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 · Haemoglobin: oxygen affinity

Sample item

Fetal haemoglobin has a higher oxygen affinity than adult haemoglobin at the same 2,3-bisphosphoglycerate concentration. What is the molecular reason?

AHbF is partly saturated with carbon monoxide in uteroBHbF has only one heme group per molecule, which binds oxygen avidlyCGamma chains lack histidine 143, so HbF binds 2,3-BPG poorlyDHbF contains ferric iron that holds oxygen more tightly

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

The answer and the reasoning

C. Gamma chains lack histidine 143, so HbF binds 2,3-BPG poorly

2,3-BPG binds in the central cavity of deoxyhaemoglobin through positively charged residues including histidine 143 of the beta chains. In fetal haemoglobin (alpha2 gamma2) that residue is replaced by serine, so BPG binds weakly, the curve shifts left and oxygen is transferred from maternal blood across the placenta.

Option by option

A. HbF is partly saturated with carbon monoxide in utero
The fetus is not saturated with carbon monoxide; the higher affinity is intrinsic to HbF.
B. HbF has only one heme group per molecule, which binds oxygen avidly
HbF is a tetramer with four heme groups, just like HbA.
C. Gamma chains lack histidine 143, so HbF binds 2,3-BPG poorly (correct)
Correct. Weak BPG binding by gamma chains raises oxygen affinity.
D. HbF contains ferric iron that holds oxygen more tightly
Ferric iron in haemoglobin cannot bind oxygen at all.

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 C, Gamma chains lack histidine 143, so HbF binds 2,3-BPG poorly. 2,3-BPG binds in the central cavity of deoxyhaemoglobin through positively charged residues including histidine 143 of the beta chains.

Why is "HbF is partly saturated with carbon monoxide in utero" not the answer?

The fetus is not saturated with carbon monoxide; the higher affinity is intrinsic to HbF.

Why is "HbF has only one heme group per molecule, which binds oxygen avidly" not the answer?

HbF is a tetramer with four heme groups, just like HbA.

Why is "HbF contains ferric iron that holds oxygen more tightly" not the answer?

Ferric iron in haemoglobin cannot bind oxygen at all.

What concept does this question test?

It tests Haemoglobin: oxygen affinity in Biochemistry. In fetal haemoglobin (alpha2 gamma2) that residue is replaced by serine, so BPG binds weakly, the curve shifts left and oxygen is transferred from maternal blood across the placenta.

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 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.