Enzymes (College Board AP® Biology): Exam Questions

An experiment was conducted to measure the reaction rate of the human salivary enzyme α-amylase. Ten mL of a concentrated starch solution and 1.0 mL of α-amylase solution were placed in a test tube. The test tube was inverted several times to mix the solution and then incubated at 25°C. The amount of product (maltose) present was measured every 10 minutes for an hour. The results are given in the table below.

Construct a graph for this data on the axes provided and calculate the rate of the reaction for the time period 0 to 30 minutes. 

Explain why a change in the reaction rate was observed after 30 minutes. 

Draw and label another line on the graph to predict the results if the concentration of α-amylase was doubled. Explain your predicted results. 

Identify two environmental factors that can change the rate of an enzyme-mediated reaction. Discuss how each of those two factors would affect the reaction rate of an enzyme. 

Fireflies emit light when the enzyme luciferase catalyzes a reaction in which its substrate, D-luciferin, reacts to form oxyluciferin and other products (Figure 1). In order to determine the optimal temperature for this enzyme, scientists added ATP to a solution containing D-luciferin, luciferase, and other substances needed for the reaction. They then measured the amount of light emitted during the first three seconds of the reaction when it was carried out at different temperatures.

D-Luciferin + O₂ + ATP  Oxyluciferin + CO₂ + AMP + PP_i + Light

Figure 1. Light is emitted as a result of the reaction catalyzed by luciferase.

Describe a characteristic of the luciferase enzyme that allows it to catalyze the reaction.

Identify the dependent variable in the experiment.

State the null hypothesis for the experiment.

A student claims that, as temperature increases, there will be an increase in the amount of light given off by the reaction in the first three seconds. Support the student's claim.

Researchers investigated the activity of the enzyme lactase, which breaks down lactose into glucose and galactose. They measured reaction rates at different substrate concentrations and recorded mean values with standard errors, as shown in Figure 1 below:

Using the data in Figure 1, describe the effect of substrate concentration on lactase activity.

With reference to the data in Figure 1, justify why the reaction rate does not increase indefinitely as substrate concentration rises.

The researchers claim that increasing substrate concentration from 3.0 mM to 4.0 mM does not significantly increase reaction rate.

Provide evidence, from Figure 1, to support this claim.

Lactose intolerance is caused by insufficient levels of the enzyme lactase. When undigested lactose reaches the large intestine, gut bacteria ferment it, producing gases, such as hydrogen, methane, and carbon dioxide. This results in flatulence and bloating.

Using the data provided in Figure 1, explain how a lactase deficiency can lead to the symptoms observed in individuals with lactose intolerance.

A team of scientists studied the effect of pH on the activity of trypsin, a protease enzyme that catalyzes the breakdown of protein in the small intestine. They compared the activity of the wild-type enzyme to a mutant version of the enzyme, which contained a nucleotide base substitution in its active site. The scientists measured reaction rates (μmol product/min) for both enzyme variants at different pH levels and recorded their findings, including standard error values to account for variability.

Describe how mutations can affect enzyme specificity.

Using the data in Figure 1, describe how the enzyme activity has changed due to the mutation.

Predict how the mutation is likely to affect protein digestion in the small intestine.

Give reasons to justify your answer to part (c).

A team of scientists studied the effects of toxic compounds, cyanide and carbon monoxide (CO) on cellular respiration.

Both substances negatively affect the activity of cytochrome c oxidase, a key enzyme in the electron transport chain of cellular respiration. The interaction between both inhibitors and cytochrome C is represented in Figure 1.

To investigate their impact, the scientists measured enzyme activity under control conditions (no inhibitor), in the presence of carbon monoxide, and in the presence of cyanide across varying substrate concentrations. Figure 2 shows the results of the investigation.

Describe the role of oxygen in ATP production in eukaryotic cells.

Based on Figures 1 and 2, explain why an increase in oxygen concentration after 2 mM, does not result in an increase in ATP production in the presence of cyanide.

Predict what would happen to the oxygen consumption rate and ATP synthesis if the concentration of carbon monoxide were doubled, based on the data.

Justify your prediction with reference to the proton gradient and activity of ATP synthase.

A scientist is studying phosphofructokinase (PFK), a key enzyme in glycolysis that catalyzes the conversion of fructose-6-phosphate into fructose-1,6-bisphosphate using ATP. The scientist is investigating how two inhibitors, X and Y, affect the activity of PFK under different conditions.

Describe how ATP production in anaerobic and aerobic respiration relies on PFK.

The scientist measured the reaction rate (μmol ATP/min) of PFK under different conditions and recorded the data in Table 1.

Table 1: Effect of inhibitors on PFK activity

Justify which inhibitor (X or Y) is most likely a competitive inhibitor, using the data in Table 1.

ATP can act as a non-competitive inhibitor of PFK. This is a key rate-limiting step in the pathway as it is regulated by levels of ATP.

Predict how ATP production would be impacted if ADP levels increased.

Give reasoning to justify your prediction.

A team of scientists studied the role of DNA polymerases and exonucleases in DNA replication. DNA polymerases are enzymes essential for accurate DNA synthesis, and exonucleases provide proofreading activity to correct errors during replication.

The scientists compared the accuracy and efficiency of three different DNA polymerases:

• Polymerase A (lacks exonuclease activity).

• Polymerase B (possesses exonuclease activity).

• Polymerase C (high-accuracy polymerase with exonuclease activity).

They measured the rate of DNA synthesis (nucleotides per second) and mutation frequency (errors per million base pairs) across the three enzymes. The data are provided in Table 1.

Table 1: DNA replication efficiency and accuracy in different polymerases

(i) Describe the role of DNA polymerases in DNA replication.

(ii) Explain how complementary base pairing contributes to the accuracy of DNA replication.

In a follow-up investigation, the scientists studied how the accuracy of DNA replication changed with more complex templates. They measured the replication accuracy based on the percentage of nucleotide bases correctly incorporated into the new strand of DNA. Figure 1 illustrates the effect of template complexity on DNA replication accuracy for different DNA polymerases:

(i) Identify the dependent variable in the follow-up investigation.

(ii) Justify the control group used by the scientists.

(i) Based on the data in Figure 1 and Table 1, describe the relationship between the accuracy of DNA replication and the presence of exonucleases.

(ii) One set of chromosomes in a human cell contains 3 billion base pairs. The scientists calculated that during replication with polymerase C, there would be an estimated 600 mutations.

Using the data in Table 1, calculate how many more mutations are estimated to occur during replication by polymerase A compared to polymerase C in a diploid cell.

(i) Predict how increasing the complexity of the DNA template would affect both the replication rate and error frequency of the three polymerases.

(ii) Use the trends shown in Table 1 and Figure 1 to justify your prediction.

Figure 1 shows the initial stage in glycolysis involving the conversion of glucose to glucose-6-phosphate, catalyzed by hexokinase enzyme.

With reference to Figure 1, describe the role of hexokinase as a catalyst.

Based on Figure 1, explain the role of hexokinase in the production of ATP.

Phosphorylation of glucose by hexokinase is an exergonic reaction, meaning there is a net release of energy during the process. Figure 2 represents the energy profiles for two different types of reaction.

On the template provided, draw an X to indicate which line best represents the energy profile of this reaction.

Hexokinase is regulated through feedback inhibition where glucose 6-phosphate will compete with glucose for substrate binding.

With reference to Figure 1, explain why this is possible.