What you need to know
- 2.7enzymeA globular protein that acts as a biological catalyst, speeding up a specific reaction. structure, specificityAn enzyme catalyses only one reaction because only one substrate has a shape complementary to its active site. and activation energyThe minimum energy needed to start a reaction. Enzymes lower the activation energy..
- 2.8Initial-rate investigation and control of variables.
By the end, you should be able to…
- Design a control for a claim about heating enzymes.
- Distinguish low activity from absence of enzyme.
Build your understanding
A claim to investigate
A fictional product advertisement says “raw enzymeA globular protein that acts as a biological catalyst, speeding up a specific reaction. activity is always better”. The word “better” has no scientific meaning until the desired reaction, conditions and outcome are specified. The textbook heading is retained, but this investigation is original.
Design a fair comparison
Take aliquots from one enzyme preparation. Heat one, leave another unheated, then bring both to the same assay temperature and pHA measure of how acidic or alkaline a solution is, based on its H⁺ ion concentration. Below 7 is acidic, above 7 is alkaline. before adding identical substrateThe molecule that binds to an enzyme’s active site and is changed in the reaction. solutions. Match enzyme volume, substrate concentration, total volume and timing. Include a substrate-only control and independent repeats.
Interpret an initial-rate result
If the previously heated sample has a lower 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. at the same assay temperature, prior structural damage is a possible explanation. It is not explained simply by lower current kinetic energy. If the claim concerns a practical application, test performance under the intended conditions rather than extrapolating from one assay.
Make your reasoning visible
Separate treatment from assay conditions
Equal assay temperatures let you investigate lasting effects of prior heating.
Define the endpoint
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. measures early activity; product yield at a much later time is a different outcome.
Avoid this misconception
The claim: No visible reaction proves there is no enzymeA globular protein that acts as a biological catalyst, speeding up a specific reaction. protein present.
The correction: Protein may be present but inactive, denatured or tested under unsuitable conditions.
Check your understanding
Try each question before opening the suggested answer.
Practice question
Retrieval and explanation · suggested answer
Why bring heated and unheated samples to the same temperature before comparing rates?
Show suggested answer
To prevent current assay temperature from confounding the effect of prior heating.
Practice question
Retrieval and explanation · suggested answer
Why include a substrate-only control?
Show suggested answer
It shows whether the signal changes without the added enzyme, such as through spontaneous reaction or instrument drift.
Take the idea further
Apply the same specification knowledge to an unfamiliar problem. Any extra assumptions are supplied in the question.
Stretch question
A heated sample has low activity but regains it after cooling for longer. What conclusion about irreversible denaturation would be too strong?
Show a hint
The evidence includes recovery.
Explore the reasoning
It would be too strong to claim that all molecules were irreversibly denatured. Recovery could reflect reversible structural changes or surviving active molecules; further controlled measurements are needed.
Recall this lesson
Lesson recall questions
Question 1 of 5: Compare the lock-and-key model with the induced-fit model of enzyme specificity.
Enzyme action and location
Practice question
Retrieval and explanation · suggested answer
Compare the lock-and-key model with the induced-fit model of enzyme specificity.
Lesson: 2B 2: How Enzymes Work
Try this from memory first.
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)
Can an enzyme recycle a plastic bottle?
2B · EnzymesResearchers engineered FAST-PETase, an enzyme that breaks bonds in the polyester PET, and tested it on several post-consumer products. The work linked changes in amino-acid sequence with useful…Specification 2.7(i) · 2.7(ii) · 2.8Read & use the scienceClose article
Why this matters
Researchers engineered FAST-PETase, an enzyme that breaks bonds in the polyester PET, and tested it on several post-consumer products. The work linked changes in amino-acid sequence with useful activity under selected temperature and pH conditions. That is an extension of enzyme specificity and experimental rate measurements, not evidence that one enzyme digests every plastic. PET products also differ in structure and accessibility, so results depend on the material tested and any pretreatment. Recovering monomers can make chemical recycling possible, but an industrial process must additionally account for energy, separation, enzyme production and repeated performance.
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
- 2.7(ii) understand that enzymes are biological catalysts that reduce activation energy
- 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 would an enzyme that hydrolyses PET not necessarily break down polyethylene, even though both are called plastics?
Explore a possible response
Their chemical structures and bonds differ. An enzyme’s active site and catalytic mechanism are specific; polyethylene lacks PET’s ester linkages.
Explore the source
- Machine learning-aided engineering of hydrolases for PET depolymerizationLu et al., Nature (2022)