Hemocyanin

Hemocyanins (also spelled haemocyanins) are respiratory proteins in the form of metalloproteins containing two copper atoms that reversibly bind a single oxygen molecule (O₂). Oxygenation causes a color change between the colorless Cu(I) deoxygenated form and the blue Cu(II) oxygenated form. Hemocyanins carry oxygen in the hemolymph of most molluscs, and some arthropods, including the horseshoe crab, Limulus polyphemus. They are second only to hemoglobin in frequency of use as an oxygen transport molecule. Unlike the hemoglobin in red blood cells found in vertebrates, hemocyanins are not bound to blood cells but are instead suspended directly in the hemolymph.