What you need to know
- 2.7(i)enzymeA globular protein that acts as a biological catalyst, speeding up a specific reaction. action and specificityAn enzyme catalyses only one reaction because only one substrate has a shape complementary to its active site. in terms of three-dimensional structure.
- 2.8Conceptual preparation for investigating temperature, pHA measure of how acidic or alkaline a solution is, based on its H⁺ ion concentration. Below 7 is acidic, above 7 is alkaline., enzyme and substrateThe molecule that binds to an enzyme’s active site and is changed in the reaction. concentration effects on initial rateThe rate at the start of a reaction, before the substrate is used up. It is found from the gradient of a tangent at time zero..
This lesson introduces the reasoning needed for Core Practical 4. Planning, method, risk controls and evaluation are available in the linked CP4 guide. Protein structure is covered in 1A5 (specification 2.6).
By the end, you should be able to…
- Explain active-site specificity and induced fitA model in which the active site changes shape slightly as the substrate binds, giving a closer fit and putting strain on bonds..
- Relate temperature and pH effects to molecular motion and protein structure.
- Interpret concentration effects and calculate an initial rate.
Build your understanding
A specific three-dimensional site
An enzymeA globular protein that acts as a biological catalyst, speeding up a specific reaction.’s active siteThe region of an enzyme with a specific shape where the substrate binds. is a region formed by its folded structure. The arrangement of amino-acid side chains creates a shape and chemical environment suited to binding particular substrateThe molecule that binds to an enzyme’s active site and is changed in the reaction. and catalysing a reaction. specificityAn enzyme catalyses only one reaction because only one substrate has a shape complementary to its active site. depends on chemical interactions as well as a rough shape match.
A specific three-dimensional site (continued)
The substrate binds to form an enzyme–substrate complex. In an induced-fit model, binding brings about a conformational change that helps position interacting groups and stabilise the transition state. After reaction, products leave and the enzyme can catalyse another cycle.
The lock-and-key model represents complementarity but treats the active site as rigid. induced fitA model in which the active site changes shape slightly as the substrate binds, giving a closer fit and putting strain on bonds. captures the role of shape changes. Neither model means that an enzyme can reshape itself to catalyse any reaction with any molecule.
Temperature: motion and structural change
As temperature rises within a range where the enzyme remains stable, molecules have greater kinetic energy. More frequent successful encounters can increase enzyme–substrate complex formation and reaction rate. Low temperature usually slows activity rather than denaturing the enzyme.
Temperature: motion and structural change (continued)
At sufficiently high temperatures, disruption of interactions maintaining the enzyme’s structure changes its active site. The substrate may no longer bind effectively or catalysis may fail: the enzyme is denatured. Ordinary denaturationA permanent change to a protein’s tertiary structure, for example by high temperature or extreme pH, so an enzyme’s active site no longer fits its substrate. does not require peptide bonds to be broken.
The measured optimum depends on the enzyme and experimental conditions, including how long it is exposed to the temperature. Do not assume every enzyme has an optimum of 37°C or that cooling always restores activity after heating.
pH changes interactions
Changes in pHA measure of how acidic or alkaline a solution is, based on its H⁺ ion concentration. Below 7 is acidic, above 7 is alkaline. alter the ionisation of amino-acid side chains. This can change binding or catalytic groups at the active site and disrupt ionic and hydrogen-bonding interactions that help maintain structure. Extreme conditions may denature the enzyme.
Different enzymes have different pH optima because their active-site requirements and normal environments differ. A buffer helps hold pH constant during an experiment so that an intended change in another variable is not confused with a pH effect.
Concentration and initial rate
Increasing substrate concentration increases opportunities for enzyme–substrate complex formation while active sites are available. At high substrate concentrations, most sites are occupied for much of the time and the rate approaches a maximum for that enzyme concentration. Adding more substrate then has little effect.
Concentration and initial rate (continued)
Increasing enzyme concentration provides more active sites. initial rateThe rate at the start of a reaction, before the substrate is used up. It is found from the gradient of a tangent at time zero. can rise approximately in proportion if substrate is abundant and other factors remain constant. The same prediction may fail if substrate or another factor becomes limiting.
