|Hemocyanin, copper containing domain|
Single Oxygenated Functional Unit from the hemocyanin of an octopus
|Hemocyanin, all-alpha domain|
Crystal structure of hexameric haemocyanin from Panulirus interruptus refined at 3.2 angstroms resolution
|Hemocyanin, ig-like domain|
crystallographic analysis of oxygenated and deoxygenated states of arthropod hemocyanin shows unusual differences
Hemocyanins (also spelled haemocyanins) are proteins that transport oxygen throughout the bodies of some invertebrate animals. These metalloproteins contain two copper atoms that reversibly bind a single oxygen molecule (O2). 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. Oxygenation causes a color change between the colorless Cu(I) deoxygenated form and the blue Cu(II) oxygenated form.
Hemocyanins are found in two animal phyla, Mollusca and Arthropoda, but hemocyanins from the two phyla are rather different. In both types, however, the copper sites are similar. Hemocyanins are quite widespread among molluscs. A hemocyanin was first discovered in 1927 by Svedberg from the snail Helix pomatia. Among arthropods, a hemocyanin was early discovered in the horseshoe crab, Limulus polyphemus. Hemocyanins are well known among crustaceans such as lobsters and crabs. More recently they have been found among land arthropods such as the tarantula Eurypelma californicum, the scorpion Pandinus imperator, and the centipede Scutigera coleoptrata. Arthropod hemocyanins must have originated early in the evolutionary history of this phylum as they have been found from the class Onychophora. Hemocyanins seem to be rare among insects but are not completely absent. Larval storage proteins in many insects appear to be derived from hemocyanins.
Structure and mechanism
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. It has been noted that species using hemocyanin for oxygen transportation include crustaceans living in cold environments with low oxygen pressure. Under these circumstances hemoglobin oxygen transportation is less efficient than hemocyanin oxygen transportation. Nevertheless there are also terrestrial arthropods using hemocyanin, notably spiders and scorpions, that live in warm climates.
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 dissolved salt ion levels and pH.
Hemocyanin is made of many 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 may be arranged in dimers or hexamers depending on species; the dimer or hexamer complex is likewise arranged in chains or clusters with weights exceeding 1500 kDa. The subunits are usually homogeneous, or heterogeneous with two variant subunit types. Because of the large size of hemocyanin, it is usually found free-floating in the blood, unlike hemoglobin.
Hexamers are characteristic of arthropod hemocyanins. A hemocyanin of the tarantula Eurypelma californicum is made up of 4 hexamers or 24 pepide chains. A hemocyanin from the house centipede Scutigera coleoptrata is made up of 6 hexamers or 36 chains. Horseshoe crabs have an 8-hexamer (i. e. 48-chain) hemocyanin. Simple hexamers are found in the spiny lobster Panulirus interruptus and the isopod Bathynomus giganteus. Peptide chains in crustaceans are about 660 amino acid residues long, and in chelicerates they are about 625. In the large complexes there is a variety of variant chains, all about the same length; pure components do not usually self-assemble.
Hemocyanin is homologous to the phenol oxidases (e.g. tyrosinase) since both enzymes sharing type 3 Cu active site coordination. Hemocyanin also exhibits phenol oxidase activity, but with slowed kinetics from greater steric bulk at the active site. Partial denaturation actually improves hemocyanin’s phenol oxidase activity by providing greater access to the active site.
Spectroscopy of oxyhemocyanin shows several salient features:
- Resonance Raman spectroscopy shows symmetric binding
- UV-Vis spectroscopy shows strong absorbances at 350 and 580 nm
- OxyHc is EPR-silent indicating the absence of unpaired electrons
- Infrared spectroscopy shows ν(O-O) of 755 cm-1
(1) rules out a mononuclear peroxo complex (2) does not match with the UV-Vis spectra of mononuclear peroxo and Kenneth Karlin's trans-peroxo models. (4) shows a considerably weaker O-O bond compared with Karlin's trans-peroxo model.
The hemocyanin found in Concholepas concholepas blood has immunotherapeutic effects against bladder in murine models. Researchers in 2006 primed mice with C. concholepas before implantation of bladder tumor (MBT-2) cells. Mice treated with C. concholepas showed antitumor effects: prolonged survival, decreased tumor growth and incidence, and lack of toxic effects and may have a potential use in future immunotherapy for superficial bladder cancer.
|Wikimedia Commons has media related to Hemocyanin.|
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