biology : class11 Protein Phosphorylation Cascades : Receptors, Second Messengers, Gene Regulation : Hormone Action MCQ Questions and Answers
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Protein Phosphorylation Cascades
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MCQ Questions
1. A hormone activates a receptor-linked protein kinase, initiating a cascade in which kinase 1 phosphorylates kinase 2, and kinase 2 phosphorylates kinase 3. If a nonhydrolysable ATP analogue prevents kinase 1 from transferring phosphate groups, which immediate outcome is most likely?
A.
Kinase 2 remains inactive and kinase 3 is not phosphorylated
B.
Kinase 2 becomes permanently active through receptor binding
C.
Kinase 3 phosphorylates kinase 1 instead
D.
The hormone is converted directly into a transcription factor
Correct Answer:
Kinase 2 remains inactive and kinase 3 is not phosphorylated
Explanation:
Protein phosphorylation cascades depend on sequential transfer of phosphate groups. Blocking the kinase activity of kinase 1 prevents activation of downstream kinase 2 and consequently prevents phosphorylation of kinase 3.
2. In a phosphorylation cascade, an activated kinase phosphorylates a target protein on a specific serine residue. A mutation changes that serine to alanine. What is the most likely effect?
A.
The target protein will always remain phosphorylated
B.
The target protein cannot be phosphorylated at that regulatory site
C.
The kinase will phosphorylate alanine more rapidly
D.
The mutation will convert the target protein into a phosphatase
Correct Answer:
The target protein cannot be phosphorylated at that regulatory site
Explanation:
Serine contains a hydroxyl group that can accept a phosphate group during kinase action. Alanine lacks this suitable hydroxyl group, so phosphorylation at the altered site is prevented.
3. A hormone-induced cascade activates a kinase that phosphorylates a metabolic enzyme, causing the enzyme to become inactive. If a phosphatase specific for that enzyme is then activated, what is the expected result?
A.
The enzyme is dephosphorylated and may regain activity
B.
The enzyme receives an additional phosphate and remains inactive
C.
The kinase is converted into a receptor
D.
The hormone becomes permanently attached to the enzyme
Correct Answer:
The enzyme is dephosphorylated and may regain activity
Explanation:
Protein phosphatases remove phosphate groups from phosphorylated proteins. Because phosphorylation can either activate or inhibit a target, dephosphorylation of this enzyme reverses its inhibitory modification and may restore activity.
4. Why can a phosphorylation cascade produce a large cellular response even when only a small number of hormone-bound receptors are activated?
A.
Each activated kinase can phosphorylate multiple molecules of its downstream target
B.
The hormone molecules multiply after binding to receptors
C.
Phosphate groups independently activate every protein in the cytoplasm
D.
Receptors are permanently replicated during hormone binding
Correct Answer:
Each activated kinase can phosphorylate multiple molecules of its downstream target
Explanation:
Sequential kinase activation creates signal amplification. One activated kinase can modify many molecules of the next component, allowing a small initial receptor signal to generate a much larger downstream response.
5. A mutation makes the final kinase in a hormone-responsive phosphorylation cascade unable to bind ATP but leaves its substrate-binding region intact. Which function is directly lost?
A.
Recognition of the hormone by the receptor
B.
Transfer of phosphate groups to the final target protein
C.
Removal of phosphate groups from the target protein
D.
Synthesis of phosphate ions by the kinase
Correct Answer:
Transfer of phosphate groups to the final target protein
Explanation:
Kinases require ATP as the phosphate donor. Without ATP binding, the final kinase may still recognize its substrate but cannot catalyze phosphorylation of that protein.
6. Two hormones activate different receptors but converge on the same downstream kinase. Hormone X activates the kinase, whereas hormone Y activates a phosphatase that removes the kinase's phosphate group. If both hormones are present simultaneously, what is the most likely outcome?
A.
The kinase activity will reflect the balance between phosphorylation and dephosphorylation
B.
The kinase must be fully active because kinase pathways always dominate
C.
The kinase must be fully inactive because phosphatases permanently destroy kinases
D.
Neither hormone can affect the kinase because they use different receptors
Correct Answer:
The kinase activity will reflect the balance between phosphorylation and dephosphorylation
Explanation:
The activity of a cascade component depends on opposing kinase and phosphatase actions. Simultaneous signals can therefore produce an intermediate or context-dependent kinase activity based on their relative strengths and timing.
7. A constitutively active upstream kinase is expressed in a cell even when the hormone is absent. Which pattern best describes the downstream pathway?
A.
The downstream phosphorylation cascade may remain active without hormone stimulation
B.
The cascade cannot begin because hormones are required to provide phosphate groups
C.
All downstream proteins must be dephosphorylated immediately
D.
The receptor will prevent the upstream kinase from binding ATP
Correct Answer:
The downstream phosphorylation cascade may remain active without hormone stimulation
Explanation:
A constitutively active kinase continuously phosphorylates its downstream target, bypassing the normal hormone-dependent activation step and potentially causing persistent pathway signaling.
8. In a hormone-triggered cascade, kinase A phosphorylates kinase B, and active kinase B phosphorylates a transcription-regulating protein. A phosphatase removes the phosphate from kinase B but not from the transcription-regulating protein. What is the immediate effect on new signal transmission?
A.
Further activation of the transcription-regulating protein by kinase B is reduced
B.
Kinase B becomes more active because dephosphorylation always activates kinases
C.
The receptor is automatically phosphorylated by the phosphatase
D.
The transcription-regulating protein must immediately lose every phosphate group
Correct Answer:
Further activation of the transcription-regulating protein by kinase B is reduced
Explanation:
Removing the activating phosphate from kinase B reduces its ability to phosphorylate additional target molecules. Existing phosphorylation on the transcription-regulating protein may persist until a separate phosphatase removes it.
9. A researcher replaces a phosphorylatable target protein with a version that mimics its phosphorylated state. Even after the upstream kinase is inhibited, the target remains functionally active. What does this result demonstrate?
A.
The target's phosphorylated state can be sufficient to maintain its activity
B.
Kinases are unnecessary for initiating any hormone response
C.
Phosphatases add phosphate groups to inactive proteins
D.
The hormone receptor directly supplies ATP to the target
Correct Answer:
The target's phosphorylated state can be sufficient to maintain its activity
Explanation:
A phosphorylation-mimicking mutation can maintain the structural or functional state normally produced by phosphorylation. Thus, the target may remain active even when upstream kinase signaling is blocked.
10. A hormone activates a kinase cascade, but a mutation removes the phosphorylation site from the final protein that normally changes cell activity. Which statement best predicts the response?
A.
Upstream kinases may still become active, but the final cellular response will be weakened or absent
B.
The receptor cannot bind the hormone because the final protein is altered
C.
The mutation will cause every upstream kinase to lose its ATP-binding site
D.
The hormone will activate the final protein without phosphorylation
Correct Answer:
Upstream kinases may still become active, but the final cellular response will be weakened or absent
Explanation:
A phosphorylation cascade can proceed through its upstream components even if the terminal target lacks its regulatory phosphorylation site. However, the target cannot undergo the modification required to produce the normal cellular response.
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