AP® Biology review sheet from Aim for Five (aimforfive.com/bio/units/3/3-2)
Unit 3 · Topic 3.2
3.2 Environmental Impacts on Enzyme Function
An enzyme's activity depends on its surroundings. Temperature, pH, and the amounts of substrate and product all change how fast it works, and conditions far from the optimum can denature it. Inhibitors slow enzymes down in two main ways: competitive inhibitors block the active site, and noncompetitive inhibitors bind elsewhere and change the enzyme's shape.
Key terms
- optimal temperature
- optimal pH
- denaturation
- competitive inhibitor
- noncompetitive inhibitor
- allosteric site
Temperature
Warming a solution makes molecules move faster on average, so enzymes and substrates collide more often and with more energy. That's why reaction rate rises as temperature rises, up to a point.
That point is the optimal temperature, where the enzyme works fastest. Many human enzymes have an optimum near body temperature, about 37 °C. Enzymes from bacteria that live in hot springs can have optimums above 70 °C.
Above the optimum, the extra motion breaks the hydrogen bonds and other weak interactions that hold the enzyme's shape. The active site changes shape, so substrates no longer fit, and the rate falls steeply. When the structure is disrupted enough that the enzyme can no longer catalyze its reaction, the enzyme is denatured. A graph of rate vs. temperature rises gradually, peaks at the optimum, then drops sharply.
Cold slows enzymes down because there are fewer collisions, but it usually doesn't denature them. Warm them back up and they work again.
pH
Each enzyme also has an optimal pH. Changing the pH changes how many H⁺ ions are around, which can change the charges on R groups. That disrupts the hydrogen bonds and ionic bonds that hold the enzyme's shape and can change the charges in the active site.
Optimal pH matches where an enzyme works. Pepsin, which digests protein in your stomach, works best around pH 2. Trypsin, which digests protein in your small intestine, works best around pH 8. A graph of rate vs. pH is usually a hump centered on the optimum.
Some denaturation is reversible. If the change was mild and conditions return to normal, some enzymes can refold and regain their activity. Severe denaturation, like cooking an egg, is usually permanent.
Substrate, product and enzyme amounts
With a fixed amount of enzyme, adding substrate speeds the reaction at first, because active sites meet substrates more often. Eventually every active site is busy all the time, and adding more substrate doesn't help. The enzymes are saturated, and the rate levels off at a maximum. Adding more enzyme raises that maximum.
The amount of product matters too. As product builds up and substrate runs low, the forward reaction slows. In a cell, products are often used right away by the next step in a pathway (3.3), which keeps reactions moving forward.
Inhibitors
Many inhibitors bind reversibly: they attach and detach. Some poisons bind permanently.
Cells use allosteric inhibition to regulate themselves. In feedback inhibition, the end product of a pathway binds an allosteric site on an enzyme early in the pathway and slows it down. When plenty of product exists, the cell stops making more, and when product runs low, the enzyme is released and the pathway restarts.
| Feature | Competitive inhibitor | Noncompetitive inhibitor |
|---|---|---|
| Where it binds | The active site | An allosteric site (a site other than the active site) |
| How it works | Resembles the substrate and blocks it from binding | Changes the enzyme's shape, so the active site works less well |
| Effect of adding lots of substrate | Can overcome the inhibitor; substrate outcompetes it | Can't overcome it; the maximum rate stays lower |
Worked examples
Try each one yourself first, then open the solution.
- Example 1
Identifying inhibitor types from data
Reaction rates (µmol product/min) for an enzyme are measured at four substrate concentrations, with no inhibitor and with inhibitor A or inhibitor B. Rates at 1, 5, 20 and 100 mM substrate: no inhibitor: 3.3, 7.1, 9.1, 9.8. With A: 1.4, 4.5, 7.7, 9.4. With B: 1.7, 3.6, 4.5, 4.9. Which inhibitor is competitive and which is noncompetitive? Justify.
Show the solutionHide the solution
- Step 1: Look at the highest substrate concentration, where the key difference shows up.
- Step 2: Inhibitor A: at 100 mM, the rate (9.4) is almost back to the uninhibited rate (9.8). Lots of substrate is outcompeting the inhibitor for the active site.
- Step 3: Inhibitor B: at 100 mM, the rate (4.9) is still only about half the uninhibited rate. Extra substrate doesn't help, because the inhibitor isn't in the active site.
- Step 4: Conclude: A behaves like a competitive inhibitor, and B behaves like a noncompetitive inhibitor binding an allosteric site.
