A LITTLE LIGHTBULB MOMENT
Human organ systems
Hemoglobin cooperativity
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One oxygen makes hemoglobin friendlier to the next
The idea behind the cue
Hemoglobin shows positive cooperativity: binding of one oxygen increases affinity at remaining heme sites. This creates a sigmoidal oxygen-hemoglobin dissociation curve and helps loading in the lungs while supporting unloading in tissues.
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Short & simple
One oxygen makes hemoglobin friendlier to the next
Acronym
BSA — Bouncy Squirrels Applaud.
Story
A four-seat movie couch starts with one person sitting down. Suddenly the armrests fold up for the next guests, and the whole couch becomes easier to fill. Hemoglobin behaves similarly after the first oxygen binds.
Connect the pieces
- B (Bouncy) → Binding cooperativity: When one oxygen binds, hemoglobin’s remaining heme sites gain higher affinity.
- S (Squirrels) → Sigmoidal curve: Hemoglobin’s oxygen-binding curve becomes S-shaped because of cooperativity.
- A (Applaud) → Affinity increase: Cooperativity helps oxygen loading in the lungs and unloading in tissues.
- Bouncy · Binding cooperativity: When one oxygen binds, hemoglobin’s remaining heme sites gain higher affinity.
- Squirrels · Sigmoidal curve: Hemoglobin’s oxygen-binding curve becomes S-shaped because of cooperativity.
- Applaud · Affinity increase: Cooperativity helps oxygen loading in the lungs and unloading in tissues.
Know the limits
Does not apply to myoglobin, which is noncooperative and has a hyperbolic binding curve.
Further reading · OpenStax science textbooks ↗Check your understanding
What does positive cooperativity in hemoglobin mean?
A. Oxygen binds only when blood pH is exactly neutral
B. Binding of one oxygen increases affinity for additional oxygen molecules
C. Binding of one oxygen permanently blocks the other heme groups
D. Hemoglobin uses ATP to load oxygen
Reveal explanation
Binding of one oxygen increases affinity for additional oxygen molecules
Hemoglobin's affinity rises after initial oxygen binding, which is the definition of positive cooperativity.