Enzymes are the molecules that make life fast enough to happen. Without them, the chemical reactions inside your cells would be far too slow to keep you alive. In this topic you will learn what enzymes are, how they work and what changes their rate.
What enzymes are
An enzyme is a biological catalyst: a protein that speeds up a chemical reaction without being used up or changed itself.
- Because they are not used up, a small amount of enzyme can work again and again.
- Enzymes are made of protein, so their function depends on a precise three-dimensional shape.
- Each enzyme is specific – it catalyses only one type of reaction or one type of substrate.
- Examples include amylase (breaks down starch), protease (breaks down protein) and catalase (breaks down hydrogen peroxide).
The lock and key model
Enzymes work because the shape of the enzyme fits the shape of its substrate.
- The substrate fits into a region of the enzyme called the active site, like a key fitting a lock.
- The enzyme and substrate form an enzyme–substrate complex, the reaction takes place, and products are released.
- Only a substrate with the complementary shape fits the active site, which explains enzyme specificity.
- If the active site changes shape, the substrate can no longer fit and the enzyme stops working.
Effect of temperature
Temperature has a strong effect on the rate of an enzyme reaction.
- As temperature rises, molecules move faster and collide more often, so the rate increases.
- The rate is fastest at the optimum temperature (about 37°C in humans).
- Above the optimum, the enzyme’s shape is permanently damaged – it becomes denatured – and the active site no longer fits the substrate.
- A denatured enzyme cannot recover, so the rate falls sharply to zero.
Effect of pH
Each enzyme also has an optimum pH at which it works fastest.
- Most enzymes work best near a neutral pH (around 7), but there are exceptions.
- Stomach protease (pepsin) works best in acidic conditions, while enzymes in the small intestine prefer slightly alkaline conditions.
- A pH far from the optimum changes the shape of the active site and denatures the enzyme.
- Plotting rate against pH gives a curve that peaks at the optimum and falls away on either side.