AP Biology: Enzyme Catalysis — Feedback Regulation Practice Questions
Test yourself on AP Biology: Enzyme Catalysis — Feedback Regulation with 12 original AP Exams practice questions. Pick an answer to see instant feedback and a full explanation.
Free original practice questions for study purposes. Open Exam Prep is an independent study resource and is not affiliated with, endorsed by, or sponsored by the makers of AP Exams.
advertisement
Answer the questions below — you get instant feedback and a full explanation for each.
1. In an invented biosynthetic pathway, enzymes convert precursor S into intermediate T and then end product U. When U accumulates, it binds the first enzyme and reduces conversion of S to T. Which description best fits this regulation?
Explanation. The pathway's output reduces an earlier step in its own production. That arrangement is end-product feedback inhibition. Positive feedback would reinforce production rather than reduce it; neither enzyme replacement nor a gene conversion is described.
2. A regulatory molecule binds a purified enzyme at a site distinct from the substrate-binding active site and changes catalytic activity. Which term describes this type of regulation?
Explanation. Allosteric regulation involves binding at a distinct regulatory site that affects enzyme function. The described experiment acts on an existing purified protein; it does not describe making RNA, translating a protein, or copying DNA.
3. In a model pathway, end product R reversibly inhibits the first enzyme. Cells consume R, so its free concentration falls and fewer inhibitor molecules remain bound. Enzyme amount, precursor availability, and other conditions stay constant. What is the expected immediate effect?
Explanation. The stated model links inhibitory occupancy to free R concentration. Less bound inhibitor relieves inhibition and allows more activity under the otherwise unchanged conditions. This prediction concerns regulation of existing enzyme, not a change in gene content.
4. A purified enzyme is inhibited within seconds after a pathway end product is added. The tube contains no cells, DNA, ribosomes, or protein-synthesis machinery. Which explanation is consistent with this setup?
Explanation. The tube already contains the enzyme but lacks gene-expression machinery. Regulation of the existing protein can explain the rapid change. This observation alone does not locate the binding site or establish a specific kinetic inhibition model.
5. In matched assays, a normal enzyme and a variant have initial rates of 20 units without end product Z. With Z, the normal enzyme's rate is 5 units and the variant's rate is 19 units. Protein amount and assay conditions are controlled. Which conclusion is best supported?
Explanation. Both preparations have the same measured baseline rate, but Z causes a much smaller decrease for the variant. That supports reduced sensitivity in this assay. Rate data alone do not establish whether altered binding or another structural effect explains the difference.
6. An enzyme's initial rate is 24 units without a feedback inhibitor and 9 units with it. What percentage of the uninhibited activity remains in the inhibited assay?
Explanation. Activity remaining is the inhibited rate divided by the uninhibited rate, multiplied by 100: 9 divided by 24 times 100 equals 37.5 percent. The 62.5 percent value is the percentage decrease, which is a different quantity.
7. In a hypothetical branched pathway, S becomes branch point T. One branch converts T to product U, and a separate branch converts T to product V. U inhibits only the first enzyme on the T-to-U branch. Which step does U directly reduce?
Explanation. The stem specifies that U targets the first enzyme on its own branch, so that reaction is directly reduced. Changes in shared metabolite availability could indirectly affect other reactions, but the question asks which enzyme is directly regulated.
8. A student adds an end-product stock dissolved in buffer to an enzyme assay. Which comparison best separates the end product's effect from the effect of the added buffer and volume?
Explanation. A matched vehicle control accounts for the stock buffer and added volume. Matching enzyme, substrate, temperature, and final pH helps isolate end-product presence as the intended difference. Deliberately changing those conditions introduces alternative explanations.
9. A model cell makes 14 nanomoles of product P per minute and consumes 10 nanomoles per minute. Feedback reduces synthesis to 6 nanomoles per minute while consumption stays at 10. With no other inputs or losses, what is the net rate of change in the amount of P just after feedback acts?
Explanation. Immediately after feedback acts, the balance is synthesis minus consumption: 6 minus 10 equals minus 4 nanomoles per minute. The negative sign means the amount decreases. The question fixes the rates at that instant and does not ask for a long-term steady state.
10. For an illustrative reversible regulation model, rate v equals 30 divided by (1 plus I divided by 4), where I is inhibitor concentration in micromolar and v is in rate units. What rate does this specified model predict at I = 8 micromolar?
Explanation. Substitute I = 8: the denominator is 1 plus 8 divided by 4, or 3. Therefore v is 30 divided by 3, or 10 rate units. This supplied teaching model is not a universal rate law for all feedback-regulated enzymes.
11. A purified enzyme's rate falls when compound K is added and recovers when K is removed without changing enzyme concentration or other assay conditions. Which conclusion is supported by these observations alone?
Explanation. Recovery after removal supports reversibility under the tested conditions. The observations give no pathway identity for K, so they do not by themselves establish end-product feedback. They also do not identify the binding site or a precise kinetic mechanism.
12. A pathway's end product reduces the activity of an upstream enzyme at high product concentration. A student argues that this guarantees the product concentration will stay exactly constant at every moment. Which evaluation is most accurate?
Explanation. Reducing synthesis can counter product accumulation without forcing an unchanging concentration. The net change still depends on production and use, and regulation may take time. A stabilizing response therefore does not guarantee exact constancy at every instant.
📘 Want a full structured course and official-style practice tests? Browse top-rated AP Exams prep books and courses. Some links are affiliate links; we may earn a commission at no cost to you.
advertisement
FAQ
How does feedback inhibition differ from allosteric regulation?
Feedback describes a pathway relationship: an output inhibits an earlier step in its production. Allosteric regulation describes an effect of binding at a distinct regulatory site. Many feedback examples use allosteric inhibition, but the terms answer different questions; a lower rate alone does not identify either relationship or binding site.
Does feedback inhibition require making a new enzyme?
No. Binding of a regulatory molecule can change the activity of enzyme already present. Regulation of enzyme production through gene expression is another mechanism and requires evidence appropriate to that mechanism.
Where do the pathway examples and numerical values in these questions come from?
The lettered pathways, assay values, and supplied rate model are invented for independent practice. They are not measurements from a named laboratory or reproductions of real exam questions. Use the assumptions stated in each question.