Enzymes as catalysts
pp. 380–395A catalyst speeds up a reaction without being used up. Without enzymes, many reactions would be too slow to sustain life. Heat would speed them up, but it would also damage proteins.
Guiding questions
- How do enzymes act on, and respond to, other molecules?
- Which parts of metabolism depend on one another?
Slides
Notes
Written for this site from the class slides. The chapter is on pages 380–395 of the course textbook. Image credits are on slide 18.
Why enzymes matter
The role of enzymes in metabolism
pp. 380–395Metabolism is the complex network of interdependent reactions in an organism. Enzymes are specific, so organisms need many different ones. A cell can control metabolism through its enzymes.
Anabolic and catabolic reactions
pp. 380–395| Anabolic: builds | Catabolic: breaks down |
|---|---|
| Protein synthesis, by condensation | Digestion, by hydrolysis |
| Glycogen formation | Oxidation of substrates in respiration |
| Photosynthesis | Decomposition |
How enzymes work
Globular proteins with an active site
pp. 380–395The active site contains only a few amino acids. Interactions between amino acids throughout the protein hold it in shape, and give it the properties needed for catalysis.
Induced-fit binding
pp. 380–395
Both the substrate and the enzyme change shape as binding occurs. This places stress on bonds in the substrate.
Molecular motion and collisions
pp. 380–395Movement is needed for a substrate and active site to meet. In solution, random motion causes collisions. Large substrates can be immobilized, and enzymes can be fixed in membranes. In these cases, only the other molecule moves.
Active site, specificity and denaturation
pp. 380–395The active site has a shape and chemical properties that are complementary to the substrate, and this gives specificity. In denaturation, heat or extreme pH changes the active site, so the substrate can no longer bind.
Rates of reaction
How temperature, pH and substrate affect the rate
pp. 380–395| Factor | Below the optimum, or low | Above the optimum, or high |
|---|---|---|
| Temperature | Rate rises, with more collisions | Rate falls sharply, due to denaturation |
| pH | Rate falls on either side of the optimum, as bonds holding the active site are disrupted | |
| Substrate | Rate rises, with more collisions | Rate levels off, as active sites are saturated |
Measuring reaction rates
pp. 380–395Rate is the change in substrate or product per unit of time. A curve is steepest at the start, so use the initial rate. For example, 6 cm³ of oxygen in 20 seconds is 0.3 cm³ per second.
Enzymes lower activation energy
pp. 380–395Energy is needed to break bonds in the substrate, and energy is released as bonds form in the products. An enzyme lowers the activation energy, without changing the total energy released.
Videos
Select a video to play it here. Select it again to close it.
Note-taking sheet
Print this guide and complete it as you work through the slides and videos. It asks for your own explanations, so close the notes while you try each section.
Quick self-check
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Practice questions
A printable worksheet covering all ten statements, from quick recall to data questions and a longer answer. It has 44 marks and takes about 45 minutes, or you can do one section at a time.
Your teacher has the mark scheme.
Data-based questions
Questions built on realistic experimental data, in the style of Paper 1B and Paper 2. Work them on paper and show your working. 8 marks in total. Your teacher has the mark scheme.
A cofactor and an immobilised enzyme
8 marks
Hexokinase from yeast was tested free in solution and immobilised in alginate beads. ATP was kept at 1.0 mmol dm−3, and the concentration of magnesium chloride (MgCl2) was varied. Activity is given as a percentage of the highest activity measured for that form of the enzyme.
- State the relative activity of the free enzyme at 1.0 mmol dm−3 MgCl2. [1]
- Identify the MgCl2 concentration that gives the highest activity for each form of the enzyme. [2]
- Calculate the ratio of ATP to MgCl2 concentration at which the free enzyme is most active. [1]
- Explain why there is almost no activity without magnesium ions. [2]
- Suggest why the immobilised enzyme needs a higher MgCl2 concentration to reach its highest activity. [1]
- State one advantage of immobilising enzymes for use in industry. [1]
Practical
Temperature and catalase activity
You will need: Potato or yeast suspension, 3% hydrogen peroxide, water baths, measuring cylinder, boiling tubes
- Warm enzyme and peroxide separately to each temperature
- Mix them, and measure the height of foam or gas collected after one minute
- Repeat at five temperatures
- Plot rate against temperature and explain the shape of the curve
Safety: Eye protection. Hydrogen peroxide irritates, so wash off any skin contact.
Carry this out with your teacher's supervision and follow your school's lab rules.
Nature of science and TOK
Nature of science
Models
Fischer's lock-and-key model of 1894 was refined by Koshland's induced-fit model in 1958, as evidence showed that enzymes change shape.
Theory of knowledge
Is a simpler model ever better than a more accurate one?
Lock and key is still taught first. Consider when usefulness and accuracy pull in different directions.
Syllabus statements
Summarised in our own words. Use each code to find the exact wording in your textbook or the IB subject guide.
- C1.1.1Enzymes act as catalystsp. 380
- C1.1.2What enzymes do in metabolismp. 381
- C1.1.3Anabolic reactions compared with catabolic reactionsp. 382
- C1.1.4Enzymes: globular proteins with an active site where catalysis happensp. 383
- C1.1.5Induced fit: how substrate and active site interact as they bindp. 384
- C1.1.6Why molecular motion and collisions between substrate and active site matterp. 386
- C1.1.7How the shape of the active site explains specificity and denaturationp. 387
- C1.1.8How temperature, pH and substrate concentration change the rate of an enzyme reactionp. 387
- C1.1.9How to measure enzyme-catalysed reactionsp. 394
- C1.1.10How enzymes lower activation energyp. 395
HL extension
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