All living organisms need oxygen for metabolic processes that generate energy. This oxygen must be acquired from the environment and delivered efficiently to every cell. For smaller life forms, simple diffusion across their body surface is sufficient for gas exchange. However, as organisms increase in size and complexity, specialized internal transport systems and molecules become necessary to meet the oxygen demands of their tissues. These molecules capture and release oxygen, ensuring distant cells receive an adequate supply.
What is Hemerythrin?
Hemerythrin is a unique protein responsible for oxygen transport and storage in certain marine invertebrates. Unlike other oxygen carriers, hemerythrin lacks a heme group (a porphyrin ring with an iron atom). Instead, its oxygen-binding site consists of two iron atoms directly bound to the protein itself.
This protein is primarily found in specific phyla of marine invertebrates, including sipunculids (peanut worms), priapulids (penis worms), brachiopods (lamp shells), and a single genus of annelid worm, Magelona. Hemerythrin typically exists as an octamer, composed of eight protein subunits (13-14 kDa each). Some species also have dimeric, trimeric, or tetrameric forms.
How Hemerythrin Carries Oxygen
The mechanism by which hemerythrin binds and releases oxygen is distinct. Its active site features a pair of iron centers, anchored to the protein by carboxylate side chains of aspartic and glutamic acid, and five histidine residues.
In its deoxygenated state, both iron atoms are in the ferrous (Fe2+) oxidation state and are bridged by a hydroxyl group. When oxygen binds, it attaches to one iron atom (the pentacoordinate Fe2+ center) at a vacant coordination site. This binding event causes a two-electron oxidation of both ferrous ions, resulting in a binuclear ferric (Fe3+, Fe3+) center and the formation of a hydroperoxide (OOH-) complex. The hydrogen atom from the bridging hydroxyl group transfers to the bound oxygen, stabilizing this peroxo nature.
This transformation also produces a color change. Deoxygenated hemerythrin is colorless, but turns reddish-purple or violet-pink when oxygenated. This color change indicates its oxygenation status.
Comparing Hemerythrin to Other Oxygen Carriers
Hemerythrin stands apart from other major oxygen-carrying proteins like hemoglobin and hemocyanin. Hemoglobin, found in vertebrates and some invertebrates, utilizes a heme prosthetic group with a single iron atom for oxygen binding. This iron in hemoglobin reversibly binds oxygen in an “end-on” fashion, and its oxygenated state appears bright red.
Hemocyanin, present in mollusks and arthropods, uses copper atoms at its active site for oxygen transport. Unlike hemerythrin’s iron, hemocyanin’s copper centers bind oxygen in a “side-on” bridging formation between two copper atoms. Deoxygenated hemocyanin is colorless, but it turns blue when oxygenated due to the copper’s interaction with oxygen.
Why Certain Organisms Use Hemerythrin
The presence of hemerythrin in specific marine invertebrates is linked to evolutionary adaptations to their environments and physiological needs. These organisms, such as sipunculids and brachiopods, often inhabit marine environments with varying oxygen levels. Hemerythrin’s tight oxygen binding may offer advantages in these conditions.
Most hemerythrins do not exhibit cooperative oxygen binding (where oxygen binding to one site increases affinity of others), though some brachiopods do. Cooperative binding allows for more efficient oxygen uptake and release over a range of oxygen concentrations. Additionally, hemerythrin has a lower affinity for carbon monoxide (CO) compared to oxygen, unlike hemoglobin, which is highly susceptible to CO poisoning. This reduced CO affinity may be beneficial in environments where carbon monoxide is present, reflecting hydrogen bonding’s role in its oxygen binding.