Carbohydrate metabolism: 2,3-bisphosphoglycerate synthesis: practice question

A Biochemistry question in the style of the NEET PG paper. Answer it first, then read why the right option is right and why each of the others is wrong.

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Biochemistry · Carbohydrate metabolism: 2,3-bisphosphoglycerate synthesis

Sample item

Red cells generate 2,3-bisphosphoglycerate by diverting an intermediate of glycolysis, bypassing the first ATP-generating step. Which enzyme catalyses the diversion?

Original practice item, not a recalled exam question. Nothing here is recorded.

The answer and the reasoning

B. Bisphosphoglycerate mutase

In the Rapoport-Luebering shunt, bisphosphoglycerate mutase converts 1,3-bisphosphoglycerate to 2,3-bisphosphoglycerate. The shunt skips phosphoglycerate kinase, so no ATP is made at that step, but the product lowers haemoglobin oxygen affinity and favours tissue oxygen delivery.

Option by option

  • A. Phosphoglycerate mutase

    Phosphoglycerate mutase converts 3-phosphoglycerate to 2-phosphoglycerate and is a normal glycolytic step.

  • B. Bisphosphoglycerate mutase (correct)

    Correct. Bisphosphoglycerate mutase makes 2,3-BPG from 1,3-BPG and bypasses the ATP-generating kinase step.

  • C. Enolase

    Enolase dehydrates 2-phosphoglycerate to phosphoenolpyruvate and is also inhibited by fluoride.

  • D. Phosphoglycerate kinase

    Phosphoglycerate kinase is the step being bypassed; it converts 1,3-BPG to 3-phosphoglycerate and makes ATP.

Source note: Original NextPrep sample item — pending expert review. This item is original practice material awaiting independent expert review, so confirm important facts in a standard textbook.

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Questions about this Biochemistry question

What is the answer to this Biochemistry question?

The best answer is B, Bisphosphoglycerate mutase. In the Rapoport-Luebering shunt, bisphosphoglycerate mutase converts 1,3-bisphosphoglycerate to 2,3-bisphosphoglycerate.

Why is "Phosphoglycerate mutase" not the answer?

Phosphoglycerate mutase converts 3-phosphoglycerate to 2-phosphoglycerate and is a normal glycolytic step.

Why is "Enolase" not the answer?

Enolase dehydrates 2-phosphoglycerate to phosphoenolpyruvate and is also inhibited by fluoride.

Why is "Phosphoglycerate kinase" not the answer?

Phosphoglycerate kinase is the step being bypassed; it converts 1,3-BPG to 3-phosphoglycerate and makes ATP.

What concept does this question test?

It tests Carbohydrate metabolism: 2,3-bisphosphoglycerate synthesis in Biochemistry. The shunt skips phosphoglycerate kinase, so no ATP is made at that step, but the product lowers haemoglobin oxygen affinity and favours tissue oxygen delivery.

How hard is this question?

We tag it as an application question: it asks you to use a fact on a patient or a finding. The tag is our own editorial judgement, not an official difficulty rating, so read the reason for every option, not only the answer.

How should I study Carbohydrate metabolism: 2,3-bisphosphoglycerate synthesis?

Build one table of enzyme deficiency, disease, inheritance and key finding, then revise it by spaced recall. Then use the flashcards and the other Biochemistry questions on this site to test what stuck.

Is this a real NEET PG question?

No. It is an original practice item (editorial status: sample, version 1). NextPrep does not publish recalled or leaked questions, and independent faculty review of this item has not been completed yet, so confirm important facts in a standard textbook.