Enzymes Notes
Cambridge IGCSE Biology: Revision notes
Key facts
- An enzyme is a protein that works as a biological catalyst: it speeds up a reaction and is unchanged.
- The substrate fits the enzyme's active site because their shapes are complementary, forming an enzyme-substrate complex.
- Activity rises with temperature up to the optimum, then falls sharply as the enzyme denatures.
- Each enzyme has an optimum pH: pepsin about 2, trypsin about 8, most cell enzymes 7 to 7.4.
- Investigations must control temperature, pH, enzyme and substrate concentration.
Enzymes as catalysts
An enzyme speeds up a reaction by lowering the activation energy, and is not used up.
At body temperature many reactions would be far too slow. An enzyme is a protein that acts as a biological catalyst. It lowers the activation energy, speeds the reaction up and is unchanged at the end, so it can be reused. Say 'remain unchanged' rather than 'used up'.
- 1
Reaction is slow
At body temperature
- 2
Enzyme added
Lowers the activation energy
- 3
Reaction speeds up
Enzyme is unchanged and can be reused
What happens to an enzyme during the reaction it catalyses?
Active site
The substrate fits the enzyme's active site because their shapes are complementary, which is why enzymes are specific.
The substrate approaches the active site and binds, forming the enzyme-substrate complex. Bonds in the substrate are destabilised, the substrate is converted into products, and the products leave. The enzyme is released unchanged. Each enzyme works on only one substrate because its active site has a specific shape.
- 1
Approach
Substrate approaches the active site
- 2
Bind
Complementary shapes form the enzyme-substrate complex
- 3
React
Bonds are destabilised, lowering activation energy
- 4
Product forms
Substrate is converted
- 5
Release
Product leaves; enzyme is regenerated
Worked example
Describe how amylase acts on starch.
- 1
Starch is the substrate; its shape is complementary to the amylase active site.
- 2
An enzyme-substrate complex forms.
- 3
Starch is converted into sugars, the products.
- 4
Amylase is released unchanged and can catalyse another reaction.
Why does an enzyme act on only one substrate?
Temperature
Activity rises to an optimum temperature, then falls sharply as the enzyme denatures.
At low temperatures more kinetic energy gives more frequent effective collisions, so activity increases. At the optimum the rate is highest. Above the optimum, heat breaks bonds, the active site changes shape and the substrate no longer fits: the enzyme is denatured. This is permanent, so cooling does not reactivate it.
| Low | Optimum | Above optimum | |
|---|---|---|---|
| Effect | Activity increases as it rises | Maximum activity | Activity falls sharply |
| Reason | More kinetic energy: more effective collisions | Highest rate; active site fully complementary | Enzyme denatures; substrate no longer fits |
Low
- Effect:
- Activity increases as it rises
- Reason:
- More kinetic energy: more effective collisions
Optimum
- Effect:
- Maximum activity
- Reason:
- Highest rate; active site fully complementary
Above optimum
- Effect:
- Activity falls sharply
- Reason:
- Enzyme denatures; substrate no longer fits
A liver enzyme has an optimum of 37 °C. Why is its activity low at 20 °C?
pH
Each enzyme has an optimum pH, and values far from it change the shape of the active site and reduce activity.
Extreme pH changes the shape of the active site, so the enzyme works less well and can be denatured. The optimum depends on where the enzyme works.
- Pepsin (stomach): optimum pH 2, very acidic.
- Trypsin (small intestine): optimum pH 8, slightly alkaline.
- Most cell enzymes: optimum pH 7 to 7.4.
Why does pepsin work well in the stomach?
Investigations
Change one variable, measure the activity, and keep every other variable constant.
Control enzyme concentration, substrate concentration, volumes and time. Keep pH constant when changing temperature, and temperature constant when changing pH. Plot activity against the variable to find the optimum.
- 1
Water baths
10, 20, 30, 40, 50, 60, 70 °C
- 2
Measure activity
Time for reaction, gas volume or colour change
- 3
Plot a graph
Of activity against temperature
- 4
Find the optimum
And note denaturation at high temperatures
- 1
Buffer solutions
pH 4 to 9
- 2
Constant temperature
Keep it fixed
- 3
Measure activity
At each pH
- 4
Plot a graph
To find the optimum
Worked example
Describe how to investigate the effect of temperature on amylase.
- 1
Keep pH, substrate concentration and enzyme concentration constant.
- 2
Set water baths at 10, 20, 30, 40 and 50 °C.
- 3
Measure the time taken for starch to break down, using iodine to test for starch.
- 4
Plot a graph: activity peaks at about 37 °C, then falls as heat denatures the enzyme.
When investigating the effect of temperature, which variable must be kept constant?
Try an exam question
Describe and explain the effect of increasing temperature from 20 °C to 60 °C on the activity of an enzyme with an optimum temperature of 37 °C.
[4 marks]
- [1]Activity increases up to about 37 °C.
- [1]Particles have more kinetic energy, so more effective collisions.
- [1]Above the optimum, activity falls sharply.
- [1]The enzyme denatures: the active site changes shape and the substrate no longer fits.
That's the notes covered.
Carry on to the next subtopic.