1.4.1Biological Molecules
Many Proteins are Enzymes — AQA A Level Biology 7402 Revision Notes & Topical Questions
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1.4.1 Many Proteins are Enzymes — AQA A Level Biology 7402 notes: Define enzymes as biological catalysts, explain how they lower activation energy, and compare the lock-and-key and induced-fit models.An enzyme is a globular protein that acts as a biological catalyst: it speeds up…
- intracellularly
- extracellularly
- active site
- enzyme-substrate complex
- Exam-style question 1
- Exam-style question 2
Section 1 of 9Enzymes as biological catalysts
An enzyme is a globular protein that acts as a biological catalyst: it speeds up a metabolic reaction without being used up.
Section 2 of 9Activation energy
Reacting molecules must collide with enough energy to break existing bonds; this minimum energy is the activation energy.
- or the products formed, only the rate at which equilibrium is reached.
Enzymes lower activation energy in several ways: bringing substrates close together in the right orientation, straining bonds in the substrate, and providing a micro-environment (charge or pH) in the active site that favours the reaction.
Section 3 of 9The active site
The active site is a small region, usually a cleft or pocket, on the surface of the enzyme formed by the folding of the polypeptide (tertiary structure).
- so the enzyme is free to be reused.
Section 4 of 9Models of enzyme action
| Feature | Lock-and-key | Induced fit |
|---|---|---|
| Active site shape | Fixed, exactly complementary to substrate | Not exactly complementary at first; moulds around the substrate |
| Explains | Specificity | Specificity and how bonds are strained to lower activation energy |
| Evidence | Early experiments | Better fits modern data, e.g. enzymes that act on several related substrates |
The induced-fit model is now preferred. The substrate causes small changes in the shape of the active site, so the R groups fit around it more tightly and put strain on the substrate bonds. After the products leave, the active site returns to its original shape.
Section 5 of 9Types and examples
Because the active site depends on the tertiary structure, anything that damages that structure (heat, extreme pH) alters the enzyme's activity. Link forward to the lessons on temperature, pH, concentration and inhibitors.
Section 6 of 9Enzymes in context
Enzymes control the rate of nearly every reaction in a cell, so living organisms can regulate metabolism by controlling them.
- which are activated only where they are needed; this prevents the enzymes digesting the cells that make them.
Enzymes have many uses in industry.
- and immobilised enzymes in alginate beads can be reused and separated easily from the product.
For the exam, an effective definition of an enzyme includes three ideas: it is a protein, it is a catalyst, and it works by lowering activation energy. Naming the enzyme-substrate complex shows you understand the mechanism, and mentioning that the enzyme is unchanged at the end shows you understand catalysis.
Section 7 of 9Exam-style questions
Explain how an enzyme speeds up a reaction. [3]3 marks
The substrate binds to the active site to form an enzyme-substrate complex [1]. This lowers the activation energy, for example by straining bonds or holding substrates in the correct orientation [1]. So more molecules have sufficient energy to react in a given time [1].
Describe the induced-fit model and explain why it is preferred to lock and key. [3]3 marks
The active site is not exactly complementary before binding; it changes shape as the substrate binds [1], so that it fits tightly and puts strain on bonds [1]. It explains how enzymes lower activation energy and how some enzymes act on a range of related substrates, which a rigid site cannot [1].
The enzyme catalase was added to hydrogen peroxide and 12 cm3 of oxygen was collected in 30 seconds. Calculate the mean rate and state what would happen to the final volume of oxygen if the enzyme concentration were doubled. [2]2 marks
Rate = 12 / 30 = 0.4 cm3 s-1 [1]. The final volume would be unchanged, since substrate is the same, but it would be reached faster [1].
Section 8 of 9Key terms
| Term | Meaning |
|---|---|
| Enzyme | A protein that acts as a biological catalyst. |
| Substrate | The molecule that an enzyme acts upon. |
| Active site | The region of the enzyme where the substrate binds. |
| Activation energy | The minimum energy needed for a reaction to occur. |
| Enzyme-substrate complex | The temporary structure formed when substrate binds to the active site. |
| Induced-fit model | Model where the active site changes shape to fit the substrate. |
| Intracellular enzyme | An enzyme that works inside the cell. |
| Extracellular enzyme | An enzyme secreted to work outside the cell. |
Section 9 of 9Exam technique
Marked practiceQuizzes for this lesson
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