Enzymes · Year 12
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.