on)
3. The substrate fits into the active site like a key fitting into a lock
4. When the substrate binds to the active site, it forms an enzyme-substrate complex
5. The reaction is catalysed — the substrate is converted into the product(s)
6. The products are released from the active site, leaving the enzyme unchanged and ready to catalyse another reaction

Enzyme + Substrate -> Enzyme-Substrate Complex -> Enzyme + Product(s)

Specificity:

• Each enzyme’s active site has a specific shape that will only fit one particular substrate (or a few closely related substrates)
• If the substrate does not have a shape complementary to the active site, it cannot bind and no reaction occurs
• This is why one enzyme (e.g. amylase) only breaks down starch and cannot break down proteins or lipids

The active site is complementary to the substrate — not “the same as” or “similar to”. Complementary means the shapes fit together precisely.

3. Effect of Temperature on Enzyme Activity

Temperature has a major effect on the rate of enzyme-catalysed reactions.

The temperature-rate graph:

• As temperature increases from a low starting point, the rate of reaction increases steadily
• The rate reaches a maximum at the optimum temperature
• For most human enzymes, the optimum is around 37 degrees C (body temperature)
• Beyond the optimum, the rate decreases sharply to zero

Explanation:

Key definitions:

• Optimum temperature: The temperature at which the enzyme works at its maximum rate
• Denaturation: Permanent change in the 3D shape of the enzyme protein, causing the active site to lose its complementary shape to the substrate. The enzyme can no longer function.

4. Effect of pH on Enzyme Activity

The pH-rate graph:

• Each enzyme has an optimum pH at which it works fastest
• On either side of the optimum, the rate is lower
• At extremes of pH, the enzyme is denatured

Explanation:

• pH is a measure of H+ ion (or OH- ion) concentration
• Changes in pH alter the charges on the amino acid side chains in the enzyme’s active site
• This disrupts the ionic and hydrogen bonds holding the enzyme’s precise 3D shape
• The active site changes shape → substrate can no longer bind → rate decreases
• Small changes in pH are usually reversible (if the pH returns to optimum, the enzyme regains function)
• Large/extreme changes in pH cause permanent denaturation

5. Effect of Substrate Concentration

The substrate concentration-rate graph:

• As substrate concentration increases, the rate of reaction increases proportionally
• Eventually the rate plateaus (levels off) at a maximum rate

Explanation:

• At low substrate concentration: many active sites are empty → increasing substrate concentration fills more active sites → more enzyme-substrate complexes → rate increases
• At high substrate concentration: all active sites are saturated (continuously occupied) → adding more substrate has no effect — the enzyme is working at its maximum rate (Vmax)
• The only way to increase rate beyond this plateau is to add more enzyme

6. Effect of Enzyme Concentration

The enzyme concentration-rate graph:

• The graph shows a directly proportional relationship (straight line through origin)
• Rate increases linearly with enzyme concentration

Explanation:

• More enzyme molecules → more active sites available → more enzyme-substrate complexes can form per unit time → rate increases proportionally
• This assumes substrate is in excess (not limiting)

7. Enzymes in Industry

Biological Washing Powders

Biological washing powders contain enzymes that break down stains:

Advantages:

• Effective at lower washing temperatures (30-40 degrees C instead of 60 degrees C+) → saves energy (lower electricity bills)
• Less damage to delicate fabrics than hot water washing
• Enzymes are biodegradable (they are proteins)

Washing at too high a temperature (e.g. 60 degrees C+) would denature the enzymes, making the powder ineffective. Biological powders work best at warm (not hot) temperatures — around 30-40 degrees C, near the enzymes’ optimum.

Pectinase in Fruit Juice Production

• Pectin is a polysaccharide found in plant cell walls that makes fruit pulpy and cloudy
• Pectinase (an enzyme) breaks down pectin
• Adding pectinase to crushed fruit:
  • Makes the juice clearer (not cloudy) — more appealing to consumers
  • Increases juice yield — more juice is released from the fruit cells since their cell walls are partially broken down

Lactase and Lactose-Free Milk

• Lactose is the sugar found in milk
• Some people are lactose intolerant — they lack the enzyme lactase and cannot digest lactose (causes bloating, diarrhoea)
• Lactase enzyme breaks lactose down into glucose and galactose (sweeter, digestible sugars)
• Immobilised lactase (lactase trapped in alginate beads) is used:
  • Milk is passed through a column containing immobilised lactase beads
  • The lactase breaks down the lactose
  • The enzyme remains in the beads — it is not mixed with the milk, so the milk is not contaminated with enzyme
  • The beads can be reused for many batches (saves money)

Two key advantages of immobilised enzymes: (1) the enzyme can be reused, (2) the product is not contaminated with enzyme.

8. Enzymes in Living Organisms (Digestion)

Digestive enzymes break down large, insoluble food molecules into small, soluble molecules that can be absorbed into the blood.

Other key enzymes:

Sources

• BBC Bitesize GCSE Biology — Enzymes and digestion, BBC (free educational resource)
• OpenStax Biology 2e — Ch. 6 Metabolism / Enzymes, Rice University (free, CC BY 4.0)
• Cambridge IGCSE Biology 0610 — Syllabus 5: Enzymes, Cambridge Assessment International Education
• CK-12 Biology for High School — Enzymes, CK-12 Foundation (free, CC BY-NC 3.0)

Related Notes

• Biological Molecules — Enzymes are proteins; digestion breaks down carbohydrates, lipids, and proteins
• Human Nutrition — The role of digestive enzymes (amylase, protease, lipase) in breaking down food
• Movement Into and Out of Cells — How products of enzyme digestion are absorbed (diffusion, active transport)
• Cell Structure and Organisation — Ribosomes (where enzymes are made), mitochondria (where respiration occurs with enzyme involvement)
• IGCSE-Bio-Index — Full IGCSE Biology index

Common Misconceptions