Initial rate is the rate near the start, before substantial substrate depletion or product accumulation changes conditions. For a product–time graph, use the gradient of a tangent at the start, or a sufficiently short initial straight section. Rate = change in product ÷ time, with appropriate units. A whole-experiment average is not necessarily the initial rate.
NOT: induced fit does not mean the enzyme can reshape itself for any substrate.
- Lock and key shows complementarity but treats the site as rigid
- Low temperature slows collisions; it does not denature
- Ordinary denaturation does not break peptide bonds
- At high substrate concentration the enzyme, not the substrate, limits the rate
Two traps this diagram blocks
- Do not give every enzyme an optimum of 37°C. The optimum depends on the enzyme and the experiment.
- Do not say the rate keeps rising in proportion to substrate concentration after the enzyme is saturated.
From the diagram alone
Question
Explain why the initial rate of an enzyme-catalysed reaction levels off at high substrate concentration. [3]
Show a model answer
At low substrate concentration, many active sites are free, so more substrate means more enzyme–substrate complexes. At high concentration essentially all active sites are occupied. The enzyme concentration then limits the rate, so more substrate does not increase it.
| Change | Expected effect under suitable conditions | Important limit |
|---|---|---|
| More substrate | More enzyme–substrate complexes | Finite active-site capacity |
| More enzyme | More available active sites | Substrate must remain sufficient |
| Higher temperature | Faster molecular motion | Structural instability at high temperatures |
Make your reasoning visible
Complete the denaturation explanation
Temperature or pHA measure of how acidic or alkaline a solution is, based on its H⁺ ion concentration. Below 7 is acidic, above 7 is alkaline. changes interactions maintaining structure → active-site structure changes → binding or catalysis is impaired. Avoid saying that an enzymeA globular protein that acts as a biological catalyst, speeding up a specific reaction. “dies”.
Explain a plateau using the limiting factor
At high substrateThe molecule that binds to an enzyme’s active site and is changed in the reaction. concentration, available enzyme active sites and their turnover limit rate. The substrate has not necessarily run out.
Choose the correct gradient
An initial-rate calculation must use the start of the curve. A tangent’s rise and run can use well-separated points on the tangent, not unrelated points on the curve.
Key explanations
- In induced fitA model in which the active site changes shape slightly as the substrate binds, giving a closer fit and putting strain on bonds., binding causes a small change in shape that positions groups and stabilises the transition state.
- The lock-and-key model shows a complementary shape but treats the site as rigid. Induced fit includes the shape change.
Avoid this misconception
The claim: More substrate always makes an enzyme-controlled reaction faster.
The correction: At a fixed enzyme concentration, active sites become limiting. Rate approaches a maximum under those conditions.
Check your understanding
Try each question before opening the suggested answer.
Practice question
Retrieval and explanation · suggested answer
An illustrative initial straight portion of a graph rises from 0 to 12 μmol of product in 20 seconds. Calculate the initial rate.
Show suggested answer
12/20 = 0.60 μmol s⁻¹, assuming that interval represents the initial linear portion.
Practice question
Retrieval and explanation · suggested answer
Explain why doubling enzyme concentration can approximately double initial rate when substrate is abundant. Why does this not necessarily double the total product formed by the end of the reaction?
Show suggested answer
Doubling enzyme provides twice as many active sites, so more enzyme–substrate complexes can react per unit time when other conditions remain constant. It does not add substrate. If the same amount of substrate is eventually converted, the final amount of product is unchanged; it is reached sooner.
Practice question
Retrieval and explanation · suggested answer
Why might activity return after warming a cold enzyme sample but not after cooling a strongly heated one?
Show suggested answer
Cold conditions may only slow molecular motion while leaving structure intact. Strong heating may denature the enzyme so that the functional active-site structure does not recover on cooling.
Take the idea further
Apply the same specification knowledge to an unfamiliar problem. Any extra assumptions are supplied in the question.
Stretch question
Two enzyme samples are assayed at 25°C. One was kept at 25°C; the other was heated strongly and then cooled to 25°C. The second has lower activity. Why is different current kinetic energy not an adequate explanation?
