In the cooperative binding of oxygen to hemoglobin, which structural change in the hemoglobin molecule is responsible for the allosteric effect?

Explanation

## Cooperative Binding and Allosteric Transitions in Hemoglobin ### The T ↔ R State Model **Key Point:** Hemoglobin exhibits cooperative (allosteric) binding of oxygen through conformational changes between two quaternary states: the **T (tense) state** and the **R (relaxed) state**. Oxygen binding to one subunit increases the affinity of remaining subunits through rotation of the β₁β₂ dimer relative to the α₁α₂ dimer. ### Structural Mechanism | Feature | T State (Tense) | R State (Relaxed) | |---------|-----------------|-------------------| | **Oxygen affinity** | Low | High | | **Quaternary structure** | Constrained | Open | | **Dimer rotation** | Minimal | ~15° rotation of β-dimer | | **Heme iron position** | Out of plane | In plane with porphyrin | | **Stabilizing factors** | Salt bridges, H-bonds | Oxygen binding weakens stabilizers | ### Molecular Events During Oxygen Binding 1. **First O₂ molecule** binds to one subunit → pulls iron into the plane of the porphyrin 2. **Conformational strain** develops within the T-state quaternary structure 3. **Dimer rotation** occurs: the β₁β₂ dimer rotates ~15° relative to α₁α₂ 4. **T → R transition:** Salt bridges and hydrogen bonds stabilizing the T state are disrupted 5. **Remaining subunits** now have higher O₂ affinity (R state) 6. **Cooperative binding** results: sigmoid oxygen-binding curve (not hyperbolic) **High-Yield:** The **quaternary structure change** (T ↔ R transition with dimer rotation) is the physical basis of cooperativity. This is distinct from the **tertiary structure change** (iron movement into the heme plane), which occurs within a single subunit. **Clinical Pearl:** Hemoglobin's sigmoidal binding curve (Hill coefficient ~2.8) reflects this cooperativity. Myoglobin, which lacks quaternary structure, shows hyperbolic binding (Hill coefficient = 1). **Mnemonic:** **TENSE-to-RELAX** — **T**ense state (low affinity, constrained) → **R**elaxed state (high affinity, open). Oxygen binding **relaxes** the molecule.