Myoglobin looks like a single subunit of hemoglobin and binds one O2 at one heme. But it is not cooperative - with only one subunit, there is no one to cooperate with. This single structural difference produces radically different oxygen-binding behavior.
The oxygen dissociation curve, and every shift
Hemoglobin
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Scroll sideways to see the whole map.
Myoglobin: hyperbolic, one site Hemoglobin: sigmoid, four sites Hemoglobin, shifted right P₅₀: the pressure at half saturation
Why the curve is S-shapedThe first oxygen binds with difficulty because hemoglobin starts in the tense T state. Binding pulls the subunit into the relaxed R state and drags its neighbors with it, so the second, third, and fourth oxygens bind progressively more easily. That is cooperativity, and it is the whole reason the curve is sigmoid rather than hyperbolic.
Why the shape is usefulA sigmoid curve is steep in the middle, so a small drop in pO₂ between artery and working tissue causes a large drop in saturation. Myoglobin's hyperbolic curve is nearly flat at those pressures, which is exactly why myoglobin makes a good store and a poor carrier.
The Bohr effectHard-working tissue is warm, acidic, and full of CO₂. All three shift the curve right, so hemoglobin releases more oxygen precisely where it is needed. The tissue asks for oxygen by changing the chemistry around it, not by any signal.
Right means release, left means load. Anything that says a tissue is working hard shifts the curve right and dumps more oxygen. Anything that says oxygen must be held, like fetal hemoglobin pulling oxygen across the placenta, shifts it left.
Side-by-Side
| Property | Myoglobin | Hemoglobin |
|----------|-----------|------------|
| Structure | Monomer (1 subunit) | Tetramer (4 subunits) |
| Heme groups | 1 | 4 |
| Cooperativity | None | Positive |
| Curve shape | Hyperbolic | Sigmoidal |
| Role | Oxygen storage in muscle | Oxygen transport in blood |
| Affinity for O2 | Very high (holds tight) | Variable (loads in lungs, unloads in tissue) |
Why a Hyperbolic Curve for Myoglobin
Myoglobin has one binding site. Binding follows a simple equilibrium just like Michaelis-Menten with one substrate. The plot of fraction bound vs. pO2 is hyperbolic - it rises fast at low pO2 and levels off.
Why the Curves Matter
Myoglobin’s high affinity means it holds O2 even at relatively low tissue pO2. That is perfect for a reservoir - muscles can quickly tap myoglobin when demand spikes (sprinting). But it is terrible for transport, because you would need very low pO2 before it would release O2.
Hemoglobin’s cooperativity means it releases O2 over a small pO2 range, which matches the pO2 gradient between lungs (high) and tissues (lower). It is built for long-distance transport with quick offloading on arrival.
Whale and Seal Muscle
Diving mammals have dramatically more myoglobin in their muscles than humans (their muscle is dark red or even black from the myoglobin content). That enormous myoglobin pool lets them hold enough local O2 to stay submerged for 20+ minutes without breathing. It is the anatomical signature of evolution solving a gas-supply problem.
Why does myoglobin have a hyperbolic O2-binding curve while hemoglobin has a sigmoidal one?
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Myoglobin is a single-subunit protein with one heme, so there are no neighboring subunits with which to show cooperativity. The binding follows a simple equilibrium and produces a hyperbolic curve. Hemoglobin is a four-subunit tetramer with cooperativity between hemes, producing the S-shaped curve.
Which has higher O2 affinity at a given pO2, myoglobin or hemoglobin, and why does that match physiology?
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Myoglobin - its curve sits far to the left of hemoglobin’s. This is appropriate because myoglobin’s job is to hold O2 as a muscle reservoir. Hemoglobin’s lower affinity lets it release O2 to tissues; a too-tight binder (myoglobin-level affinity) would never give up its oxygen once it reached a tissue.
Why do diving mammals have especially high myoglobin content in their muscles?
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Their muscles store large amounts of O2 bound to myoglobin so they can sustain aerobic activity during long dives without breathing. The dark color of whale and seal muscle reflects this dense myoglobin packing. It is an evolutionary solution to intermittent oxygen availability.
