Enzymes
Senior 2 · Biology
Enzymes
Lesson 3 of 4
22 min
Let's begin
A very high fever is dangerous partly because the body's enzymes start to lose their shape. Food lasts longer in a cold fridge because the enzymes of decay bacteria slow down. The stomach is strongly acidic, yet its enzyme pepsin works perfectly there.
Each of these is explained by how temperature and pH affect enzymes. Examiners often ask you to read and explain the graphs.
The graph of rate against temperature therefore rises gradually to a peak at the optimum and then falls steeply.
Each enzyme has an optimum pH. On either side of it, the charges on the active site change, the substrate binds less well and the rate falls. Extreme pH denatures the enzyme.
| Enzyme | Where it works | Optimum pH |
|---|---|---|
| Pepsin (a protease) | stomach, where hydrochloric acid is present | about 2 (acidic) |
| Salivary amylase | mouth | about 7 (neutral) |
| Trypsin (a protease) | small intestine | about 8 (slightly alkaline) |
Amylase stops working once food reaches the acidic stomach, and pepsin stops once food moves into the alkaline small intestine.
Question: Starch and amylase were mixed at different temperatures. Every 30 seconds a drop of the mixture was tested with iodine, and the time taken for the starch to disappear (iodine staying yellow-brown) was recorded.
| Temperature (°C) | 10 | 20 | 30 | 40 | 50 | 60 |
|---|---|---|---|---|---|---|
| Time for starch to disappear (min) | 10 | 5 | 2.5 | 2 | 8 | starch still present after 20 min |
(a) Calculate the rate at 20 °C and at 40 °C, using rate = 1 ÷ time. (b) State the optimum temperature. (c) Explain the result at 60 °C.
Solution:
(a) At 20 °C: rate = 1 ÷ 5 = 0.2 per min. At 40 °C: rate = 1 ÷ 2 = 0.5 per min. A shorter time means a faster rate, which is why we use 1 ÷ time.
(b) The fastest reaction (the shortest time) was at about 40 °C. The true optimum could lie anywhere between 30 °C and 50 °C, so testing more temperatures in that range, such as 35 °C and 45 °C, would find it more precisely.
(c) At 60 °C the amylase was denatured. Its active site changed shape, so starch could no longer fit and form enzyme-substrate complexes, and the starch was never broken down.
To make the test fair, keep the volumes and concentrations of starch and amylase, and the pH, the same at every temperature. Warm the starch and the enzyme separately to the test temperature before mixing them.
Tip
Graph technique: describe it in three parts: (1) the rate rises as temperature increases up to the optimum, because of more kinetic energy and more collisions; (2) there is a peak at the optimum; (3) the rate falls steeply above it, because of denaturation.
Common error: low temperatures do not denature enzymes; they only slow them down.
Explain why the rate of an enzyme reaction increases from 10 °C to 30 °C.
Solution: The molecules gain kinetic energy and move faster, so enzyme and substrate molecules collide more often, and more enzyme-substrate complexes form per second.
Why does food keep longer in a fridge at 4 °C, and why does it start to spoil once taken out?
Solution: At 4 °C the enzymes of decay bacteria and fungi work very slowly, so decay is slow. They are not denatured, so when the food warms up the enzymes work faster again.
Pepsin was added to egg white at pH 2 and at pH 8. At which pH would the egg white be digested? Explain.
Solution: At pH 2. Pepsin's optimum pH is about 2, the acidic conditions of the stomach. At pH 8 its active site loses its proper shape, so it works very poorly or not at all.
Starch disappeared in 4 minutes at 35 °C. Calculate the rate.
Solution: rate = 1 ÷ 4 = 0.25 per min.
Summary
Sources and credits
The explanations of the temperature and pH effects, the experiment data, the worked examples and the practice questions are written by Ganzaa.
