Hemocyanin

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1ll1 :110-373 1hc1A:136-393 1hcyD:136-393 1hc6B:136-393 1hc4C:136-393 1hc3C:136-393 1hc5C:136-393 1hc2C:136-393

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 biological popularity of use in oxygen transport.[1] 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.

Contents

Explanation

Although the respiratory function of hemocyanin is similar to that of hemoglobin, there are a significant 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 coordinated by histidine residues. Species using hemocyanin for oxygen transportation are commonly crustaceans living in cold environments with low oxygen pressure. Under these circumstances hemoglobin oxygen transportation is less efficient than hemocyanin oxygen transportation.

Most hemocyanins bind with oxygen non-cooperatively and are roughly one-fourth as efficient as hemoglobin at transporting oxygen per amount of blood. Hemoglobin binds oxygen cooperatively due to steric conformation changes in the protein complex, which increases hemoglobin's affinity for oxygen when partially oxygenated. In some hemocyanins of horseshoe crabs and some other species of arthropods, cooperative binding is observed, with Hill coefficients of 1.6 - 3.0. Hill coefficients vary depending on species and laboratory measurement settings. Hemoglobin, for comparison, has a Hill coefficient of usually 2.8 - 3.0. In these cases of cooperative binding hemocyanin was arranged in protein sub-complexes of 6 subunits (hexamer) each with one oxygen binding site; binding of oxygen on one unit in the complex would increase the affinity of the neighboring units. Each hexamer complex was arranged together to form a larger complex of dozens of hexamers. In one study, cooperative binding was found to be dependent on hexamers being arranged together in the larger complex, suggesting cooperative binding between hexamers. Hemocyanin oxygen-binding profile is also affected by dissolve-salt ion levels and pH.

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