Teaching Enzymes With Practicals

Enzymes are one of the best topics for practical work in school science: the reactions are safe, cheap and fast enough to measure in a lesson, and the results make clear graphs. They’re also a topic loaded with misconceptions — “enzymes die”, “the enzyme is used up”, “heat makes molecules bigger”. This guide sets out a four-lesson sequence for students aged roughly 14–17 that uses practicals and simple models to build a clear, chemical picture of how enzymes work, with notes on what students commonly get wrong.

Learning goals

By the end of the sequence, students should be able to:

1. Describe enzymes as protein catalysts that lower activation energy.
2. Explain specificity using the active site and the induced-fit model.
3. Explain the effects of temperature, pH and substrate concentration on rate.
4. Plan, carry out and evaluate an enzyme investigation, including calculating rates.
5. Distinguish competitive and non-competitive inhibition (for older groups).

Prior knowledge: proteins are chains of amino acids; the idea of reaction rate and collision theory; simple graph skills.

Lesson 1: what do catalysts do?

Starter demo: catalase in action (10 minutes)

Pour 20 cm³ of 3 % hydrogen peroxide into each of three measuring cylinders, each with a drop of washing-up liquid. Add: nothing to the first; a spatula of manganese(IV) oxide to the second; a few cubes of raw potato or liver to the third. Foam rises in the second and third.

Ask: “Where did the foam come from? What do the manganese(IV) oxide and the potato have in common?” Then show the equation:

2H₂O₂ → 2H₂O + O₂

Both are catalysts. The potato contains the enzyme catalase. Recover the manganese(IV) oxide by filtering to show that catalysts aren’t used up — a point to return to often.

Explanation (15 minutes)

Draw an energy profile diagram with and without a catalyst (see reaction rates and catalysts). Emphasise that a catalyst provides an alternative pathway with a lower activation energy; it doesn’t change the overall energy change.

Then introduce enzymes as biological catalysts, almost all proteins, each with an active site (see enzymes explained).

Model: the active site (15 minutes)

Give pairs cut-out shapes: an “enzyme” with a notch and several “substrate” shapes, only one of which fits. Then give a floppy foam or plasticine version to show induced fit — the active site moulds around the substrate. Discuss what the model gets right (shape matters, specificity) and wrong (real active sites use charges and hydrogen bonds, not just shape; see lock-and-key vs induced fit).

Lesson 2: the effect of temperature (amylase and starch)

Practical: amylase with starch

Principle: amylase hydrolyses starch into maltose. Iodine turns blue-black with starch and stays orange-brown when starch has gone. Timing how long it takes for starch to disappear gives a measure of rate.

Method (per group):

1. Place drops of iodine solution in each well of a spotting tile.
2. Put 2 cm³ of 1 % starch and 2 cm³ of 1 % amylase in separate tubes in a water bath at the chosen temperature for 5 minutes to equilibrate.
3. Mix, start the timer, and every 30 seconds transfer one drop of the mixture to a fresh iodine well.
4. Record the time when the iodine no longer turns blue-black.
5. Repeat at 20, 30, 40, 50 and 60 °C (share temperatures across groups).

Rate = 1 / time (s⁻¹). Students plot rate against temperature.

Safety: eye protection; iodine stains; amylase can be a sensitiser — avoid skin contact and don’t generate dust from powder.

Discussion

• Rising part of the curve: more kinetic energy, more frequent and more energetic collisions.
• Falling part: denaturation — bonds holding the tertiary structure break, the active site changes shape (see denaturation).

Stress the language: the enzyme is denatured, not killed. It was never alive.

Lesson 3: pH and substrate concentration (catalase)

Practical: catalase and hydrogen peroxide

Principle: measure the volume of oxygen produced in a fixed time, or the time for a filter-paper disc soaked in catalase (from potato extract) to rise through a tube of hydrogen peroxide. Faster rising = faster reaction.

Floating-disc method:

1. Blend potato with cold water and filter to make catalase extract.
2. Dip a 5 mm paper disc into the extract for a fixed time, drain it, then drop it into a tube containing 10 cm³ of hydrogen peroxide mixed with a pH buffer.
3. Time how long the disc takes to rise to the surface. Repeat three times.

