Protein chemistry: haemoglobin oxygen affinity: 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 · Protein chemistry: haemoglobin oxygen affinity

Sample item

In vigorously exercising muscle, blood has higher carbon dioxide, lower pH and higher 2,3-bisphosphoglycerate. How does the oxygen dissociation curve of haemoglobin change, and what is the effect?

ARightward shift with a lower P50, reducing oxygen unloadingBLeftward shift with a lower P50, increasing oxygen unloadingCNo shift, because haemoglobin affinity is independent of pHDRightward shift with a higher P50, increasing oxygen unloading to tissues

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

The answer and the reasoning

D. Rightward shift with a higher P50, increasing oxygen unloading to tissues

Protons and carbon dioxide stabilise the T (deoxy) state of haemoglobin through the Bohr effect, and 2,3-bisphosphoglycerate does the same by binding between the beta chains. The curve shifts to the right, which means a higher P50 and lower oxygen affinity, so oxygen is released more readily where metabolism is active.

Option by option

A. Rightward shift with a lower P50, reducing oxygen unloading
A right shift is by definition accompanied by a higher, not lower, P50.
B. Leftward shift with a lower P50, increasing oxygen unloading
A left shift raises affinity and would reduce unloading, as in HbF or alkalosis.
C. No shift, because haemoglobin affinity is independent of pH
Haemoglobin affinity does depend on pH, as shown by the Bohr effect.
D. Rightward shift with a higher P50, increasing oxygen unloading to tissues (correct)
Correct. A right shift raises P50 and favours oxygen release in active tissue.

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.

Questions about this Biochemistry question

What is the answer to this Biochemistry question?

The best answer is D, Rightward shift with a higher P50, increasing oxygen unloading to tissues. Protons and carbon dioxide stabilise the T (deoxy) state of haemoglobin through the Bohr effect, and 2,3-bisphosphoglycerate does the same by binding between the beta chains.

Why is "Rightward shift with a lower P50, reducing oxygen unloading" not the answer?

A right shift is by definition accompanied by a higher, not lower, P50.

Why is "Leftward shift with a lower P50, increasing oxygen unloading" not the answer?

A left shift raises affinity and would reduce unloading, as in HbF or alkalosis.

Why is "No shift, because haemoglobin affinity is independent of pH" not the answer?

Haemoglobin affinity does depend on pH, as shown by the Bohr effect.

What concept does this question test?

It tests Protein chemistry: haemoglobin oxygen affinity in Biochemistry. The curve shifts to the right, which means a higher P50 and lower oxygen affinity, so oxygen is released more readily where metabolism is active.

How hard is this question?

We tag it as a multi-step question: it asks you to combine more than one idea. 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 Protein chemistry: haemoglobin oxygen affinity?

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.