Hemoglobin, which is the principal oxygen-carrier in humans, has four subunits in which the iron(II) ion is coordinated by the planar macrocyclic ligand protoporphyrin IX (PIX) and the imidazole nitrogen atom of a histidine residue.
Hemocyanins carry oxygen in the blood of most mollusks, and some arthropods such as the horseshoe crab.