Q.102 In an individual, three distinct proteins bind oxygen depending on the location and development
stage. While hemoglobin is the major oxygen binding protein in adults, myoglobin is present in
skeletal muscles and fetal hemoglobin is present in fetal stage only. The following graph shows the
oxygen binding capacity of these proteins. The A, B and C plots represent oxygen binding capacity
of
(A) hemoglobin, fetal hemoglobin and myoglobin, respectively
(B) fetal hemoglobin, hemoglobin and myoglobin, respectively
(C) hemoglobin, myoglobin and fetal hemoglobin, respectively
(D) myoglobin, fetal hemoglobin and hemoglobin, respectively

Introduction

Oxygen transport in humans depends on three major proteins — myoglobin (Mb), adult hemoglobin (HbA), and fetal hemoglobin (HbF). Each shows a distinct oxygen-binding affinity and produces characteristic oxygen dissociation curves.

By analyzing the shape and position of saturation vs partial pressure (pO₂) graphs, we can identify which curve belongs to which protein.

This concept is frequently tested in biochemistry, physiology, and CSIR/NET/GATE exams.

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Concept Behind the Graph

The graph shows:

• X-axis → Oxygen partial pressure (pO₂)

• Y-axis → Fractional saturation

Three curves:

• A (leftmost, highest affinity)

• B (middle)

• C (rightmost, lowest affinity)

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Step-by-Step Biological Logic

 Myoglobin (Mb)

• Monomeric protein

• No cooperativity

• Hyperbolic curve

• Very high O₂ affinity

• Saturates at very low pO₂

• Appears leftmost

👉 Therefore → Curve A

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 Fetal Hemoglobin (HbF)

• Tetramer (α₂γ₂)

• Higher affinity than adult Hb

• Binds 2,3-BPG weakly

• Helps fetus extract O₂ from mother

• Sigmoidal curve

• Slightly left of adult Hb

 Therefore → Curve B

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 Adult Hemoglobin (HbA)

• Tetramer (α₂β₂)

• Shows cooperativity

• Lower affinity than HbF

• Releases O₂ to tissues easily

• Right-shifted sigmoidal curve

 Therefore → Curve C

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 Curve Identification

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 Matching With Options

(A) hemoglobin, fetal hemoglobin and myoglobin

 Wrong — order incorrect

(B) fetal hemoglobin, hemoglobin and myoglobin

 Wrong — myoglobin should be first

(C) hemoglobin, myoglobin and fetal hemoglobin

 Wrong — incorrect affinity sequence

(D) myoglobin, fetal hemoglobin and hemoglobin

 Correct — matches A, B, C

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Quick Exam Tricks

Remember:

Affinity order:

$$\text{Myoglobin}>\text{Fetal Hb}>\text{Adult Hb}$$

Curve rule:

• Left shift → high affinity

• Right shift → low affinity

• Hyperbolic → myoglobin

• Sigmoidal → hemoglobin

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 Final Answer

(D) myoglobin, fetal hemoglobin and hemoglobin, respectively