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.