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. Each hemocyanin monomer holds a pair of copper(I) cations in place via interactions with the
imidazole rings of six 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. The molecule is conformationally stable and fully functioning at temperatures up to 90 degrees C. 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. hemocyanin. It is a homodecamer of five dimers arranged into a 31 nm diameter cylinder. Each monomer has a string of eight individual subunits each with a Cu2O2 binding site. Hexamers are characteristic of arthropod hemocyanins. A hemocyanin of the tarantula
Eurypelma californicum 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. == Catalytic activity ==