Enzyme
Summary: Enzymes are biological catalysts — proteins that speed up specific biochemical reactions without being consumed. They lower activation energy and are denatured by high temperatures and extreme pH. Tags: igcse chemistry biology catalysis proteins Created: 2026-07-14 Last Updated: 2026-07-23
Definition
An enzyme is a biological catalyst — a protein molecule that increases the rate of a specific biochemical reaction without being used up or chemically changed at the end.
How Enzymes Work
Enzymes work by providing an alternative reaction pathway with a lower activation energy. More substrate molecules have sufficient energy to react at a given temperature, so the reaction rate increases.
Lock and Key Model
- The substrate (reactant) fits into the enzyme’s active site
- The active site has a specific 3D shape complementary to the substrate
- The enzyme-substrate complex forms
- The reaction occurs, products are released
- The enzyme is unchanged and free to catalyse more reactions
Properties of Enzymes
| Property | Description |
|---|---|
| Specificity | Each enzyme catalyses only one type of reaction / substrate |
| Catalytic efficiency | Very small amounts catalyse large quantities of substrate |
| Temperature sensitivity | Have an optimum temperature (~37°C for human enzymes) |
| pH sensitivity | Have an optimum pH (varies by enzyme location) |
| Reusability | Not consumed in the reaction — reused many times |
Temperature and Enzymes
Rate
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Temperature →
↑ optimum
- Below optimum: Rate increases with temperature (more kinetic energy → more successful collisions)
- At optimum: Maximum rate (all active sites occupied)
- Above optimum: Enzyme is denatured — the protein’s 3D shape is permanently destroyed, the active site changes shape, substrate no longer fits, and the enzyme stops working
pH and Enzymes
Each enzyme has an optimum pH:
- Pepsin (stomach): optimum pH ~2 (acidic)
- Amylase (saliva): optimum pH ~7 (neutral)
- Lipase (small intestine): optimum pH ~8 (slightly alkaline)
Extreme pH also causes denaturation by disrupting the ionic and hydrogen bonds that hold the protein in its specific 3D shape.
Denaturation
Denaturation is the permanent loss of an enzyme’s 3D structure, destroying the shape of the active site. It is:
- Irreversible — the enzyme cannot recover
- Caused by: high temperature and extreme pH
- The primary structure (amino acid sequence) remains intact, but the folded shape is lost
Why IGCSE Chemistry Includes Enzymes
Enzymes are covered in the rates of reaction topic as an example of biological catalysts. The key comparison:
| Inorganic Catalyst | Enzyme | |
|---|---|---|
| Nature | Metal / metal oxide (e.g. MnO₂, Fe) | Protein |
| Specificity | Not specific | Highly specific |
| Denaturation | Not denatured by heat | Denatured at high temp |
| Conditions | Works at high T and P | Works at mild conditions |
Key Facts
- Enzymes are proteins that act as biological catalysts
- They lower activation energy, increasing rate without being consumed
- Lock and key model: specific substrate fits specific active site
- Denatured by high temperature (permanent shape change)
- Optimum temperature for most human enzymes: ~37°C
- Optimum pH varies by enzyme location in the body
- Denaturation is permanent — cooling does NOT restore activity