Why is the hemoglobin curve sigmoidal but the myoglobin curve hyperbolic?

Hemoglobin has 4 subunits that bind O2 cooperatively — binding the first O2 shifts the whole tetramer toward a higher-affinity conformation, making the second, third, and fourth O2 bind more easily. That positive cooperativity produces the S-shaped (sigmoidal) curve, described by a Hill coefficient around 2.8. Myoglobin has only one subunit and one binding site, so there's no cooperativity to speak of (Hill coefficient of 1), giving a simple hyperbolic curve identical in shape to a non-cooperative enzyme's Michaelis-Menten curve.

Why does fetal hemoglobin have a higher affinity than adult hemoglobin?

Fetal hemoglobin (HbF) binds 2,3-BPG more weakly than adult hemoglobin does, and since 2,3-BPG binding is what lowers hemoglobin's oxygen affinity, weaker 2,3-BPG binding means HbF holds onto oxygen more tightly (lower P50). This left-shifted curve relative to maternal (adult) hemoglobin is exactly what lets the fetus extract oxygen from the mother's blood at the placental interface, where both bloodstreams sit at the same pO2 but fetal Hb still ends up more saturated.

Why does carbon monoxide poisoning shift the curve left in addition to reducing total oxygen capacity?

CO binds the same heme iron site as O2, with roughly 200x higher affinity, so it directly reduces how many sites are available for O2 (lowering the maximum percent saturation achievable). But CO bound to one subunit also increases the remaining subunits' affinity for O2 through the same cooperative mechanism that makes normal O2 binding cooperative — left-shifting the curve for whatever O2-binding capacity remains, so what oxygen is loaded gets held onto more tightly and released less easily at the tissues, compounding the problem.

Oxygen-Hemoglobin Dissociation Curve Calculator, Free

pO₂ (mmHg) — drag to read saturation off the curve

%Saturation at this pO₂

—

Why the shapes are different

Hemoglobin has 4 subunits that bind O₂ cooperatively — binding the first O₂ shifts the whole tetramer toward a higher-affinity shape, making the next ones bind more easily. That positive cooperativity is what produces the S-shaped (sigmoidal) curve (Hill coefficient n ≈ 2.8). Myoglobin has just one subunit, no cooperativity (n = 1), and a much higher affinity (P50 ≈ 2.8 mmHg vs. hemoglobin's ≈ 26.6 mmHg) — fitting its job as an oxygen-storage protein in muscle rather than a transport protein. Fetal hemoglobin sits between the two: still cooperative (n ≈ 2.8) but higher-affinity than adult Hb (P50 ≈ 19 mmHg), letting it pull oxygen across the placenta from maternal blood.

The Bohr effect (and the other right-shifters)

Strictly, the Bohr effect is the drop in hemoglobin's oxygen affinity as CO₂ and H⁺ (falling pH) rise — exactly the conditions inside actively metabolizing tissue — which shifts the curve to the right (higher P50) and makes hemoglobin release more of its bound oxygen right where it's needed. In the lungs, low CO₂ and higher pH shift the curve back left, favoring loading. Rising temperature and 2,3-BPG (elevated at high altitude and in chronic hypoxia) right-shift the curve the same way, but they're usually named as separate modulators rather than part of the Bohr effect proper — a distinction a picky examiner may want.

Right shift (lower O2 affinity, easier unloading) — tissue conditions: ↑ CO2 ↓ pH (more acidic) [Bohr effect] ↑ Temperature ↑ 2,3-BPG [separate right-shifters] Left shift (higher O2 affinity, easier loading) — lung conditions: ↓ CO2 ↑ pH (more alkaline) ↓ Temperature ↓ 2,3-BPG Mechanism: CO2, H+, and 2,3-BPG all bind hemoglobin away from the heme iron and stabilize the low-affinity "tense" (T) conformation over the high-affinity "relaxed" (R) conformation.

FAQ

Why is the hemoglobin curve sigmoidal but myoglobin's is hyperbolic?
Hemoglobin's 4 subunits bind O₂ cooperatively (n ≈ 2.8) — the S-shape. Myoglobin has one subunit and no cooperativity (n = 1), giving a simple hyperbolic curve, the same shape as a non-cooperative enzyme's Michaelis-Menten curve.

Why does fetal hemoglobin have higher affinity?
HbF binds 2,3-BPG more weakly than adult Hb, and weaker 2,3-BPG binding means higher O₂ affinity (lower P50). That left-shifted curve is exactly what lets the fetus pull oxygen from maternal blood at the placenta.

What is the Bohr effect?
Strictly, it's the drop in hemoglobin's O₂ affinity as CO₂ and H⁺ (falling pH) rise — shifting the curve right and releasing more oxygen where active tissue needs it. Rising temperature and 2,3-BPG right-shift it the same way but are usually named as separate modulators, not the Bohr effect itself.

What causes the Bohr effect mechanistically?
CO₂ and H⁺ bind hemoglobin away from the heme iron and stabilize the low-affinity T conformation over the high-affinity R conformation. 2,3-BPG does the same, binding the deoxygenated tetramer's central cavity.

Why does CO poisoning shift the curve left too?
CO binds the same heme site as O2 with ~200x higher affinity, reducing available sites — but CO bound to one subunit also raises the remaining subunits' O2 affinity through the same cooperative mechanism, left-shifting the curve for whatever capacity remains, so what oxygen is loaded gets released less easily at the tissues.