Enzymes and metabolism
IB Biology SLΒ· Topic 2.5: EnzymesΒ· 25 min read
1. Core Properties of Enzymesβ β ββββ± 8 min
Enzyme
Globular proteins that act as biological catalysts, lowering the activation energy of chemical reactions without being altered or consumed in the process.
Example:
Catalase, the enzyme that breaks down toxic hydrogen peroxide in mammalian liver cells.
All enzymes have a unique 3D tertiary structure that forms a pocket called the active site, which is complementary in shape and chemical properties to one specific substrate molecule. This specificity means most enzymes only catalyze one single metabolic reaction, and do not interact with unrelated molecules.
Identify which of the following statements correctly describes a property of enzymes: A) Enzymes are consumed after 10 reaction cycles, B) Enzymes lower the activation energy of a reaction, C) Enzymes raise the final free energy of product molecules, D) Enzymes work equally well across all pH values.
- 1
Eliminate option A: Enzymes are not consumed at any point in the reaction, so they can be reused indefinitely for thousands of cycles.
- 2
Eliminate option C: Enzymes only affect activation energy, not the total free energy of reactants or final products.
- 3
Eliminate option D: Enzymes have a narrow optimal pH range, and extreme pH values denature their active site permanently.
- 4
Confirm option B is correct: The core function of all enzymes is to reduce activation energy to speed up reaction rates.
2. Models of Enzyme Actionβ β β βββ± 7 min
Test your understanding of the two models:
Which model explains why the active site is not a perfect pre-formed match for the substrate?
Reveal answer
Induced fit model βThe induced fit model accounts for the dynamic, flexible nature of the globular protein structure that forms the active site.
Exam tip:
When asked to 'describe the induced fit model' for 3 marks, you must mention: 1) active site is not perfectly complementary pre-binding, 2) substrate triggers conformational change, 3) this change strengthens the enzyme-substrate complex to reduce activation energy.
3. Factors Affecting Enzyme Reaction Rateβ β β βββ± 6 min
Factor | Effect on rate as value increases | Explanation of plateau |
|---|---|---|
Temperature | Rises to optimum, then drops sharply to zero | Above optimum, hydrogen bonds in tertiary structure break, denaturing the active site |
pH | Peaks at narrow optimum, drops on either side | H+ or OH- ions alter the charge of amino acid R-groups in the active site |
Substrate concentration | Rises linearly, then plateaus at maximum Vmax | All available active sites are fully saturated with substrate at any given time |
Explain the shape of a catalase reaction rate curve as substrate concentration increases from 0 to 1.0 mol dmβ»Β³.
- 1
Start at 0 reaction rate when there is no substrate present to bind to active sites.
- 2
At low substrate concentrations, rate increases proportionally with substrate, as more active sites have molecules to bind.
- 3
At ~0.4 mol dmβ»Β³, the curve levels off to a flat horizontal line at Vmax.
- 4
Explain the plateau: every enzyme active site is occupied at all times, so adding more substrate cannot increase the rate further.
4. Inhibitors and Metabolic Pathway Regulationβ β β β ββ± 6 min
End-product inhibition
A regulatory mechanism for metabolic pathways where the final product of the reaction sequence binds to an allosteric site on the first enzyme in the pathway to stop over-production of the product.
Example:
ATP inhibiting phosphofructokinase in the glycolysis pathway.
Competitive inhibitors bind to the active site directly, and can be overcome by increasing substrate concentration
Non-competitive inhibitors bind to an allosteric site separate from the active site, and cannot be overcome by adding more substrate
5. Common Pitfalls
Wrong move:
Stating that enzymes are denatured at temperatures below their optimum
Why:
Low temperature only reduces molecular kinetic energy, it does not break the bonds that maintain the active site shape
Correct move:
Write that low temperature reduces reaction rate due to fewer successful collisions between substrate and active site, with no permanent denaturation.
Wrong move:
Describing the induced fit model as 'the substrate changes shape to fit the active site'
Why:
The substrate is usually a small, rigid molecule β the enzyme's active site is the part that undergoes conformational change
Correct move:
Explicitly state that the active site of the enzyme alters its shape to form a tight, complementary fit around the bound substrate.
Wrong move:
Claiming that increasing enzyme concentration will never increase reaction rate
Why:
If substrate is in excess, adding more enzyme will increase the number of available active sites, raising Vmax
Correct move:
Note that Vmax only plateaus when all active sites are saturated, so extra enzyme will raise the maximum rate if substrate is abundant.
Wrong move:
Classifying end-product inhibition as a form of competitive inhibition
Why:
The end product binds to a separate allosteric site, not the active site of the first enzyme in the pathway
Correct move:
Label end-product inhibition as an example of allosteric non-competitive inhibition, as required by IB mark schemes.
Wrong move:
Writing that enzymes 'raise the activation energy' of a reaction
Why:
This is the exact opposite of their core function, and is one of the most common mark scheme penalties
Correct move:
Always confirm you write that enzymes lower activation energy to speed up the conversion of substrates to products.
6. Quick Reference Cheatsheet
Key term | IB mark scheme definition |
|---|---|
Enzyme | Globular biological catalyst that lowers activation energy, not consumed in reaction |
Active site | Region of enzyme complementary to substrate shape and chemical properties |
Induced fit | Active site changes shape to tightly bind substrate after initial contact |
Vmax | Maximum reaction rate when all enzyme active sites are fully saturated |
End-product inhibition | Allosteric regulation of metabolic pathways by final product binding to first enzyme |
7. Frequently Asked
Do I need to memorize both lock-and-key and induced fit models for the exam?
Yes, mark schemes explicitly award marks for stating that the induced fit model is the currently accepted, updated version that accounts for conformational change of the active site, while lock-and-key is the older, simplified model that assumes a rigid active site.
Why is end-product inhibition classified as non-competitive in IB mark schemes?
IB examiners define end-product inhibition as a form of allosteric non-competitive inhibition, where the end product binds to a site separate from the active site to alter enzyme shape, so you must not classify it as competitive to avoid losing marks.
When this came up on past exams
AI-estimated based on syllabus patterns β cross-check with official past papers for accuracy. Use only as revision-focus signals.
- 2025 Β· P1
Enzyme pH effect multiple choice
- 2024 Β· P2
Induced fit model 6 mark question
- 2023 Β· P1
End product inhibition identification
Going deeper
What's Next
Mastering enzymes and metabolism is critical for scoring well on both Paper 1 multiple choice and Paper 2 extended response questions, as this topic frequently overlaps with content on cellular respiration, photosynthesis, and molecular biology lab practicals. The concepts you have learned here will also directly support your work on the required enzyme catalase IA investigation, where you will design and execute your own experiment to test the effect of a variable on reaction rate. This foundational knowledge of biological catalysts will help you contextualize all metabolic processes covered later in the syllabus.