Question:
Consider a chain reaction with the sequence S1 to S2 to S3 to S4, where S4 is converted to P1 and P1 is converted to P2. The enzymes catalyzing the three reactions S1 to S2, S2 to S3, and S3 to S4 are EA, EB, and EC respectively. Enzyme EA is under positive feedback and enzyme EB is under negative feedback. If enzyme EC is absent, which of the following is true?
Consider a chain reaction with the sequence S1 to S2 to S3 to S4, where S4 is converted to P1 and P1 is converted to P2. The enzymes catalyzing the three reactions S1 to S2, S2 to S3, and S3 to S4 are EA, EB, and EC respectively. Enzyme EA is under positive feedback and enzyme EB is under negative feedback. If enzyme EC is absent, which of the following is true?
Show Hint
Loss of EC lets S3 rise, which switches off EB by negative feedback; EA keeps feeding S2, so S2 backs up.
Updated On: Jul 8, 2026
- S1 accumulates
- S2 accumulates
- P1 accumulates
- P2 accumulates
Show Solution
Verified By Collegedunia
The Correct Option is B
Solution and Explanation
Step 1: Understanding the Question.
The pathway runs S1 to S2 (enzyme EA), S2 to S3 (enzyme EB), and S3 to S4 (enzyme EC), with S4 going on to P1 and then P2. EA is switched on by positive feedback, and EB is switched off by negative feedback. The question asks what builds up if EC is missing.
Step 2: Trace what happens right after EC is lost.
Without EC, S3 cannot move forward to S4. This means S3, the product of the EB reaction, starts to rise because it has nowhere to go.
Step 3: Apply the negative feedback on EB.
Negative feedback on EB means that as its own product (S3) climbs, S3 acts back on EB and slows it down. So once S3 starts to build up from the missing EC step, EB is switched off before S3 has a chance to keep climbing without limit. This stops the flow of S2 into S3.
Step 4: Apply the positive feedback on EA.
EA does not have this brake. Its positive feedback keeps it running, or even running harder, so S1 keeps being turned into S2 at a steady or increased pace.
Step 5: Put the two effects together.
S2 keeps being made by EA, but it cannot move on to S3 because EB has been throttled by the S3 buildup triggering its negative feedback. With production continuing and the exit route blocked, S2 is the one that piles up.
Step 6: Rule out the other options.
(A) S1 accumulates: S1 is still being used up normally by the unblocked, positively driven EA reaction. Incorrect.
(C) P1 accumulates: P1 sits downstream of the missing EC step, so with S3 to S4 blocked, little to no new S4 or P1 is even being formed. Incorrect.
(D) P2 accumulates: P2 is even further downstream than P1, so it is affected the same way and does not build up. Incorrect.
Step 7: Final Answer.
With EA still active from positive feedback and EB shut down by negative feedback once S3 rises, the substrate stuck between them, S2, is what accumulates.
\[ \boxed{\text{S2 accumulates}} \]
The pathway runs S1 to S2 (enzyme EA), S2 to S3 (enzyme EB), and S3 to S4 (enzyme EC), with S4 going on to P1 and then P2. EA is switched on by positive feedback, and EB is switched off by negative feedback. The question asks what builds up if EC is missing.
Step 2: Trace what happens right after EC is lost.
Without EC, S3 cannot move forward to S4. This means S3, the product of the EB reaction, starts to rise because it has nowhere to go.
Step 3: Apply the negative feedback on EB.
Negative feedback on EB means that as its own product (S3) climbs, S3 acts back on EB and slows it down. So once S3 starts to build up from the missing EC step, EB is switched off before S3 has a chance to keep climbing without limit. This stops the flow of S2 into S3.
Step 4: Apply the positive feedback on EA.
EA does not have this brake. Its positive feedback keeps it running, or even running harder, so S1 keeps being turned into S2 at a steady or increased pace.
Step 5: Put the two effects together.
S2 keeps being made by EA, but it cannot move on to S3 because EB has been throttled by the S3 buildup triggering its negative feedback. With production continuing and the exit route blocked, S2 is the one that piles up.
Step 6: Rule out the other options.
(A) S1 accumulates: S1 is still being used up normally by the unblocked, positively driven EA reaction. Incorrect.
(C) P1 accumulates: P1 sits downstream of the missing EC step, so with S3 to S4 blocked, little to no new S4 or P1 is even being formed. Incorrect.
(D) P2 accumulates: P2 is even further downstream than P1, so it is affected the same way and does not build up. Incorrect.
Step 7: Final Answer.
With EA still active from positive feedback and EB shut down by negative feedback once S3 rises, the substrate stuck between them, S2, is what accumulates.
\[ \boxed{\text{S2 accumulates}} \]
Was this answer helpful?
0
0
Top NEET PG Biochemistry Questions
- Which of the following clotting factors is needed for gamma-glutamyl carboxylase?
- A patient presented with diarrhea, dermatitis, and dementia. Which of the following vitamin deficiencies would you suspect in this patient?
- A young boy presents to the OPD with hypoglycemia and muscle cramps, on exertion or while playing. Then he becomes normal after resting for a while. These episodes are recurrent after a period of activity. He has decreased serum lactate and glucose levels. Which of the following diseases is he most likely to be suffering from?
- A family consumes only polished rice. Which of the following combinations of vitamin deficiency and enzymatic defect will be present in this family?
- A chronic alcoholic patient presented to the casualty with confusion, ataxia, and painful eye movements. What could be the vitamin deficiency seen in this patient?
View More Questions
Top NEET PG Regulation of major metabolic pathways Questions
- True About Noncompetitive antagonist :
- According to NCEP-ATP III, which among the following have not been included in metabolic syndrome ?
- A man survives 5 days without food. What is the brain's primary energy source during this prolonged fasting period?
- Which of the following dietary fiber is insoluble in water ?
- Which of the following is not the source of cytosolic NADPH ?
Top NEET PG Questions
- A patient hailing from Delhi presents with fever, arthralgia, and extensive petechial rash for 3 days. Lab investigations revealed a hemoglobin of 9 g/ dL, a white blood cell count of 9000 cells/mm3, a platelet count of 20000 cells/mm3, and a prolonged bleeding time. The clotting time was normal. What is the most likely diagnosis?
- Which of the following statements is true about Trichomonas vaginalis?
- A lady from West Rajasthan presented with an ulcer surrounded by erythema on the right leg. Microscopy of the biopsy from the edge of the ulcer showed organisms with dark staining nuclei and kinetoplast. What is the most likely causative agent?
- A child presents with a fever and a rash. Urine examination showed cells with owl's eye appearance. What is the most likely diagnosis?
- Infection with Clonorchis sinensis is associated with an increased risk of__?
View More Questions