Show a hint
Both reactions are now at the same temperature.
Explore the reasoning
The present temperature is the same, so a difference in current temperature cannot explain the result. Prior heating may have denatured some enzyme molecules, reducing the number of functional active sites. Appropriate controls are needed to exclude other changes caused by the heating treatment.
Recall this lesson
Lesson recall questions
Question 1 of 5: Describe, in order, lock-and-key binding of a substrate.
Enzyme action and location
Practice question
Retrieval and explanation · suggested answer
Describe, in order, lock-and-key binding of a substrate.
Lesson: 2B 2: How Enzymes Work
Build the sequence from memory first. Then reveal the model order.
Tap the steps in the order you recall. Tap a placed step to send it back.
- 01Next step
- 02Waiting
- 03Waiting
Tap to place
Put your biology to work
Explore where these ideas appear in the world. Enrichment, not extra required recall.
Selecting enzymes for a new task
2B · How Enzymes WorkFrances Arnold’s directed evolution of enzymes was recognised in the 2018 Nobel Prize in Chemistry. Researchers generate variants and screen their performance, retaining variants useful under the chosen…Specification 2.7(i) · 2.8Read & use the scienceClose article
Why this matters
Frances Arnold’s directed evolution of enzymes was recognised in the 2018 Nobel Prize in Chemistry. Researchers generate variants and screen their performance, retaining variants useful under the chosen conditions. “Better” must be defined by the test: greater initial activity, longer stability or useful activity in a new solvent are different targets. A variant chosen under one set of conditions may perform poorly in another. This connects the Raw Enzymes extension to a real research method without duplicating its content. Researchers do not instruct a protein to improve; they create variation and use measurements to select it. The assay determines which properties are rewarded and which trade-offs may remain hidden.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 1, pp. 15–18. The application extends beyond required recall.
- 2.7(i) understand the mechanism of action and the specificity of enzymes in terms of their three-dimensional structure
- 2.8 CORE PRACTICAL 4 Investigate the effect of temperature, pH, enzyme concentration and substrate concentration on the initial rate of enzyme-catalysed reactions.
Use the science
Why should an enzyme selected for activity at one temperature also be tested under the conditions of its intended use?
Explore a possible response
A useful change in one condition may not improve performance in another. Temperature, pH, substrates and stability requirements can differ, so selection results are conditional on the assay.
Explore the sources
- Nobel Prize: directed evolution of enzymesNobel Prize
- Innovation by Evolution: Bringing New Chemistry to LifeFrances H. Arnold, Nobel Lecture (2018)
The enzyme inside a glucose sensor
2B · How Enzymes WorkMany glucose sensors use an enzyme reaction to turn a chemical concentration into an electrical signal. The enzyme helps the device respond to its target, while calibration connects…Specification 2.7(i) · 2.8Read & use the scienceClose article
Why this matters
Many glucose sensors use an enzyme reaction to turn a chemical concentration into an electrical signal. The enzyme helps the device respond to its target, while calibration connects the measured signal with concentration. This makes specificity, reaction conditions and the useful measurement range engineering problems. A sensor that works well in one test solution may behave differently when temperature changes or interfering substances are present. The laboratory scientists developing it must test performance across realistic conditions and compare results with a reference method. Enzyme biology therefore helps explain both the power and the limitations of a familiar medical technology.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 1, pp. 15–18. The application extends beyond required recall.
- 2.7(i) understand the mechanism of action and the specificity of enzymes in terms of their three-dimensional structure
- 2.8 CORE PRACTICAL 4 Investigate the effect of temperature, pH, enzyme concentration and substrate concentration on the initial rate of enzyme-catalysed reactions.
Use the science
Why could a change in temperature alter a sensor’s output even if the sample’s glucose concentration stays the same?
Explore a possible response
Temperature can affect the enzyme’s reaction rate and hence signal generation. Calibration or compensation is needed to separate concentration from other influences.
Explore the source
- Glucose biosensors in clinical practice: principles, limits and perspectives of currently used devicesPullano et al., Theranostics (2022)