Hemerythrin

Hemerythrin is a non-heme, iron-containing oxygen-binding protein found in the muscles and tissues of a variety of marine invertebrates, including certain species of annelids (segmented worms), mollusks, and the brachiopods. Unlike the more commonly known oxygen-carrying proteins hemoglobin and myoglobin, which contain heme as their iron-containing prosthetic group, hemerythrin carries oxygen by directly binding it to iron atoms in its structure.

Structure and Function

Hemerythrin is typically found as an octamer or a dimer, though its oligomeric state can vary among different species. Each monomeric unit of hemerythrin contains two iron atoms that are not incorporated into a heme group but are instead directly coordinated to the protein by histidine residues and possibly oxygen or hydroxide ligands. This iron center is responsible for the reversible binding of oxygen, which changes the color of the protein from colorless to pink or red upon oxygenation.

The primary function of hemerythrin is to transport oxygen from the organism's respiratory surfaces to its tissues, similar to the role of hemoglobin in vertebrates. However, hemerythrin is found in much lower concentrations in the body fluids of invertebrates than hemoglobin in vertebrates, suggesting differences in oxygen transport efficiency and mechanism.

Evolutionary Significance

The presence of hemerythrin in only a few, unrelated groups of marine invertebrates suggests an interesting case of convergent evolution. The evolution of different oxygen-binding proteins, such as hemerythrin, hemoglobin, and hemocyanin (another non-heme, copper-containing oxygen carrier found in some invertebrates), highlights the diverse strategies life has developed to solve the problem of oxygen transport within multicellular organisms.

Comparative Aspects

Hemerythrin's oxygen-binding capacity is generally lower than that of hemoglobin, but it has a higher affinity for oxygen, which allows these invertebrates to thrive in low-oxygen environments. Unlike hemoglobin, hemerythrin does not exhibit cooperative binding of oxygen (a phenomenon where the binding of one oxygen molecule increases the affinity for the next), which is a characteristic feature of the quaternary structure of hemoglobin.

Clinical and Biotechnological Applications

Research into hemerythrin is ongoing, with studies focusing on its unique structure and oxygen-binding mechanism. Understanding these aspects could have implications for the development of artificial blood substitutes and oxygen carriers, as well as providing insights into the evolutionary biology of oxygen transport proteins.

See Also

• Myoglobin
• Hemoglobin
• Hemocyanin
• Oxygen transport