An Introduction to Enzymes

Enzymes first make an appearance in Year 9 and although most students at this level quickly grasp that these globular proteins speed up chemical reactions there are always a number of stubborn misconceptions about exactly what they are and how they work. Below are a few ideas for class practicals (tried and tested – enzyme experiments are notoriously fickle) and activities that can help at Key Stage 3 and beyond.

Practical 1 – Factors affecting the activity of catalase

Science is about discovery and students should be given opportunities to actually be scientists by discovering things for themselves. Too often teachers feel that they have to tell students everything, explaining exactly what will happen in an experiment and leaving nothing to be explored. So instead I turn the topic of enzymes on its head and start with this class practical investigating factors which affect the activity of catalase.

At the end of the lesson ask the students to describe what has happened and make some simple deductions; they will have seen that both liver and potato share a curious ability to break down hydrogen peroxide and release bubbles of gas, but that boiling them removes their ability to do so – why? By the time you start talking about enzymes, the lock and key theory and denaturing, the students’ curiosities will have been stirred and they will want to know how on Earth it all works.

You will need per group:

  • 6 boiling tubes in a test tube rack
  • 4 watch glasses
  • 30 cm3 hydrogen peroxide (20%) solution
  • 10 cm3 measuring cylinder
  • Forceps
  • Dropping pipette
  • Boiling water bath
  • Ruler
  • Raw liver, cut into  5 g cubes
  • Raw potato, skin removed and cut into 5 g cubes
  • Glass rod
  • Pestle and mortar
  • Stopwatch

Steps:

  1. Label the boiling tubes A, B and C and the watch glasses B and C.
  2. Measure out 5 cm3 of hydrogen peroxide solution into each boiling tube.
  3. Place a 5 g cube of raw liver into the boiling water bath and leave for two minutes.
  4. Use the forceps to carefully remove the cube from the water bath, and place it on watch glass B.
  5. Grind one raw liver cube with the pestle and mortar, and transfer the paste to watch glass C.
  6. Add the remaining raw cube of liver to boiling tube A and after one minute record the height of froth in the boiling tube.
  7. Repeat with the boiled cube in boiling tube B and the raw liver paste in boiling tube C.
  8. Repeat the experiment using potato cubes instead of liver (the potato cubes are difficult to grind in a pestle and mortar so you may need to cut them up into smaller pieces first).
  9. Record the results in a suitable table.

Modelling enzyme action

Follow the class practical by modelling the protein structure of enzymes, discussing the lock and key theory and demonstrating what happens when an enzyme is denatured.

Locked out

Begin the lesson by purposely locking yourself and the students out of the classroom. Produce a big handful of keys and make a fuss about finding the right key to fit the lock. Once inside, introduce the lock and key theory of enzyme action.

Complementary pairs

Another good starter activity is to cut large pieces of paper into complementary enzyme and substrate molecules then hand one out to each student as they enter the classroom and ask them to find their partner.

complementary sites

Amino acid necklace

Give each student a shoelace or a long piece of string and a handful of different coloured beads. Ask them to thread the beads onto the shoelace in any order they wish in order to make a colourful necklace. Explain that enzymes are large protein molecules made of many amino acids joined together in a long chain, a bit like the beads on their necklace.

beads

Ask the students to screw the necklace into a tight ball to make an ‘enzyme’. Highlight that everyone in the class has made a different type of enzyme because the sequence of ‘amino acids’ on their necklace and the 3D shape of the balls are all different.

necklace

Plasticine models

Make two or three enzymes with different shaped active sites using plasticine or modelling clay. Demonstrate that the substrate molecule only fits the active site of one type of enzyme, before reshaping the plasticine to show what happens when the enzyme is denatured.

Student enzyme models

Nominate two or three students to play the role of enzymes by standing up and putting out their hands in front of them to model an active site. Use a piece of scrap paper as the substrate molecule and move from one student to another until you find the complementary enzyme (in truth this can be any one of the students but it helps to reinforce the specificity of enzyme action).

Model digestive enzymes by gesturing for the chosen student to rip the paper in two before throwing the products dramatically into the air so that their active site is free to accept a second substrate molecule.

found-two-bits-of-paper-while-doing-some-sorting-out-yesterday-that-uEnPFm-clipart

You can also model the action of anabolic enzymes by asking the student to hold two ‘substrate molecules’ together in their active site while a bond forms between them (using sellotape). Again, encourage the student to throw the product dramatically into the air, leaving their active site free to repeat the process.

High five collisions

This is a very simple yet effective way of demonstrating the effect of temperature on an enzyme-catalysed reaction. With reference to a graph of enzyme activity against temperature explain that at low temperatures the average kinetic energy of the enzyme and substrate molecules is low and as such they move very slowly and collide only infrequently. Model this by asking the students to trudge slowly around the classroom and to high five one another on the odd occasion that they meet.

Now turn up the temperature. Ask the students to move around the room a little more quickly, again high fiving when they collide. The students should be able to hear that the number of successful collisions has now increased.

Increase the temperature further still. The students will now be whizzing around the room (careful!) and high fiving almost constantly. The noise of substrate molecules and enzymes colliding will be deafening. This is of course the optimum temperature and the enzyme’s catalytic activity is at its greatest.

Finally, raise the temperature beyond the optimum, denaturing the enzyme and inhibiting the formation of enzyme-substrate complexes. Ask the students to lower their hands so that they can no longer high five. They will still be whizzing about (in fact, faster than before) and will certainly collide but the collisions will no longer be successful. The classroom will fall silent, the reaction has stopped.

Flowmap donuts, zoetropes and flickbooks

Instead of using linear flowmaps to illustrate the stages of an enzyme-catalysed reaction use flowmap donuts, zoetropes or flickbooks to highlight that enzymes remain unchanged by the reaction and can be used again. The students could even animate their plasticine models using stop motion applications such as Stop Motion Studio.

zoetrope 2

Practical 2 – Investigating the effect of temperature on the activity of lipase

A simple protocol which provides reliable, unambiguous results. The investigation can be carried out as a demonstration at two different temperatures, or in groups of five or six students with each student working at a different temperature, allowing enough time to collect repeat data. A nice extension is to add washing-up liquid to the solution in order to emulsify the fats and provide a larger surface area for enzyme action (demonstrating the effect of bile salts in the digestive system).

Full teaching notes and student sheets are available to download from the Nuffield Foundation.

Practical 3 – Investigating the effect of pH on amylase activity

Another reliable class practical from the Nuffield Foundation, this time measuring the time taken for amylase to completely break down starch at different pHs. Again, students can work in groups of five or six with each student working at a different pH before pooling results.

Virtual lab

However, if time is tight, one alternative is to use the excellent Virtual Lab from McGraw-Hill Education, in which students can investigate both the effect of pH and substrate concentration on an enzyme-catalysed reaction from their computer or tablet.

Screen Shot 2559-08-13 at 12.52.26

BBC Bitesize

A nice video from BBC Bitesize which can be used to summarise much of the Key Stage 3 and Key Stage 4 content on enzymes.

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