Answer: A is competitive (high substrate nearly restores the normal rate); B is noncompetitive (the maximum rate stays about half, no matter how much substrate is added).
- Example 2
Designing an enzyme experiment
A student wants to test how temperature affects the activity of an enzyme from yeast. Identify the independent variable, the dependent variable, two variables to keep constant, and a negative control. Then predict the shape of the results.
Show the solutionHide the solution
- Step 1: Independent variable (what the student changes): temperature, for example 10, 20, 30, 40, 50 and 60 °C.
- Step 2: Dependent variable (what is measured): reaction rate, for example the amount of product formed per minute.
- Step 3: Constants: the amount of enzyme, the substrate concentration, pH, total volume and reaction time.
- Step 4: Negative control: tubes with substrate but no enzyme (or enzyme denatured by boiling) at each temperature, to show the reaction doesn't go fast without active enzyme.
- Step 5: Prediction: the rate rises with temperature up to an optimum, then drops sharply at higher temperatures as the enzyme denatures.
Answer: IV: temperature. DV: reaction rate. Constants: e.g., enzyme amount, substrate concentration, pH. Negative control: no enzyme or boiled enzyme. Expected result: rate rises to an optimum, then falls steeply.
- Example 3
Is cold denaturing? (classic trap)
An enzyme's activity is measured at 4 °C, then the same sample is warmed to 37 °C and measured again. Activity at 4 °C is very low, but at 37 °C it's back to normal. A student says the enzyme was denatured in the cold and then 'fixed' itself. Give a better explanation.
Show the solutionHide the solution
- Step 1: Recall what low temperature does: molecules move more slowly, so enzymes and substrates collide less often and with less energy.
- Step 2: That lowers the rate without breaking the bonds that hold the enzyme's shape.
- Step 3: Check the evidence: activity returned fully when the sample was warmed. That fits slowed collisions, not damage to the structure.
- Step 4: Denaturation from high heat usually reduces activity even after cooling, because the shape is disrupted.
Answer: The cold didn't denature the enzyme; it just slowed molecular motion and collisions. Warming restored the normal collision rate, so activity returned.
Common mistakes
- Saying high temperature 'kills' an enzyme, or that cold denatures it. Heat beyond the optimum denatures by disrupting bonds; cold mainly slows collisions.
- Saying denaturation breaks peptide bonds. It disrupts hydrogen bonds and other interactions that hold the shape; the amino acid sequence is unchanged.
- Mixing up inhibitor types. Competitive = active site, beaten by more substrate. Noncompetitive = allosteric site, not beaten by more substrate.
- Explaining a leveling-off curve as the enzyme 'getting tired.' The rate levels off because all active sites are occupied (saturation).
On the exam
- Expect graphs of rate vs. temperature, pH or substrate concentration. Describe the trend, then explain it with collisions, active-site shape or saturation.
- Inhibitor questions often give data with and without extra substrate. Whether extra substrate restores the rate is the key evidence.
- Experimental design questions on enzymes ask for variables, controls and predictions. Name a specific negative control, such as boiled enzyme.
Connected topics
Videos
Check yourself
5 questions on 3.2 Environmental Impacts on Enzyme Function. Pick an answer to see if you got it, and why.
| Temperature (°C) | Reaction rate (µmol product/min) |
|---|---|
| 10 | 12 |
| 20 | 25 |
| 30 | 44 |
| 37 | 52 |
| 45 | 30 |
| 55 | 6 |
| 65 | 0 |
Experimental data: the activity of an enzyme from a mammal was measured at different temperatures with the same enzyme and substrate concentrations.
Which of the following best explains the change in rate from 10 °C to 37 °C?
Which of the following best explains the change in rate from 45 °C to 65 °C?
A student wants to test whether the loss of activity at 65 °C is reversible. Which procedure would best test this?
| Substrate concentration (mM) | Rate, no inhibitor | Rate with inhibitor X | Rate with inhibitor Y |
|---|---|---|---|
| 1 | 20 | 7 | 10 |
| 5 | 53 | 26 | 26 |
| 10 | 67 | 41 | 33 |
| 20 | 77 | 56 | 38 |
| 40 | 83 | 69 | 41 |
| 80 | 86 | 78 | 43 |
Experimental data: reaction rates (µmol/min) for an enzyme measured at several substrate concentrations, with no inhibitor or with the same concentration of inhibitor X or inhibitor Y.
Which of the following is the best interpretation of the data for inhibitor X?
Inhibitor Y cuts the rate roughly in half at every substrate concentration. Which of the following best explains this result?
0 of 5 answered