Hemerythrin

Hemerythrin (also spelled haemerythrin; from Greek words αίμα = blood and ερυθρός = red) is an oligomeric protein responsible for oxygen (O2) transportation in the marine invertebrate phyla of sipunculids, priapulids, brachiopods, and in a single annelid worm, magelona. Recently, hemerythrin was discovered in methanotrophic bacterium Methylococcus capsulatus. Myohemerythrin is a monomeric O2-binding protein found in the muscles of marine invertebrates. Hemerythrin and myohemerythrin are essentially colorless when deoxygenated, but turn a violet-pink in the oxygenated state.

Hemerythrin does not, as the name might suggest, contain a heme. The names of the blood oxygen transporters hemoglobin, hemocyanin, hemerythrin and vanabins, do not refer to the heme group (only found in globins), but are derived from the Greek word for blood.

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 residues. Hemerythrin and myohemerythrin are often described according to oxidation and ligation states of the iron center:

The uptake of O2 by hemerythrin is accompanied by two-electron oxidation of the reduced binuclear center to produce bound peroxide (OOH-). The chemistry of O2 binding is summarized in scheme:

Deoxyhemerythrin contains two high-spin ferrous ions bridged by hydroxyl group (A). One iron is hexacoordinate and another is pentacoordinate. The bridging hydroxyl serves as the proton donor for peroxide after O2 binding, resulting in the formation of a single oxygen atom (μ-oxo) bridge in oxy- and methemerythrin. O2 binds to the pentacoordinate Fe2+ at the vacant coordination site (B). Then electrons are transferred from the ferrous ions to generate the binuclear ferric (Fe3+,Fe3+) center with bound peroxide (C).

Hemerythrin typically exists as an homooctamer or heterooctamer composed of α- and β-type subunits of 13-14 kDa each, although some species have dimeric, trimeric and tetrameric hemerythrins. Each subunit has a four-α-helix fold binding a binuclear iron center. Because of its size hemerythrin is usually found in cells or "corpuscles" in the blood rather than free floating.

Unlike hemoglobin, most hemerythrins lack cooperative binding to oxygen making it roughly 1/4 as efficient as hemoglobin. In some brachiopods though, hemerythrin shows cooperative binding of O2. 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.

Hemerythrin affinity for carbon monoxide (CO) is actually lower than its affinity for O2 (unlike hemoglobin which has a very high affinity for CO) making hemerythrin immune to CO poisoning. This is due to hemerythrin binding mechanism (shown above) which cannot bind CO with stability.