Hemocyanin
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Hemocyanins (also spelled haemocyanins) are respiratory proteins 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 blood of most molluscs, and some arthropods such as the horseshoe crab. They are second only to hemoglobin in biological popularity of use in oxygen transport.
Although the respiratory function of hemocyanin is similar to that of hemoglobin, there are a number of differences in its molecular structure and mechanism. Whereas hemoglobin carries its iron atoms in porphyrin rings (heme groups), the copper atoms of hemocyanin are bound as prosthetic groups comprised of histidine peptides. Hemocyanin binds with oxygen non-cooperatively and is only one-fourth as efficient as hemoglobin at transporting oxygen. Hemoglobin binds oxygen cooperatively due to steric conformational changes in the protein complex, which increases hemoglobin's affinity for oxygen when partially oxygenated. Hemocyanin does not have an increased affinity for oxygen when only partially oxygenated.
Hemocyanin is made of individual subunit proteins, each of which contains two copper atoms and can bind one oxygen molecule (O2). Each subunit weighs about 75 kilodaltons (kDa). Subunits are arranged in chains or bundles in weights exceeding 1500 kDa. Because of the large size of hemocyanin, it is usually found free-floating in the blood, unlike hemoglobin, which must be contained in cells because its small size would lead it to clog and damage blood filtering organs such as the kidneys. This free-floating nature allows for higher densities of hemocyanin in the blood (as compared to hemoglobin), and helps offset its low efficiency.