AP Biology: Enzyme Catalysis Practice Questions
Test yourself on AP Biology: Enzyme Catalysis with 12 original AP Exams practice questions. Pick an answer to see instant feedback and a full explanation.
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1. An enzyme speeds up a reaction without changing its reactants or final products. Which change explains the increased reaction rate?
Explanation. An enzyme provides a reaction pathway with a lower activation-energy barrier. More reactant molecules can reach the transition state per unit time. Catalysis does not change the free energies of the reactants or products, the reaction's free-energy difference, or its equilibrium constant.
2. A reversible reaction is already at equilibrium in a closed container. An enzyme that catalyzes this reaction is added without changing temperature, volume, or reactant and product concentrations. What happens?
Explanation. The enzyme accelerates both directions of the reaction. At equilibrium their rates remain equal, so there is no net change in composition. The enzyme does not change the equilibrium constant under the stated conditions.
3. At constant enzyme concentration, a student measures initial reaction rates as substrate concentration increases. The rate eventually approaches a plateau. Temperature and pH are constant, and the enzyme remains stable. What best explains the plateau?
Explanation. At high substrate concentration, the available enzyme molecules spend most of their time processing substrate. Adding still more substrate has little effect because the fixed amount of enzyme is operating near its maximum rate. Enzymes are regenerated during catalysis rather than consumed as reactants.
4. An assay contains abundant substrate and measures the initial rate before appreciable substrate depletion. If the concentration of active enzyme is doubled while substrate remains saturating and temperature and pH stay constant, what is the expected effect?
Explanation. With substrate saturating in both assays, doubling the active enzyme provides about twice as many working active sites. The initial maximum rate therefore approximately doubles. This does not change the equilibrium constant or the free-energy change per reaction.
5. An enzyme preparation reacts slowly at 5 degrees Celsius but regains its original activity when returned to 25 degrees Celsius. No enzyme or substrate was added during warming. Which explanation best fits these observations?
Explanation. The recovery of activity shows that the low-temperature treatment did not permanently inactivate the preparation. Lower temperature can reduce molecular motion and the frequency of successful catalytic events. The data support reversible slowing rather than complete irreversible denaturation.
6. An enzyme has high activity at pH 7 and much lower activity at pH 3. Which molecular mechanism could explain the lower activity at pH 3?
Explanation. pH changes the protonation and charge of ionizable groups. These changes can alter active-site interactions, catalytic chemistry, or protein structure. Reduced activity alone does not establish that the amino-acid sequence changed or that the enzyme was permanently denatured.
7. A reversible inhibitor competes with substrate for an enzyme's active site. Under a simple competitive-inhibition model, which change should reduce the inhibitor's effect on the initial reaction rate?
Explanation. More substrate increases the chance that substrate rather than inhibitor occupies the active site. In simple reversible competitive inhibition, sufficiently high substrate concentration allows the rate to approach the same maximum rate as the uninhibited enzyme. This conclusion does not apply to irreversible inhibition.
8. A regulatory molecule binds to an enzyme at a site separate from the active site. Binding changes the enzyme's shape and reduces catalytic activity. Which description is best supported?
Explanation. Binding at a separate regulatory site that changes enzyme activity is allosteric regulation; here the effect is inhibition. This description does not by itself establish a particular kinetic model such as pure noncompetitive inhibition. A conformational change is not a change in amino-acid sequence.
9. A student tests whether an enzyme accelerates product formation. Which comparison best controls for product that might form without the enzyme?
Explanation. The enzyme-free buffer control preserves total volume and buffer conditions while removing the enzyme. Comparing product formation over the same time interval isolates the contribution of the enzyme. Changing substrate concentration, temperature, or pH would introduce another variable.
10. During the initial linear phase of an enzyme assay, product concentration is 0 micromolar at 0 seconds, 12 micromolar at 20 seconds, and 24 micromolar at 40 seconds. What is the initial rate of product formation?
Explanation. Rate is the change in product concentration divided by elapsed time: (24 - 0) micromolar divided by (40 - 0) seconds equals 0.6 micromolar per second. The first 20-second interval gives the same value, consistent with the stated linear phase.
11. A purified enzyme is heated strongly and then cooled to its original assay temperature. When tested with fresh substrate under the original pH and buffer conditions, it has almost no activity. An unheated sample remains active. Which interpretation is most consistent with the result?
Explanation. Persistent loss of activity after cooling and addition of fresh substrate is consistent with heat-induced denaturation or aggregation that disrupts functional structure. The assay controls make simple temperature slowing and substrate depletion poor explanations. Activity data alone do not identify the exact structural damage.
12. In a biosynthetic pathway, the final product binds to a regulatory site on the enzyme catalyzing the first committed step and decreases its activity. What is the most direct consequence when final-product concentration becomes high?
Explanation. Feedback inhibition links pathway activity to product abundance. Inhibiting the first committed step reduces entry into that pathway and limits unnecessary product synthesis. It regulates existing enzyme activity without requiring deletion of a gene or shutdown of unrelated reactions.
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FAQ
Do enzymes change equilibrium or make an unfavorable reaction favorable?
Enzymes lower activation-energy barriers and speed up reactions in both directions. They do not change the reaction's free-energy difference or equilibrium constant. Coupling to another reaction can change the overall energetics, but catalysis alone does not.
Does allosteric inhibition always mean pure noncompetitive inhibition?
No. Allosteric describes regulation through binding at a site separate from the active site. Pure noncompetitive inhibition is a specific kinetic model. The binding-site description alone is insufficient to establish that model.
How should an enzyme experiment compare rates fairly?
Measure initial product formation over a linear time interval, change one intended variable, and hold other conditions constant. Include an appropriate enzyme-free or treatment control and replicate measurements. Check that substrate depletion or signal saturation is not causing an apparent rate plateau.