What changes the rate of an enzyme-controlled reaction

Would an enzyme still work in water hot enough to scald you? The tempting answer is no: anything much hotter than your body must wreck an enzyme. But in the nineteen-sixties, scientists found a bacterium living in the hot springs of Yellowstone, in the United States. Its enzymes work in water at around seventy degrees. One of them, called Taq, now copies DNA in laboratories all over the world. Why that enzyme thrives where ours would be wrecked comes down to one curve.

In an enzyme's active site, a substrate that fits is changed into products. The rate of reaction is how much of that happens per second, or per minute. Use two phrases for it: rate of reaction, and enzyme activity. A higher rate means more product made every second. Here are readings for one enzyme, from ten to sixty degrees C. Plotted, they climb to a peak at forty, then fall away sharply. Draw them as one smooth curve through the points, never straight lines from point to point. The readings are samples of a smooth curve. The peak is this enzyme's optimum temperature, forty degrees C. That is where its activity is highest. Now a prediction: from ten to forty degrees, what is making the rate climb? Collisions. Warmer molecules move faster, so substrate molecules hit active sites more often, and the rate rises. Past forty, the reason is different. The heat makes the active site change shape, so the substrate no longer fits. The enzyme is being denatured, and the rate collapses. The two sides of the peak have two different reasons, and no reason word is shared: collisions on the left, denatured on the right. Run your eye to the far left of the curve, where the rate is very low. At five degrees, is this enzyme denatured, or is something else going on? Something else. Molecules move slowly, so collisions are rare and the rate is low, but every active site keeps its shape. Warm it up and the rate comes back. A cold enzyme is slowed, never denatured. One examiner's report, on a Higher paper, records two slips about this curve. For marking point two, it was often stated that the enzyme had an optimum temperature, but it wasn't always stated what this meant in terms of enzyme activity, so just giving half the story. And it names the cold side as the least well understood marking point. Many think that low temperature also denatures the enzyme. Instead, finish every optimum sentence with what it means for activity, and give each side of the peak its own reason. Denatured never appears on the cold side.

Every enzyme has its own optimum. Many human enzymes work best near body temperature, thirty-seven degrees C, but that number belongs to us, not to all enzymes. One specification prints the opposite belief as a common misconception. Learners also think that all enzymes have an optimum temperature of thirty-seven degrees C. The enzyme in the table peaks at forty. Taq, from the hot spring, works best at around seventy-five to eighty degrees C. Enzymes from animals in icy seas work best far lower. That is why the careful phrase is this enzyme's optimum, never the optimum for enzymes. pH works the same way. Each enzyme has an optimum pH, and either side of it the active site changes shape, so the rate falls on both sides. Different enzymes have very different optimum pHs: some work best in strong acid, some near neutral, some in alkaline conditions. Which side of the temperature peak has the same reason as both sides of the pH peak? The right side. Past the optimum, the active site changes shape and the enzyme is denatured, just as it is either side of the pH optimum.

Substrate concentration is how many substrate molecules are packed into each bit of the solution. Raise it, and molecules meet active sites more often, so the rate rises. But the rate does not climb for ever. Past a point, adding more substrate changes nothing, and the curve levels off. On the flat part, which limits the rate: the substrate, or the active sites? The active sites. Every one is full, so extra substrate waits its turn. On the rising part, the substrate was the limit. Enzyme concentration works the other way round. More enzyme molecules mean more active sites, so the rate rises. Here's another: adding more enzyme stops raising the rate. What has run short? The substrate. All of it is already being worked on, so the new enzyme has nothing to do. That is a general argument: whatever is in shortest supply sets the rate, and changing anything else does nothing until that is fixed. It comes back with photosynthesis. Side by side, the four typical shapes tell the story. Temperature and pH each rise to a peak, and temperature falls more steeply. Substrate concentration climbs, then flattens. Enzyme concentration keeps climbing while the substrate lasts. One examiner's report, on a Foundation paper, comments on the lines drawn through enzyme results. Although it was pleasing to see few candidates joining points with straight lines, it was disappointing to note that many candidates were unfamiliar with the typical graphs associated with different factors affecting enzyme action. The habit that fixes it: know the four shapes and the reason behind each, then draw a smooth line through the points.

A rate can be calculated from readings. Rate of reaction equals the change in quantity, divided by the time taken. Say it once more, slowly: rate equals change in quantity over time taken. Here the quantity is a volume of oxygen, so the unit is centimetres cubed per second. Catalase from celery is mixed with hydrogen peroxide, and the oxygen given off is collected in a syringe. After thirty seconds there are twelve centimetres cubed of oxygen. After ninety seconds there are twenty-four, and there it stops, because the hydrogen peroxide has run out. Write the unit on its own line first: centimetres cubed per second. Work out the rate between thirty and ninety seconds, in centimetres cubed per second. First the change in volume: twenty-four take away twelve is twelve centimetres cubed. Then the time taken: ninety take away thirty is sixty seconds. Twelve divided by sixty is nought point two. The rate is nought point two centimetres cubed per second. Dividing twenty-four by ninety would use the whole run instead of this interval, and gives a different, wrong answer. Now the same rate per minute. A minute is sixty seconds, so multiply by sixty. Nought point two times sixty is twelve, so the rate is twelve centimetres cubed per minute. A second run is done at a lower temperature. It also reaches twenty-four centimetres cubed, but only after six minutes. Is the catalase in that colder run denatured? Say how the numbers tell you. No. It still made all the oxygen the first run made, so its enzyme kept working, only more slowly. A denatured enzyme would have stopped short. Measuring how pH and temperature change enzyme activity is a required practical, and it has a video of its own.

A student's four-sentence answer about a temperature curve needs marking, one sentence at a time. One: the rate goes up to thirty-seven degrees C because that is the optimum temperature for all enzymes. Two: above that the enzyme denatures. Three: below twenty degrees C the enzyme also denatures, which is why the rate is low. Four: adding more substrate at the end would speed it up again. Which of the four sentences still earns credit, and what is wrong with each of the others? Only sentence two earns credit as it stands: above this enzyme's optimum, it is denatured. Sentence one is wrong twice. Thirty-seven is the optimum for many human enzymes, not all of them, and it never says what an optimum means for activity. Sentence three puts denatured on the cold side. Cold slows an enzyme; it does not denature it. Sentence four is right in the wrong place. Once the enzyme is denatured, extra substrate has no working active site to go into. Now rewrite 'it took less time' as a sentence about the rate. The rate of reaction was higher. Time describes the run; rate describes how fast the enzyme works. One examiner's report, on a Foundation paper, ends its comment on a question like this with one line. It was pleasing to see that no candidate referred to the enzyme being killed. Not one student in that group wrote killed, so keeping to denatured is a habit that can be learned.

Three checks to finish, and the last one goes back to the hot spring. An enzyme's rate has levelled off as substrate is added. What is limiting it now? Every active site is busy, so the enzyme concentration sets the rate. Now a sum: twenty centimetres cubed of gas in forty seconds. What is the rate? Twenty divided by forty is nought point five, so the rate is nought point five centimetres cubed per second. One more, back at the hot spring: Taq at thirty degrees C. Denatured, or slowed? Slowed. Thirty is far below its optimum, so collisions are fewer, but its active site keeps its shape.

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