Hemerythrin

Hemerythrin is used to transport O₂ in a variety of marine invertebrates. It is an octamer (eight subunits), with each subunit containing two iron atoms and binding one molecule of O₂. Deoxyhemerythrin contains two Fe²⁺ ions per subunit and is colorless, whereas oxyhemerythrin contains two Fe³⁺ ions and is bright reddish violet. These invertebrates also contain a monomeric form of hemerythrin that is located in the tissues, analogous to myoglobin. The binding of oxygen to hemerythrin and its release can be described by the following reaction, where the HO₂⁻ ligand is the hydroperoxide anion derived by the deprotonation of hydrogen peroxide (H₂O₂):

Thus O₂ binding is accompanied by the transfer of two electrons (one from each Fe²⁺) and a proton to O₂.