Enzymes
Enzymes are biological catalysts that speed up chemical reactions in living organisms. Understand the lock and key model, factors affecting enzyme activity, and denaturation.
What are enzymes?
Enzymes are biological catalysts — they speed up chemical reactions in living organisms without being used up in the process. They are large proteins made up of long chains of amino acids folded into specific three-dimensional shapes.
Every enzyme has a region called the active site. This is the part of the enzyme where the substrate (the molecule being acted upon) binds. The shape of the active site is complementary to the shape of the substrate — this is known as the lock and key model.
An enzyme is a biological catalyst — a protein that speeds up a specific chemical reaction without being changed or used up itself.
The lock and key model
The lock and key model explains how enzymes work:
- The substrate (key) has a specific shape
- The active site (lock) of the enzyme has a complementary shape
- The substrate fits into the active site, forming an enzyme–substrate complex
- The reaction takes place, and the products are released
- The enzyme is unchanged and ready to catalyse another reaction
Because the active site has a specific shape, each enzyme can only catalyse one type of reaction. This is called enzyme specificity.
Factors affecting enzyme activity
Temperature
As temperature increases, enzyme activity increases because molecules have more kinetic energy and collide more frequently with the active site. However, above the optimum temperature (around 37°C for human enzymes), the enzyme begins to denature — the active site changes shape and can no longer bind to the substrate.
pH
Each enzyme has an optimum pH at which it works fastest. Moving away from this optimum in either direction reduces activity. Extreme pH values cause denaturation.
- Most human enzymes work best at pH 7 (neutral)
- Pepsin (stomach enzyme) works best at pH 2 (acidic)
- Lipase (from the pancreas) works best in alkaline conditions
Don't say the enzyme is "killed" at high temperatures. Enzymes are not alive — they are proteins. The correct term is **denatured**, meaning the active site has permanently changed shape.
Types of enzyme
There are three main types of digestive enzyme you need to know:
- Carbohydrases (e.g. amylase) — break down carbohydrates into simple sugars
- Proteases (e.g. pepsin) — break down proteins into amino acids
- Lipases — break down lipids (fats) into glycerol and fatty acids
| Enzyme type | Substrate | Products | Where produced |
|---|---|---|---|
| Amylase | Starch | Maltose (a sugar) | Salivary glands, pancreas |
| Protease | Protein | Amino acids | Stomach, pancreas |
| Lipase | Lipids (fats) | Glycerol + fatty acids | Pancreas |
Denaturation is the permanent change in the shape of an enzyme's active site, caused by high temperatures or extreme pH. Once denatured, the substrate can no longer fit into the active site, so the enzyme cannot catalyse its reaction.
The lock and key model suggests the active site is a rigid, fixed shape. In reality, the active site can change shape slightly as the substrate binds — this is called the 'induced fit' model. At GCSE level, the lock and key model is sufficient.
Test your understanding
Try our free quiz on enzymes to check you've got the key concepts covered.
Find this in your course
Jump to this topic in your specific course
Related concepts
Photosynthesis
Photosynthesis is the process by which green plants convert light energy into chemical energy stored in glucose. Learn the equation, limiting factors, and the required practical.
BioenergeticsRespiration
Respiration is the exothermic process that transfers energy from glucose in every living cell. Learn the difference between aerobic and anaerobic respiration.
Bioenergetics
