3 · Enzymes
Cambridge A Level Biology — Full F-drive source: 3 · Enzymes — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions
5 min read 8 parts Lesson 17 of 17 Unit quiz
Lesson summary
3 · Enzymes — Factors Affecting Enzyme Action — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Explain how temperature, pH, enzyme concentration and substrate concentration affect enzyme activity, and how to read rate graphs.Below the optimum — faster molecules colli…
- Above the optimum
- Low temperature
- Optimum pH
- Extreme pH
- Rising section
- Peak then fall
Section 1 of 8Temperature
Below the optimum
- Below the optimum faster molecules collide more often and with more energy, so more complexes form.
Above the optimum
- Above the optimum hydrogen bonds break, the active site is permanently damaged and the enzyme denatures.
Low temperature
- Low temperature slows the reaction but does not denature the enzyme.
Section 2 of 8pH
Optimum pH
- Optimum pH each enzyme has its own, e.g. pepsin at pH 2.
Extreme pH
- Extreme pH breaks hydrogen and ionic bonds, changing the active site and causing denaturation.
Buffers
- Buffers keep pH constant in investigations.
Section 3 of 8Enzyme and substrate concentration
Enzyme concentration
- Enzyme concentration the rate rises in proportion while substrate is in excess.
Substrate concentration
- Substrate concentration the rate rises, then plateaus when all active sites are saturated.
Plateau
- Plateau the limiting factor changes to enzyme concentration (for substrate) or substrate (for enzyme).
Section 4 of 8Reading rate graphs
1Rising section
- Rising section the factor on the x-axis is limiting.
2Plateau
- Plateau another factor is limiting; name it.
3Peak then fall
- Peak then fall the optimum, followed by denaturation.
4Initial rate
- Initial rate use the gradient at t = 0.
Section 5 of 8Reasoning about graph questions
1
Describe the shape of the graph.
2
Name the factor limiting at that stage.
3
Explain using active sites and collisions.
4
Mention denaturation for high temperature or extreme pH.
Section 6 of 8More from the specification
More from the specification
Arrhenius thermal rate determination
- Arrhenius thermal rate determination plotting reaction rates against reciprocal absolute temperature provides a graphical means to compute activation energy.
Enzyme Catalysis and Enzyme-Substrate Complexes
- In an enzyme-catalysed reaction, substrate molecules bind specifically to the active site to form an enzyme-substrate complex, stabilizing the transition state and accelerating product formation.
Competitive Enzyme Inhibition
- Competitive inhibitors have a molecular structure resembling the natural substrate, reversibly competing for active site binding;
- their inhibitory effect is overcome by increasing substrate concentration.
Non-Competitive Enzyme Inhibition
- A non-competitive inhibitor binds to an allosteric site on the enzyme, inducing a conformational change in the active site that decreases maximal reaction rate (Vmax) regardless of added substrate.
Michaelis-Menten Kinetics and Immobilised Enzymes
- The Michaelis-Menten constant (Km) is the substrate concentration at half-maximal velocity;
- immobilising enzymes onto inert solid supports increases operational stability against thermal and pH fluctuations.
Enzyme Catalysis and Enzyme-Substrate Complexes
- In an enzyme-catalysed reaction, substrate molecules bind specifically to the active site to form an enzyme-substrate complex, stabilizing the transition state and accelerating product formation.
Competitive Enzyme Inhibition
- Competitive inhibitors have a molecular structure resembling the natural substrate, reversibly competing for active site binding;
- their inhibitory effect is overcome by increasing substrate concentration.
Non-Competitive Enzyme Inhibition
- A non-competitive inhibitor binds to an allosteric site on the enzyme, inducing a conformational change in the active site that decreases maximal reaction rate (Vmax) regardless of added substrate.
Michaelis-Menten Kinetics and Immobilised Enzymes
- The Michaelis-Menten constant (Km) is the substrate concentration at half-maximal velocity;
- immobilising enzymes onto inert solid supports increases operational stability against thermal and pH fluctuations.
Successful collision
- A collision with enough energy and correct orientation.
Non-competitive inhibitor
- An inhibitor that binds elsewhere and changes the active site.
Immobilised enzyme
- An enzyme fixed to a surface or trapped in a matrix so it can be reused.
Buffer solution
- A solution that holds the pH at a fixed value.
Enzyme-substrate complex
- The temporary structure formed when a substrate binds to an enzyme's active site.
Inhibitor
- A molecule that reduces enzyme activity.
Lactose
- A disaccharide of glucose and galactose.
Affinity
- How readily haemoglobin binds oxygen.
Section 7 of 8Key terms
| Term | Meaning |
|---|---|
| Optimum | The condition giving maximum enzyme activity. |
| Denaturation | Permanent loss of active site shape. |
| Limiting factor | The factor restricting the rate. |
| Buffer | A solution resisting pH change. |
| Saturation | All active sites occupied. |
| Initial rate | The rate at the start of the reaction. |
Section 8 of 8Exam technique
1
Saying low temperature denatures enzymes — it only slows them.
2
Saying enzymes die — they are not alive.
3
Not naming the limiting factor on a plateau — always state it.
4
Forgetting to keep pH constant with a buffer — pH must be controlled.
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