2.2.2Unit 2: Cells, Development, Biodiversity and Conservation
Enzymes – Roles and Modes of Action — IAL Edexcel Linear Revision Notes & Topical Questions
5 min read 4 parts Lesson 14 of 26 Unit quiz
Part 1 of 4Enzymes as Catalysts
Without enzymes most reactions in cells would be too slow to sustain life.
Biological catalysts
- Biological catalysts Enzymes are globular proteins that speed up metabolic reactions and are unchanged at the end of the reaction.
Activation energy
- Activation energy This is the energy needed to start a reaction, and enzymes lower it by providing an alternative reaction pathway.
Specificity
- Specificity Each enzyme usually catalyses one reaction because the shape of its active site matches only one type of substrate.
Intracellular enzymes
- Intracellular enzymes These work inside cells, for example catalase breaking down toxic hydrogen peroxide into water and oxygen.
Extracellular enzymes
- Extracellular enzymes These are released outside cells, for example amylase in saliva which hydrolyses starch in the mouth.
Enzyme–substrate complex
- Enzyme–substrate complex The substrate binds to the active site briefly, products are released, and the enzyme is free to be used again.
Part 2 of 4Models of Enzyme Action
Models of Enzyme Action
Lock-and-key model
- Lock-and-key model The active site has a fixed shape that is exactly complementary to the substrate, like a key fitting a single lock.
Induced-fit model
- Induced-fit model The active site is slightly flexible and changes shape as the substrate binds, giving a tighter fit.
Why induced fit is favoured
- Why induced fit is favoured It explains how binding can strain bonds in the substrate, which lowers the activation energy even further.
Active site
- Active site A small region formed by the tertiary structure, so a change to the primary sequence can change its shape.
Part 3 of 4Factors Affecting Rate
1
As temperature rises towards the optimum, molecules gain kinetic energy, so collisions and enzyme–substrate complexes form more often.
2
Above the optimum, vibrations break the hydrogen and ionic bonds holding the tertiary structure, the active site changes shape and the enzyme is denatured.
3
Each enzyme works best at its optimum pH, and a large change in pH alters the charges on R groups and disrupts the active site.
4
Increasing substrate concentration raises the rate until all active sites are occupied, after which the enzyme concentration is limiting.
5
Increasing enzyme concentration raises the rate while substrate is in excess, because there are more active sites available.
Part 4 of 4Exam Checklist & Tips
2.2.2 – Key Points for the Exam
- Enzyme A globular protein that acts as a biological catalyst by lowering activation energy.
- Active site The region of an enzyme where the substrate binds and the reaction takes place.
- Substrate The molecule that an enzyme acts upon.
- Denaturation Permanent change in the tertiary structure of a protein, so that it can no longer function.
- Optimum The temperature or pH at which an enzyme has its maximum rate of reaction.
Common Mistakes
- Saying enzymes are killed by high temperature; proteins are not alive, so they are denatured.
- Saying the active site and substrate are identical in shape, when they are complementary.
- Claiming that enzymes provide energy for reactions; they only lower the activation energy.
- Forgetting that a low temperature slows an enzyme but does not usually denature it.
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