Metals in biology: how haemoglobin carries oxygen

On 8 November 1845 Faraday measured the magnetism of dried blood and noted: “Must try recent fluid blood.” He never did. Ninety years later Pauling and Coryell (1936) did: oxyhaemoglobin and carbonmonoxyhaemoglobin have no unpaired electrons, but deoxyhaemoglobin has four per haem — so the magnetic susceptibilities of fully oxygenated and fully deoxygenated blood differ by as much as 20 %.

One iron ion binds one molecule of oxygen and its whole spin state flips. This lesson starts from that iron in haem: why it binds O₂ reversibly without being oxidised, how four haems “consult” one another to load and unload oxygen together, why carbon monoxide kills, and how a small device clipped to a finger reads the oxygen in your blood.

Haem is a porphyrin ring whose four nitrogen atoms surround an Fe²⁺ ion. In myoglobin and in each chain of haemoglobin, a histidine of the protein (the proximal histidine) supplies a fifth ligand from below; the site above is left for O₂.

In the deoxy form the iron has only five ligands and a fairly weak field: d⁶ Fe²⁺ is high spin (t₂g⁴e_g², four unpaired electrons). High-spin Fe²⁺ is too large for the porphyrin’s central hole and sits a little below the ring plane. When O₂ binds, the sixth ligand pushes the field past the pairing energy: the iron becomes low spin (t₂g⁶, no unpaired electrons), shrinks, slides into the ring plane and tugs on the proximal histidine and the helix that carries it — a movement passed on to the other subunits, which is the switch behind cooperative binding (Perutz, 1970).

The iron stays +2 throughout. Oxidised to Fe³⁺ (methaemoglobin, brown) it can no longer bind O₂. The distal histidine and the water-poor pocket both stabilise the bound O₂ and protect the iron from oxidation.

Myoglobin has a single haem and stores oxygen in muscle. Its binding curve is a hyperbola, S = p/(p + P₅₀) — exactly the form of Langmuir’s isotherm for surface adsorption: one site, a dynamic balance of filling and emptying. Horse myoglobin at 37 °C and pH 7.0 has P₅₀ = 2.39 Torr.

Haemoglobin is a tetramer with four haems. The first O₂ binds hardest; once it has, the protein shifts from its “tense” state (T) towards its “relaxed” state (R), and the other sites gain affinity. The result is a sigmoid curve: little binding at low pressure, a steep rise in the middle, saturation at high pressure. Hill’s equation is the usual way to describe that steepness.

In 1904 Bohr, Hasselbalch and Krogh found that the more carbon dioxide, the less oxygen blood holds. H⁺ and CO₂ bind better to deoxyhaemoglobin, stabilise the T state and push the curve to the right — the Bohr effect. Working muscle is both acidic and warm, so oxygen is released where it is most needed; in the lungs CO₂ is breathed out, the pH rises and haemoglobin takes a firm grip on oxygen again.

Severinghaus’s equations give Δlg P₅₀/ΔpH ≈ −0.52 near P₅₀: lowering the pH from 7.40 to 7.20 raises P₅₀ from 26.8 to 34.1 Torr. Each degree of warming raises P₅₀ by about 7 %. 2,3-Bisphosphoglycerate in red cells also stabilises the T state — one reason fetal haemoglobin, which binds it weakly, can take oxygen from the mother’s blood.

CO competes with O₂ for the same site, with far higher affinity. Haldane and colleagues (1912) summed it up: at equilibrium the ratio of carbonmonoxy- to oxyhaemoglobin equals M times the ratio of the two partial pressures. For human haemoglobin in the relaxed state, M = 241 ± 29 (about 123 in the tense state). Just 0.05 % CO in air can occupy nearly half the haem at equilibrium. Worse, haemoglobin carrying CO sits in the R state, so its remaining sites hold O₂ more tightly, the curve shifts left and the tissues find it even harder to get oxygen.