3.1.2Additional lessons
Enzyme Action — IB Biology SL Revision Notes & Topical Questions
5 min read 4 parts Lesson 51 of 142
Section 1 of 4How an Enzyme Works
1
The substrate collides with the active site of the enzyme, a region with a particular shape and chemistry.
2
The substrate binds to form an enzyme-substrate complex, and the active site changes shape slightly to fit it closely.
3
This induced fit puts strain on bonds in the substrate, lowering the activation energy of the reaction.
4
The substrate is converted into products, which have a different shape and leave the active site.
5
The enzyme is unchanged and can bind another substrate molecule.
Section 2 of 4Factors Affecting Rate
Factors Affecting Rate
Temperature
- Rate rises as temperature increases, because molecules collide more often and with more energy, up to an optimum.
High temperature
- Above the optimum, bonds holding the tertiary structure break, so the enzyme denatures and activity falls rapidly.
pH
- Each enzyme has an optimum pH; extremes change the charges in the active site and can denature the enzyme.
Substrate concentration
- Rate rises with substrate concentration until all active sites are occupied, then levels off at a maximum.
| Factor | Low value | Optimum | Very high value |
|---|---|---|---|
| Temperature | Slow, few collisions | Maximum rate | Denatured, no activity |
| pH | Reduced activity | Maximum rate | Denatured at extremes |
| Substrate | Rate proportional to concentration | Near saturation | Plateau at maximum |
Section 3 of 4Enzyme Experiments
Enzyme Experiments
Temperature coefficient
- Over the lower part of the range, a rise of 10 °C often roughly doubles the rate of an enzyme reaction.
Immobilised enzymes
- Enzymes fixed to beads can be reused and are separated easily from the product.
Applications
- Lactase is used to make lactose-free milk, and proteases are added to some washing powders.
Section 4 of 4Exam Checklist & Tips
3.1.2 – Key Points for the Exam
- Active site The region of an enzyme to which a substrate binds.
- Enzyme-substrate complex The temporary structure formed when a substrate is bound to the active site.
- Denaturation A permanent change in the shape of a protein so that it loses its function.
- Optimum The temperature or pH at which an enzyme works fastest.
Common Mistakes
- Saying that enzymes are killed by heat; enzymes are not alive and are denatured.
- Writing that cold temperatures denature enzymes, when low temperature only slows the reaction.
- Stating that the rate keeps rising with substrate concentration, ignoring the plateau when sites are saturated.
- Believing that the substrate fits the active site like a rigid key in all cases.
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