Groups vary pH using buffers (pH 4–9; see buffers explained), or vary hydrogen peroxide concentration (0.5 %, 1 %, 1.5 %, 2 %, 3 %).

Safety: 3 % hydrogen peroxide is an irritant — eye protection, and wash spills. Avoid higher concentrations.

Analysis

• pH: a peak at the optimum; explain with ionic and hydrogen bonds changing as charges on side chains change.
• Substrate concentration: the rate rises, then levels off. Ask students why it levels off, and guide them to saturation — all active sites busy. For older students, introduce Vmax and Km (see enzyme kinetics).

Lesson 4: inhibition and real-world enzymes

Practical (optional): trypsin and milk

Trypsin, a protease, clears cloudy dilute milk powder solution as it digests the protein casein. Timing the clearing gives a rate. Add a small amount of a known inhibitor supplied by the prep room, or compare heat-treated trypsin, to discuss loss of activity.

Card sort: inhibitors

Give descriptions of inhibitors to classify as competitive or non-competitive:

• “Has a similar shape to the substrate” → competitive
• “Binds away from the active site and changes its shape” → non-competitive
• “Effect is reduced when more substrate is added” → competitive

Link to real examples: malonate and succinate dehydrogenase; many medicines such as statins (see enzyme inhibition).

Applications discussion

Biological washing powders (proteases and lipases working at lower temperatures), lactose-free milk (lactase), meat tenderisers (papain, bromelain), and why fresh pineapple stops jelly setting.

Misconceptions to expect

Misconception	How to tackle it
“Enzymes are alive / die at high temperatures.”	Enzymes are molecules; use “denatured”.
“The enzyme is used up in the reaction.”	Recover MnO₂ in lesson 1; show the same enzyme working on fresh substrate.
“Heat makes the enzyme work harder until it breaks.”	Separate collision frequency (both molecules move faster) from structure (bonds break).
“Denaturation melts the enzyme.”	Only weak bonds break; the amino acid chain is intact.
“All enzymes have an optimum of 37 °C.”	Show enzymes from thermophilic bacteria, which work at 70 °C or more (the Taq polymerase used in PCR).
“Optimum pH is always 7.”	Compare pepsin (≈2), amylase (≈7) and trypsin (≈8).
“The substrate is a key that must fit exactly.”	Discuss induced fit and flexibility.

Graded questions

Recall

1. What is an enzyme? (A biological catalyst, usually a protein.)
2. Name the part of an enzyme where the substrate binds. (Active site.)

Application 3. In the amylase practical, the iodine stays blue-black for the whole 10 minutes at 70 °C. Explain. (Amylase denatured; starch not broken down.) 4. Why were the enzyme and substrate put in the water bath separately before mixing? (So both start at the test temperature; otherwise the reaction starts at the wrong temperature.)

Analysis 5. A floating disc took 20 s at pH 7 and 50 s at pH 4. Calculate each rate and the percentage decrease. (0.050 s⁻¹ and 0.020 s⁻¹; a 60 % decrease.) 6. Suggest two sources of error in the floating-disc method and how to reduce them. (Different disc sizes or soaking times — use a hole punch and a timer; air bubbles on discs — tap gently; temperature changes — water bath.)

Challenge 7. Sketch a rate–substrate graph with and without a competitive inhibitor. Explain why the curves meet at high substrate concentration.

Differentiation

• Support: results tables with headings; graph axes pre-drawn; a word bank.
• Stretch: calculate Q₁₀; plot a Lineweaver–Burk graph; research how enzymes in thermophiles resist denaturation (more ionic bonds and disulfide bridges).

Summary for the teacher

• Use catalase for the “wow” start and for pH and concentration work; amylase for temperature.
• Keep returning to collisions (rate rises) versus denaturation (rate falls).
• Make students calculate rates (1/time) rather than just reading times.
• Tackle “enzymes die” and “enzymes get used up” head-on.

Students can consolidate with the enzymes practice questions and the full enzyme experiment guide.