Enzymes

Enzymes are biological catalysts: they speed up reactions in living things without being used up. Each enzyme is a protein with a particular shape. Digestive enzymes break large food molecules into small, soluble ones that can be absorbed into the blood.

Key facts

  • An enzyme's active site has a particular shape. Only its substrate fits, like a key in a lock (the lock and key model), so each enzyme catalyses one reaction.
  • As the temperature rises, molecules collide more often and the rate increases, up to the optimum (about 37 °C for human enzymes). Above it the active site changes shape: the enzyme is denatured and the rate falls quickly.
  • Each enzyme has an optimum pH: pepsin in the stomach about pH 2, salivary amylase about pH 7, trypsin in the small intestine about pH 8. Far from its optimum, an enzyme is denatured.
  • Amylase breaks starch into sugars (made in the salivary glands, pancreas and small intestine); proteases break proteins into amino acids (stomach, pancreas, small intestine); lipases break lipids into fatty acids and glycerol (pancreas, small intestine).
  • Bile, made in the liver and stored in the gall bladder, neutralises stomach acid and emulsifies fats into droplets, giving lipase a larger surface area. Bile is not an enzyme.
  • Rate of reaction = amount of product made ÷ time. When only a time is measured, rate = 1000 ÷ time is often used.
  • Q₁₀ = rate at (T + 10 °C) ÷ rate at T. For many enzymes it is about 2 below the optimum.

Worked examples

Example 1

Why does amylase break down starch but not protein?

  1. The active site of an enzyme has a particular shape.
  2. Starch fits amylase's active site, like a key in a lock.
  3. A protein is a different shape, so it does not fit and is not broken down.

Answer: only starch fits the shape of amylase's active site

Example 2

The graph shows how temperature affects the rate of an enzyme reaction. Explain its shape.

  1. Up to about 37 °C, molecules move faster and collide more often, so the rate rises.
  2. At the optimum the rate is highest.
  3. Above it, the bonds holding the enzyme's shape break and the active site changes shape.
  4. The substrate no longer fits: the enzyme is denatured and the rate falls to zero.

Answer: the rate rises to the optimum as collisions increase, then falls steeply as the enzyme is denatured

Example 3

In the amylase practical, a drop of the mixture stopped turning iodine blue-black after 60 seconds at pH 7. Calculate the rate using rate = 1000 ÷ time.

  1. The starch was all broken down after 60 s.
  2. Rate = 1000 ÷ time. 1000 ÷ 60 = 16.666…
  3. To 1 decimal place, the rate is 16.7.

Answer: 16.7

Example 4

Catalase released 24 cm³ of oxygen from hydrogen peroxide in 30 seconds. Calculate the mean rate of reaction.

  1. Rate = volume of oxygen ÷ time. 24 ÷ 30 = 0.8
  2. The mean rate is 0.8 cm³/s.

Answer: 0.8 cm³/s