Oxygen binding: haemoglobin versus myoglobin !

Myoglobin stores oxygen and releases it late; haemoglobin picks it up in the lung and delivers it to tissues thanks to its S-shaped curve.

Updated 2026-11-01

Key facts

Myoglobin: one chain and one haem; hyperbolic curve, P50 ≈ 2–3 mmHg (very high affinity).
Haemoglobin: four chains (α₂β₂) and four haems; sigmoidal curve from cooperativity, P50 ≈ 26–27 mmHg, Hill coefficient ≈ 2.8.
Right shift (lower affinity, more delivery): ↑ CO₂, ↑ H⁺ (↓ pH, Bohr effect), ↑ 2,3-BPG, ↑ temperature, exercise.
Left shift (higher affinity, less delivery): HbF, CO, methaemoglobin, alkalosis, hypothermia, stored blood (low 2,3-BPG).
Haldane effect: deoxygenated haemoglobin takes up more H⁺ and CO₂; in the lung, as it is oxygenated, it releases them.

With O₂ bound, the iron (Fe²⁺) moves into the haem plane and pulls the proximal histidine (F8); that movement passes to the other subunits and eases the next binding (T → R).
In the lung (PO₂ ≈ 100 mmHg) haemoglobin is about 98 % saturated; in resting tissue (≈ 40 mmHg) it stays at ≈ 75 %, and in exercising muscle it falls much further.
Myoglobin stays almost saturated at 40 mmHg: it gives up its O₂ only when muscle PO₂ falls very low, as a reserve.
2,3-BPG binds in the central cavity of deoxyhaemoglobin (between the β chains) and stabilises the T form; it rises at altitude, in anaemia and in chronic hypoxia.
HbF (α₂γ₂) binds 2,3-BPG poorly, so it has higher affinity than HbA and takes O₂ from maternal blood.
CO binds haem about 200–250 times more strongly than O₂ and also shifts the curve to the left: O₂ loading and delivery both fall.
O₂ content ≈ 1.34 × Hb (g/dL) × SaO₂ + 0.003 × PaO₂: nearly all of it travels bound to haemoglobin.

Right shift: “CADET, face right” (CO₂, Acid, 2,3-DPG, Exercise, Temperature).

In CO poisoning the pulse oximeter reads normal because it cannot tell carboxyhaemoglobin from oxyhaemoglobin; co-oximetry is needed.
At altitude (for example Bogotá, at about 2,600 m) 2,3-BPG and haemoglobin rise as an adaptation to the lower PO₂.
Blood stored for a long time has little 2,3-BPG and delivers O₂ poorly during the first hours after transfusion.

Think of the answer, then tap the question to see it.

Why is the haemoglobin curve sigmoidal and the myoglobin curve hyperbolic?

Haemoglobin has four sites that cooperate (binding one eases the others); myoglobin has a single site.

Which changes shift the haemoglobin curve to the right?

More CO₂, more H⁺ (acidosis), more 2,3-BPG and higher temperature: affinity falls and O₂ delivery to tissue rises.

Why does HbF have higher O₂ affinity than HbA?

Its γ chains bind 2,3-BPG poorly, and 2,3-BPG normally stabilises the low-affinity form.

What is the Haldane effect?

Deoxygenated haemoglobin carries more CO₂ and H⁺; when it is oxygenated in the lung it releases them, which helps clear CO₂.

Educational information, not medical advice. For any symptom, see a health professional.

Sources: PDB-101: Molecule of the Month: Hemoglobin (RCSB Protein Data Bank (PDB-101), 2026); PDB-101: Molecule of the Month: Myoglobin (RCSB Protein Data Bank (PDB-101), 2026); Blood gases: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2026); HBB gene: MedlinePlus Genetics (MedlinePlus, US National Library of Medicine, 2026); Textbook of Biochemistry for Medical Students, 6th ed. (Jaypee Brothers Medical Publishers (Vasudevan DM, Sreekumari S, Vaidyanathan K), 2011).

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Representative model, for educational purposes only.