Exercises: Enzymes as Biological Catalysts

Define the terms enzyme, substrate, and active site. Then explain in one sentence what an enzyme does to the activation energy of a reaction.

Amylase is an enzyme in saliva that breaks down starch, but it does not break down proteins. Use the term "substrate specificity" to explain why.

Which type of biological molecule are most enzymes? Is an enzyme used up in the reaction it catalyzes? Answer both questions in two or three sentences.

The enzyme catalase breaks down hydrogen peroxide into water and oxygen. A student measures the volume of oxygen collected in 60 seconds at different temperatures.

1. a) At which temperature does the enzyme work best?
2. b) Explain the increase in activity from 5 °C to 37 °C.
3. c) Explain what happened to the enzyme at 75 °C.

Pepsin works in the stomach, where the pH is about 2. Trypsin works in the small intestine, where the pH is about 8. A student places pepsin in a solution at pH 8.

1. a) Predict what will happen to the activity of pepsin and explain your prediction using the shape of the active site.
2. b) Explain why both enzymes can be healthy and functional even though they have different optimal pH values.

A researcher keeps the amount of enzyme constant and increases the substrate concentration. The reaction rate rises quickly at first and then levels off at a maximum.

1. a) Explain why the rate increases when substrate concentration is low.
2. b) Explain why the rate stops increasing at high substrate concentration.
3. c) State what change could raise the maximum rate after it has leveled off.

Lactase breaks lactose into glucose and galactose. A lab tests the rate of glucose production (micromoles per minute) at different pH values with the same amount of enzyme and substrate at 37 °C.

1. a) Identify the optimal pH and describe the shape of the data.
2. b) Using the structure of a protein, explain why the rate falls at pH 3 and pH 11.
3. c) A classmate claims that returning the enzyme from pH 11 to pH 7 will always restore full activity. Evaluate this claim.
4. d) Predict how the rate at pH 7 would change if the temperature were raised to 70 °C, and justify your prediction.

A competitive inhibitor has a shape similar to an enzyme's substrate and binds to the active site. A researcher measures the reaction rate of an enzyme with and without the inhibitor at several substrate concentrations.

1. a) Explain why the inhibitor reduces the rate at low substrate concentration.
2. b) Explain why the difference in rates becomes very small at 100 mM.
3. c) Calculate the percent decrease in rate caused by the inhibitor at 1 mM substrate. Round to the nearest whole percent.
4. d) Explain how this differs from what would happen if a denaturing change in temperature affected the enzyme.