Enzymes: how they work and what affects them

The molecule an enzyme acts on is its substrate. The substrate fits into the active site like a key into a lock, because their shapes are complementary. The reaction happens, the products are released, and the enzyme is free to work again.

Because the active site has a specific shape, each enzyme usually catalyses only one reaction. This is called specificity.

As temperature rises, molecules move faster, so the enzyme and substrate collide more often and the rate increases. At the optimum temperature (around 37 °C for most human enzymes) the rate is highest.

Above the optimum, the bonds holding the enzyme's shape break. The active site changes shape, the substrate no longer fits, and the rate falls sharply. The enzyme is denatured, and this is permanent. At low temperatures enzymes aren't denatured; they just work slowly.

Each enzyme has an optimum pH. If the pH is too far from it, the active site changes shape and the enzyme is denatured.

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

Bile is not an enzyme. It's made in the liver and stored in the gall bladder. It neutralises stomach acid and emulsifies fats into small droplets, giving lipase a larger surface area to work on. See the digestive system.

1. Put a drop of iodine solution into each well of a spotting tile.
2. Mix starch solution, amylase and a pH buffer in a test tube kept at a constant temperature in a water bath.
3. Every 30 seconds, put a drop of the mixture onto the iodine.
4. When the iodine stays orange-brown (no longer turns blue-black), all the starch has been broken down. Record the time.
5. Repeat with buffers of different pH.

The rate can be worked out as $\text{rate} = \tfrac{1000}{\text{time}}$. If the starch is gone after 50 s, the rate is $1000 \div 50 = 20\ \text{s}^{-1}$. Control variables: temperature, volumes and concentrations of starch and amylase.

Enzymes also build molecules: joining glucose into starch, glycogen and cellulose, joining amino acids into proteins, and making lipids from fatty acids and glycerol. Enzymes in mitochondria control respiration, and enzymes in chloroplasts control photosynthesis. Industry uses enzymes in biological washing powders and food production.

• Saying enzymes are "killed". They're not alive: they're denatured.
• Saying the substrate "changes shape". It's the active site that changes shape.
• Saying low temperatures denature enzymes. They just slow them down.
• Calling bile an enzyme.

1. Why does an enzyme only work on one substrate?
2. Explain why the rate of an enzyme reaction falls above 40 °C.
3. What does lipase break lipids into?
4. In the amylase practical, the starch is gone after 80 s. Calculate the rate.

Answers: 1. The active site has a specific shape that only the substrate fits. 2. The active site changes shape (the enzyme is denatured), so the substrate no longer fits. 3. Fatty acids and glycerol. 4. $1000 \div 80 = 12.5\ \text{s}^{-1}$.