What Are Hemocyanins?

Hemocyanins are respiratory proteins that are used to transport oxygen in mollusks and crustaceans, as well as in some insects. They are analogous in function to the hemoglobin found in the blood of vertebrates. Both hemoglobin and hemocyanins are metalloproteins, or proteins that include a metallic ion, but hemocyanins include copper rather than iron and are not bound to blood cells. Instead, they float freely in the hemolymph, the circulatory fluid of crustaceans and molluks. These proteins have been studied for their therapeutic benefits, including use as a potential treatment for cancer.

Many organisms use hemocyanins for oxygen transport. Mollusks in particular, a group of invertebrates that includes clams, limpets, snails, squids, and related species, rely on them. Horseshoe crabs, lobsters, crabs, and other arthropods also utilize these proteins, as do certain insects.

For crustaceans and mollusks that live in cold, low-oxygen aquatic environments, hemocyanins are more efficient than hemoglobin as a method of oxygen transport. As a free-floating protein, hemocyanin can exist in greater density within the hemolymph than hemoglobin, which must be contained in cells. The increased density of respiratory proteins allows the organism to “pick up” more oxygen, a capability that is useful in low-oxygen environments.

The disadvantage of having a high concentration of hemocyanin in the hemolymph is that it increases the viscosity of the respiratory fluid, making it more difficult for the organism to circulate the fluid around its body. For this reason, hemocyanin is most advantageous for organisms that expend less energy on movement, such as slow-moving horseshoe crabs and mollusks. In terrestrial settings with different environmental conditions, hemoglobin is the more efficient oxygen transport protein. The few insects that do have hemocyanins retain them more as an ancestral trait — an evolutionary holdover from earlier times — than as a functional advantage.

The hemocyanin metalloprotein consists chemically of two copper atoms bound to a diatomic oxygen molecule — a molecule containing two oxygen atoms. When the copper binds to the oxygen to carry it throughout the hemolymph, it becomes oxygenated and changes color, turning blue. This color change accounts for the bluish-purple coloring that can be observed in some crabs and lobsters.

Research has indicated that the hemocyanins of certain mollusks may have medicinal applications. The hemolymph of Concholepas concholepas, also known as the Chilean abalone, contains respiratory proteins that have been shown to reduce bladder tumors in mice. The keyhole limpet, Megathura crenulata, provides the keyhole limpet hemocyanin (KLH), which is purified for use in biotechnology. Specifically, KLH is used as a carrier protein for antibodies. It may also be useful as a component of cancer vaccines.

Hemocyanins are copper-containing proteins found in the blood of many invertebrates, including mollusks and arthropods. They transport oxygen throughout the body, turning a striking blue when oxygenated. Unlike hemoglobin, which carries oxygen in red blood cells of vertebrates, hemocyanins are dissolved directly in the hemolymph (invertebrate equivalent of blood) and bind oxygen less tightly, which is efficient for the metabolism of these animals.

The blue color of the blood in organisms with hemocyanins is due to the copper ions present in the protein. When hemocyanins bind with oxygen, the copper ions are oxidized, giving the blood its distinctive blue color. This contrasts with the iron in hemoglobin, which turns red when it binds with oxygen, coloring the blood of vertebrates red.

Hemocyanin can be advantageous in certain environments. It has a higher affinity for oxygen in cold and low-oxygen conditions, which can be beneficial for species living in deep-sea habitats. Additionally, hemocyanin can transport more oxygen per molecule than hemoglobin, which is useful for animals with slower metabolisms or those that experience sporadic access to oxygen.

Humans and other vertebrates exclusively use hemoglobin for oxygen transport and cannot utilize hemocyanin. The evolution of hemoglobin and hemocyanin occurred independently, and each protein is adapted to the physiology of the organisms that use them. Vertebrates lack the genetic information and physiological adaptations necessary to produce or benefit from hemocyanin.

The presence of hemocyanin affects an animal's physiology by influencing its oxygen transport and metabolic processes. Hemocyanin's lower affinity for oxygen compared to hemoglobin means that invertebrates with hemocyanin may have slower metabolisms and can thrive in environments where oxygen is less available. Their circulatory systems are also adapted to transport the protein effectively.

While hemocyanin is predominantly found in aquatic invertebrates, such as mollusks and arthropods, it is also present in some terrestrial species. For example, certain land-dwelling arthropods like spiders and scorpions possess hemocyanin. This protein's evolution reflects the diverse respiratory needs of invertebrates across various habitats, not just aquatic environments.