Proteins in Action – Haemoglobin: Cooperativity, Allostery & Physiological Modulation

Conformational Basis of Oxygen Binding

• O_2 binding flattens haem, pulls His F8 & helix F toward the haem pocket.
• Transition between $T \text{ (low affinity)}$ and $R \text{ (high affinity)}$ states underlies function.
• Any factor that keeps helix F away lowers O_2 affinity.

Cooperativity & Allostery

• Tetrameric Hb shows positive cooperativity: first O_2 bound shifts equilibrium $T \leftrightarrow R$, increasing affinity at remaining sites.
• Cooperativity is most evident when allosteric inhibitors (e.g.
  BPG, CO_2, H\^+) stabilise the $T$ state; “stripped” Hb (no inhibitors) is largely in $R$ and shows little cooperativity.

Major Allosteric Effectors

• 2,3-Bisphosphoglycerate (BPG)
  • Binds central cavity of deoxy-Hb via electrostatic contacts.
  • Stabilises $T$ state → lowers O_2 affinity.
  • [BPG] rises in peripheral tissues & at high altitude, promoting O_2 release.
• CO_2 & H\^+ (Bohr effect)
  • CO_2 forms carbamates; H\^+ protonates residues → both favour $T$ state.
  • Result: reduced O_2 affinity in metabolically active/acidic tissues.

Binding Curves & Physiological Function

• Myoglobin: hyperbolic curve; saturated at low $pO<em>2$; releases O_2 only when $pO</em>2$ is very low.
• Haemoglobin: sigmoidal curve due to cooperativity + allostery; loads O_2 in lungs (high $pO<em>2$), releases in tissues (low $pO</em>2$).

Foetal vs Adult Haemoglobin

• Foetal Hb (HbF: (\alpha2\gamma2)) replaces β with γ chains.
• γ chain has Ser instead of His in BPG site → weaker BPG binding, higher O_2 affinity.
• Enables O_2 transfer across placenta; post-natal switch to adult HbA ((\alpha2\beta2)).

Sickle Cell Haemoglobin (HbS)

• Mutation: $\beta6\,\text{Glu} \rightarrow \text{Val}$ creates hydrophobic patch.
• Deoxy-HbS polymerises → erythrocyte “sickling”, vaso-occlusion; nonetheless confers malaria resistance.
• Therapies:
  • CRISPR (CASGEVY) reactivates HbF; curative but costly & resource-intensive.
  • Voxelotor stabilises $R$ state, raises O_2 affinity; oral option where gene therapy impractical.

Summary Points

• O_2 binding induces conformational change, driving $T \rightarrow R$ shift.
• BPG, CO_2, H\^+ are key allosteric inhibitors that lower Hb O_2 affinity and reveal cooperativity.
• Foetal adaptations and pathological variants (HbS) modify these mechanisms for distinct physiological outcomes.