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.
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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.
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.
Sources and how to cite this page
Curves are generated from the Hill equation, %saturation = 100 × pO₂ⁿ / (P50ⁿ + pO₂ⁿ), using the standard physiological parameters: adult hemoglobin P50 ≈ 26.6 mmHg with a Hill coefficient n ≈ 2.8, fetal hemoglobin P50 ≈ 19 mmHg, and myoglobin P50 ≈ 2.8 mmHg with n = 1. Myoglobin's n of 1 reflects its single subunit: with no other subunits to cooperate with, its curve is a simple hyperbola rather than a sigmoid.
What the Hill coefficient is and is not. n ≈ 2.8 is an empirical measure of cooperativity, not a count of binding sites. Hemoglobin has four oxygen-binding sites, so perfectly infinite cooperativity would give n = 4 and no cooperativity would give n = 1; the measured 2.8 says binding is strongly but not completely cooperative. P50 values also shift with pH, CO₂, temperature, and 2,3-BPG, which is the Bohr effect, so treat these as reference conditions rather than fixed constants.