oxygen binding by hemoglobin
Summary
Hemoglobin binds up to four O₂ cooperatively: each O₂ bound makes the next bind more easily, giving a sigmoidal curve, unlike hyperbolic myoglobin. Most testable: a right shift (lower pH, more CO₂, higher temperature, more 2,3-BPG) means lower affinity and more O₂ release.
Detail
Meaning: Each hemoglobin tetramer has four heme groups, and each Fe²⁺ can bind one O₂. Binding is cooperative: O₂ binding to one subunit favors a shift from the low-affinity T (tense) state toward the high-affinity R (relaxed) state, so later O₂ molecules bind more easily. This gives a sigmoidal saturation curve. The concerted (MWC) and sequential (KNF) models are both used to explain it.
Equation: Y = pO₂ⁿ / (P50ⁿ + pO₂ⁿ), where Y is fractional saturation (unitless), pO₂ is the partial pressure of O₂ (Pa; curves often use mmHg), P50 is the pO₂ at half saturation (same unit), and n is the Hill coefficient (unitless; 1 for myoglobin, greater than 1 for cooperative hemoglobin).
On the MCAT: Passages show O₂ dissociation curves. Predict a right shift (higher P50, lower affinity, more unloading) with lower pH (Bohr effect), more CO₂, higher temperature or more 2,3-BPG, as in exercising muscle. Fetal hemoglobin binds 2,3-BPG less, so its curve lies left of adult hemoglobin's. Myoglobin's hyperbolic curve lies further left.
Common confusion: A right shift does not mean hemoglobin carries more O₂. It means lower affinity at a given pO₂, so more O₂ is released to tissues. A left shift means higher affinity and less release.
Sources
- Lehninger Principles of Biochemistry
- Biochemistry (Berg, Tymoczko, Gatto, Stryer)
- Biochemistry (Voet and Voet)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.