Hemoglobin (heme iron binding site) (Hb)

The heme binding site of hemoglobin refers to the iron atom (Fe²⁺) within the heme prosthetic group in each hemoglobin subunit, which reversibly binds oxygen. Each hemoglobin molecule has four such sites, allowing binding and transport of up to four oxygen molecules. The binding occurs via a coordinate covalent bond between oxygen and the iron at the heme's sixth coordination position, stabilized by histidine residues. Oxygen binding is cooperative—as each oxygen binds, the affinity for the next increases, producing a sigmoidal (S-shaped) oxygen dissociation curve. This allows efficient oxygen loading in the lungs and release in tissues. The heme site is also the target for toxic gases and some drugs, making it a pharmacological and toxicological target[1][5][8][9].

Other names

Hemoglobin heme siteHemoglobin O₂ binding siteOxyhemoglobin site

Oxygen: reversible binding to heme iron (Fe²⁺) as the sixth ligand Carbon monoxide: competitive binding to heme iron with higher affinity, preventing O₂ binding[1][5] Nitric oxide: competitive binding, can cause vasodilation and modulate O₂ delivery Some drugs (e.g., methylene blue): oxidoreductase activity restores hemoglobin iron to the ferrous state for O₂ binding in methemoglobinemia