Q5(a). Proteins as dynamic molecules: haemoglobin

Introduction. A protein's native fold is held by many weak forces (hydrogen bonds, salt bridges, hydrophobic contacts). These allow the chain, its domains and its subunits to move and change shape on binding a ligand. Function depends on these motions. Haemoglobin, an α₂β₂ tetramer with one haem per chain, is the classic example.

Two quaternary states

Haemoglobin exists in two conformations. The T (tense) state (deoxy form) is held by extra salt bridges between the subunits and has low O₂ affinity. The R (relaxed) state (oxy form) has fewer salt bridges and high affinity. Binding of O₂ shifts the balance from T to R; in doing so one αβ dimer rotates by about 15^° relative to the other.

How binding at one haem reaches the others

1. In deoxy-Hb the high-spin Fe²⁺ is too large to fit in the porphyrin plane; it sits slightly out of the plane, towards the proximal histidine (F8), and the ring is domed.
2. On binding O₂, iron becomes low-spin and smaller and moves into the plane of the ring.
3. It pulls the proximal histidine and the F helix with it. This movement breaks salt bridges at the α₁β₂ interface and shifts the whole tetramer towards R.
4. The empty haems of the other subunits now have higher affinity: cooperative (allosteric) binding.

The result is the sigmoid O₂-binding curve (Hill coefficient about 2.8), unlike the hyperbolic curve of single-chain myoglobin. Haemoglobin loads O₂ almost fully in the lungs and releases much of it in the tissues, over a small change of pressure.

Figure: Hyperbolic (myoglobin) and sigmoid (haemoglobin) binding.

Allosteric effectors also act through motion

H⁺ and CO₂ stabilise the T state (Bohr effect), so acidic, CO₂-rich tissue releases more O₂. 2,3-Bisphosphoglycerate binds in the central cavity between the β chains, which is open only in T, and lowers the affinity further. These regulators act at sites far from the haem, which is possible only because the protein changes shape.

Conclusion. Haemoglobin is not a rigid O₂ carrier: O₂ binding moves iron into the haem plane, shifts helices and subunits, and switches the tetramer from T to R. These coupled motions produce cooperative binding and its control by H⁺, CO₂ and 2,3-BPG.

Application. Sickle-cell anaemia (a single Glu→Val change that makes deoxy-Hb polymerise) and carbon monoxide poisoning (CO locks Hb in the R state) are both understood through these conformational changes.