Hemerythrin is a protein responsible for oxygen transportation in the marine invertebrate phyla of sipuniculids , priapulids, brachiopods, and in a single annelid worm, magelona . It is essentially colorless when deoxygenated, but turns a violet-pink in the oxygenated state.

Hemerythrin is not, as the name might suggest, a heme. The "heme" was at first applied to any oxygen-carrying proteins, such as hemoglobin, and when hemoglobin was analyzed, the heme structure was named for the iron-porphyrin cofactor in the transport protein (globin being a general term for a globular protein). Later, after hemerythrin was named, it was discovered that not every oxygen-carrying protein has a heme prosthetic group.

As it turns out the oxygen binding site is a binuclear iron center. The iron atoms are coordinated to the protein through the carboxylate side chains of a glutamate and aspartate, and five histidine peptides. When oxygenated the iron atoms are bridged by a μ-oxo bridge. When deoxygenated the iron atoms are bridged by a hydroxyl group. When binding a oxygen molecule the hydrogen atom from the hydroxyl bridged moves over onto the bound ligand. When releasing the oxygen molecule, the mu-oxo bridge retrieves the hydrogen atom and returns to a hydroxyl group. Thus the following:

Fe-OH-Fe + O₂ <-> Fe-O-Fe-O₂ + H⁺

Hemerythrin affinity for Carbon Monoxide (CO) is actually lower then its affinity for Oxygen (O₂) (unlike hemoglobin which has a very high affinity for CO) making Hemerythrin immune to CO poisoning.

Most Hemorythrin lacks Cooperative binding to oxygen (unlike Hemoglobin) making it roughly 1/4 as efficient as Hemoglobin. In some brachiopods though, hemerythrin shows cooperative binding of oxygen. cooperative binding is achieved by cooperation between subunits: when one subunit becomes oxygenated it increases the affinity for oxygen of the other subunits in the complex.

Hemorythrin is a small protein made of 2-8 identical subunit protiens about 13.5kD in size, totaling 108 kD (if 8 units). 8 subunits or "octamer" form is most common, though some species have 2,3 and 4 subunit groupings. Each subunit carries 2 iron atoms bond together, that can bind to one oxygen molecule. Because of its size Hemorythrin is usually found in cells or "corpuscles" in the blood rather then free floating.