By the end of this lesson you should be able to
- Describe enzymes as biological catalysts and explain the lock and key idea
- Explain the effect of temperature and pH on enzyme activity
- Name common enzymes, their substrates and their products
- Interpret and calculate rates from enzyme experiments
What are enzymes?
Chemical reactions keep a living organism alive, and most would be far too slow at body temperature without help. Enzymes provide that help. An enzyme is a protein that acts as a biological catalyst. It speeds up a reaction and is not used up in the process, so a small amount of enzyme can be used again and again.
Key ideas
Enzyme + substrate → enzyme–substrate complex → enzyme + products
- The substrate is the substance the enzyme acts on.
- The active site is the part of the enzyme where the substrate fits.
- Enzymes are specific. The shape of the active site matches only one kind of substrate, like a key fitting one lock.
Why temperature matters. As temperature rises, particles move faster and collide more often, so the reaction speeds up. This continues to the optimum temperature, which is about 37 °C for human enzymes. Above about 45 °C the enzyme’s shape changes and the active site no longer fits the substrate. The enzyme is denatured, and this change is permanent. At low temperatures the enzyme is only inactive, not denatured, and it works again when warmed.
Why pH matters. Each enzyme has an optimum pH. Extremes of pH change the shape of the active site and denature the enzyme.
Concentrations. The rate increases as the substrate concentration increases, until all the active sites are in use. After that the amount of enzyme limits the rate.
Enzymes in the body
| Enzyme | Substrate | Products | Where it works | Optimum pH |
|---|---|---|---|---|
| Amylase | Starch | Maltose | Mouth and small intestine | About 7 |
| Protease (pepsin) | Protein | Amino acids and peptides | Stomach | About 2 |
| Protease (trypsin) | Protein | Amino acids | Small intestine | About 8 |
| Lipase | Fats and oils | Fatty acids and glycerol | Small intestine | About 8 |
| Catalase | Hydrogen peroxide | Water and oxygen | Liver and many other cells | About 7 |
Pepsin works best in the acidic stomach, while the enzymes in the small intestine work best in the slightly alkaline conditions there.
Worked examples
Example 1: reading a temperature graph. An enzyme was tested at different temperatures.
| Temperature (°C) | 10 | 20 | 30 | 40 | 50 | 60 |
|---|---|---|---|---|---|---|
| Rate of reaction (arbitrary units) | 2 | 5 | 10 | 16 | 4 | 0 |
- The optimum temperature is about 40 °C, where the rate is highest.
- From 10 to 40 °C the rate rises because the molecules have more energy and collide more often.
- At 50 °C the rate falls sharply because the enzyme begins to be denatured.
- At 60 °C the rate is zero because the enzyme is completely denatured.
Example 2: measuring a rate. Amylase breaks down starch. Drops of the mixture are tested with iodine solution every minute. Iodine turns blue-black with starch, and stays orange-brown when the starch has gone. The time taken for the starch to disappear was 12 minutes at 20 °C and 3 minutes at 40 °C.
Rate is the inverse of time:
At 20 °C: per minute. At 40 °C: per minute.
The reaction is times faster at 40 °C.
Example 3: effect of pH. The rate of pepsin at different pH values was measured.
| pH | 1 | 2 | 3 | 5 | 7 |
|---|---|---|---|---|---|
| Rate (arbitrary units) | 8 | 10 | 6 | 1 | 0 |
The optimum pH is about 2, which matches the acid in the stomach. At pH 7 the rate is zero because the enzyme is denatured, so pepsin would not work in the mouth or small intestine.
Common mistakes
Saying enzymes are “killed” by heat. Enzymes are proteins, not living things. They are denatured.
Saying cold denatures enzymes. Low temperature only slows or stops the reaction. The enzyme is not damaged and works again when warmed.
Saying the enzyme is “used up”. The enzyme is not changed by the reaction. The substrate is used up.
Explaining a fall in rate as “the enzyme melts”. Say that the shape of the active site changes, so the substrate no longer fits.
Practice questions
- What is an enzyme?
- Explain what is meant by saying an enzyme is specific.
- Explain why an enzyme stops working at 70 °C.
- Explain why cooling an enzyme does not destroy it.
- Name the enzyme that breaks down (a) starch, (b) protein, (c) fats.
- In an experiment on pepsin, the rate is 8, 10, 6, 1 and 0 at pH 1, 2, 3, 5 and 7. State the optimum pH and explain why pepsin works well in the stomach.
- Starch takes 6 minutes to disappear at 30 °C and 3 minutes at 40 °C. Calculate the rate at each temperature and state how many times faster the reaction is at 40 °C.
- Explain why biological washing powders are used at about 40 °C and not in boiling water.
Show answers
- An enzyme is a protein that acts as a biological catalyst. It speeds up a chemical reaction and is not used up.
- The active site has a shape that fits only one kind of substrate, so each enzyme acts on one substance or one type of reaction.
- The high temperature changes the shape of the active site, so the substrate no longer fits. The enzyme is denatured.
- At low temperatures the molecules move slowly and collide less often, so the reaction is slow. The shape of the enzyme is not changed, and it works again when warmed.
- (a) Amylase. (b) Protease. (c) Lipase.
- The optimum pH is about 2. The stomach contains acid, which gives this pH, so pepsin works at its highest rate there.
- At 30 °C: per minute. At 40 °C: per minute. The reaction is 2 times faster at 40 °C.
- The enzymes in the powder work best at moderate temperatures. Boiling water would denature them, so they could no longer break down stains.
Summary
- Enzymes are protein catalysts that are specific and are not used up.
- Rate rises with temperature up to the optimum, then falls as the enzyme is denatured.
- Each enzyme has an optimum pH, and extremes of pH denature it.
- Rate can be found from time as .
Now practise this topic
Real questions show you what still needs work. Try a past paper under timed conditions, or take a short quiz.