Haemocyanin

Single oxygenated hemocyanin molecule of an octopus: note the two Cu atoms

The underside of the carapace of a red rock crab (Cancer productus). The purple colour is caused by haemocyanin

Haemocyanin (or hemocyanin) is a protein which transports oxygen in the bodies of some invertebrates. Their active centre has two copper atoms which reversibly bind a single oxygen molecule (O₂). Unlike the hemoglobin in red blood cells found in vertebrates, hemocyanins are not bound to blood cells. The molecules are carried in the haemolymph.

Hemocyanins are found only in the Mollusca and Arthropoda.^[1]^[2] However, the molecule is used only in a few insects. This is because insects evolved another system: the direct transfer of oxygen to tissues by a system of tubes called trachea. This delivers oxygen direct to the wing muscles and other organs.

Haemocyanin Media

Molluscan hemocyanin (4YD9)
A hemocyanin active site in the absence of O2 (each Cu center is a cation, charges not shown).
O2-bound form of a hemocyanin active site (the Cu2 center is a dication, charge not shown).

References

↑ Burmester T. 2001. Molecular evolution of the arthropod hemocyanin superfamily. Mol. Biol. Evol. 18 (2): 184–195.
↑ Voit R. et al 2000. Complete sequence of the 24-mer hemocyanin of the tarantula Eurypelma californicum. Structure and intramolecular evolution of the subunits. J. Biol. Chem. 275 (50): 39339–44. [1]

[1] Burmester T. 2001. Molecular evolution of the arthropod hemocyanin superfamily. Mol. Biol. Evol. 18 (2): 184–195.

[Voit_2000-2] Voit R. et al 2000. Complete sequence of the 24-mer hemocyanin of the tarantula Eurypelma californicum. Structure and intramolecular evolution of the subunits. J. Biol. Chem. 275 (50): 39339–44. [1]

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