How I teach enzyme kinetics (without PowerPoint)

In this post, I explain how I've taught enzyme kinetics without slides or worksheets to 14-year-olds. This lesson followed another on enzyme structure and function, which you can find here. You can read about the benefits of teaching without PowerPoint here.

Enzymes are abstract; invisible entities that appear to do work. As with anything in biology, meaning stems from relating what students learn to how it affects action in our world. This involves relating enzymes to how organisms live, and was the principal theme of Biology Made Real.

Meaning before molecules

Endotherms have an advantage

Before looking at the molecular level, I begin by discussing endothermy. This brings a natural contrast between the warmer body temperatures of endotherms and the cooler body temperatures of ectotherms. It's in this difference that they'll find meaning.

I tell the students that endothermy evolved in mammals maybe 100 million years ago. At that time, large dinosaurs dominated the landscape, and mammals found a living by burrowing underground and only appearing at night.

As temperatures drop at night, chemical reactions will slow down. Access to energy through cell respiration would be lower. Physical performance would suffer. Yet, mammals evolved to be warm all the time and could maintain their high performance during the chill of the night.

Birds also evolved to be endothermic. They evolved to fly, too, a highly energetic activity. Imagine a predator lunging at a bird on the ground, it must fly away immediately to survive. It couldn't ask the predator to wait while it warms up its muscles. These demands selected against any birds that couldn't muster the energy in time.

Nowadays, you won't find a food web in which a reptile dominates as the top predator; mammals and birds always do. There is an exception: the tiny island of Flores, where the komodo dragon (a monitor lizard) is the top predator due to the absence of mammal and bird predators.

There's more, endotherms have the energy available to grow much quicker. Mammals and birds go through a rapid growth phase to quickly reach a determined adult form. On the other hand, crocodiles will grow very slowly throughout their entire lives. Slow growth brings the disadvantage of spending more time in smaller, more vulnerable forms and having to live in a place that has food available for small, medium, and large sizes.

So what's the disadvantage? Mammals and birds must eat prodigious quantities of food. Some, like cattle, appear to spend their entire day eating. Even we humans eat regularly throughout the day. Fasting is difficult for us because 80% of what we eat may go towards heating our bodies. Without this commitment, a large crocodile may only eat every 6 months.

Beginning a diagram

Together, we drew the axes and marked the typical human body temperature. I then asked the students to predict the line (in pencil). Having just discussed the benefits of higher temperatures, some students were tempted to draw a straight positive line. Others, however, related the graph to their lives and knew we died at high temperatures.

The next question, then, was why we died if we were too cold or too hot. I drew my line and discussed how we did so due to low reaction rates. Our brains, especially, require a constant flow of energy, and if the rates drop too low, the brain begins to break down.

Explaining the mechanism

So far, we've contextualised enzymatic reactions by contrasting colder and warmer body temperatures and their ecological effects. But the graph in itself just reinforces what students know; we die if we stray too far from our optimum temperature.

This follows an input → output format:

Temperature too high → we die.

To move students forward, we need them to explain. This is a form of metacontent that I explain in Difference Maker.

We are interested in the causes of things and, therefore, need students to think with a different structure: input → cause(s) → output. We'll need two explanations, one for each side of the graph.

Explaining kinetics

To explain the first (positive) section of the graph, I used a simulation. Find the simulation here (you'll need both files). This allowed me to vary the temperature and visualise the effect on the ,enzyme and substrate. As the temperature increased the molecules moved faster and collided more often.

We added this to the diagram:

The image of the enzyme shows that is moves more quickly. But to make the mechanism explicit, I I drew the (incomplete) dot and arrow causal diagram. I wrote "↑ energy" and asked the students to discern the following causal steps.

We agreed on this:

↑ energy → ↑ movement → ↑ successful reactions

Explaining hyperthermia

So, why do we die if we get too hot? Why does the line drop again? I explain that if this were chemistry, the line would remain positive. There's something about enzymes that's different.

In the simulation, eventually the enzyme stops catalysing reactions completely. But it's looks exactly the same, so what's the happening?

I explained to students that enzymes have a specific shape due to how they fold up. We'd studied the importance of the shape of the active site in the previous lesson. The aspect of folding was new however. To show this I held my arms out wide and then slowly folded them onto my chest.

With extra energy, the enzyme doesn't just move quicker as a whole, its folds begin to move. I jiggled and jived my arms, flexing them around. As the energy increases the shape can be lost entirely, and to show this I flung out my arms wide again while continuing to wave them around.

We added a new drawing to the diagram and I began another mechanism for students to discuss and complete in pairs:

This time, the students needed a mechanism that explained why the rate of reaction decreased. After some discussion in pairs, and then as a whole class, we agreed on this:

↑ energy → ↑ active site denaturation → ↓ successful reactions

There were some finer points I wanted to make clear before we moved on. So far, we'd contrasted only not denatured with denatured so much that we die. But this is a spectrum that we needed to explore.

For example, what if we get a fever and our temperature reaches 38 degrees? Why don't we die?

I let students discuss this in pairs and then held a class discussion. To help, we all put our fingers on the line and followed it from 37 to 38 degrees. I asked the class again why we didn't die.

Is the active site becoming denatured? They agreed it was. But the rate of reaction is still high enough to keep us alive. What must be the cause? We agreed that while the active site's shape was beginning to change, it hadn't yet changed enough to significantly lower the rate.

Now that we had established a mechanism, I also explained that pH would affect the reaction rate. We'd need this idea later when studying digestion. We drew the following diagram together:

To finish the lesson, I had the students peer-explain the concept in pairs. To do this, the students take it in turns to explain to each other as if their partner hadn't been in the lesson. I walked around the lesson and answered any questions the students had. Learn more about teaching through diagrams and dialogue – without lecturing, slide decks, or worksheets – in my book Difference Maker.

Final co-constructed diagram: