Hemocyanins (also spelled haemocyanins and abbreviated Hc) 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 confined in 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.[1]
Phenoloxidase are copper containing tyrosinases. These proteins are involved in the process of sclerotization of arthropod cuticle, in wound healing, and humoral immune defense. Phenoloxidase is synthesized by zymogens and are activated by cleaving an N-terminalpeptide.[7]
Hexamerins are storage proteins commonly found in insects. These proteins are synthesized by the larval fat body and are associated with molting cycles or nutritional conditions.[8]
Pseudohemocyanin and cryptocyanins genetic sequences are closely related to hemocyanins in crustaceans. These proteins have a similar structure and function, but lack the copper binding sites.[9]
The evolutionary changes within the phylogeny of the hemocyanin superfamily are closely related to the emergence of these different proteins in various species. The understanding of proteins within this superfamily would not be well understood without the extensive studies of hemocyanin in arthropods.[10]
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. Each hemocyanin monomer holds a pair of copper(I) cations in place via interactions with the imidazole rings of six histidine residues.[11] 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.[12] 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.[13]
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.[14]
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.[15]
The 3.8 MDa structure of molluscan Japanese flying squid 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.[16]PDB: 4YD9
Hexamers are characteristic of arthropod hemocyanins.[17] A hemocyanin of the tarantula Eurypelma californicum[3] is made up of 4 hexamers or 24 peptide chains. A hemocyanin from the house centipede Scutigera coleoptrata[18] 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.[17] 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.[citation needed]
Catalytic activity
A hemocyanin active site in the absence of O2 (each Cu center is a cation, charges not shown).O2-bound form of a hemocyanin active site (the Cu2 center is a dication, charge not shown).
Hemocyanin is homologous to the phenol oxidases (e.g. tyrosinase) since both proteins have histidine residues, called "type 3" copper-binding coordination centers, as do the enzymes tyrosinase and catechol oxidase.[19] In both cases inactive precursors to the enzymes (also called zymogens or proenzymes) must be activated first. This is done by removing the amino acid that blocks the entrance channel to the active site when the proenzyme is not active. There is currently no other known modifications necessary to activate the proenzyme and enable catalytic activity. Conformational differences determine the type of catalytic activity that the hemocyanin is able to perform.[20] 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.[1][19]
Spectral properties
The underside of the carapace of a red rock crab (Cancer productus). The purple coloring is caused by hemocyanin.
Spectroscopy of oxyhemocyanin shows several salient features:[21]
Resonance Raman spectroscopy shows that O 2 is bound in a symmetric environment (ν(O-O) is not IR-allowed).
OxyHc is EPR-silent indicating the absence of unpaired electrons
Much work has been devoted to preparing synthetic analogues of the active site of hemocyanin.[21] One such model, which features a pair of copper centers bridged side-on by peroxo ligand, shows ν(O-O) at 741 cm−1 and a UV-Vis spectrum with absorbances at 349 and 551 nm. Both of these measurements agree with the experimental observations for oxyHc.[22] The Cu-Cu separation in the model complex is 3.56 Å, that of oxyhemocyanin is ca. 3.6 Å (deoxyHc: ca. 4.6 Å).[22][23][24]
Anticancer effects
The hemocyanin found in the blood of the Chilean abalone, Concholepas concholepas, has immunotherapeutic effects against bladder cancer in murine models. Mice primed with C. concholepas before implantation of bladder tumor (MBT-2) cells. Mice treated with C. concholepas hemocyanin 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.[25]
Keyhole limpet hemocyanin (KLH) is an immune stimulant derived from circulating glycoproteins of the marine mollusk Megathura crenulata. KLH has been shown to be a significant treatment against the proliferations of breast cancer, pancreas cancer, and prostate cancer cells when delivered in vitro. Keyhole limpet hemocyanin inhibits growth of human Barrett's esophageal cancer through both apoptic and nonapoptic mechanisms of cell death.[26]
Case studies: environmental impact on hemocyanin levels
A 2003 study of the effect of culture conditions of blood metabolites and hemocyanin of the white shrimp Litopenaeus vannamei found that the levels of hemocyanin, oxyhemocyanin in particular, are affected by the diet. The study compared oxyhemocyanin levels in the blood of white shrimp housed in an indoor pond with a commercial diet with that of white shrimp housed in an outdoor pond with a more readily available protein source (natural live food) as well. Oxyhemocyanin and blood glucose levels were higher in shrimp housed in outdoor ponds. It was also found that blood metabolite levels tended to be lower in low activity level species, such as crabs, lobsters, and the indoor shrimp when compared to the outdoor shrimp. This correlation is possibly indicative of the morphological and physiological evolution of crustaceans. The levels of these blood proteins and metabolites appear to be dependent on energetic demands and availability of those energy sources.[27]
^Burmester T (February 2002). "Origin and evolution of arthropod hemocyanins and related proteins". Journal of Comparative Physiology B: Biochemical, Systemic, and Environmental Physiology. 172 (2): 95–107. doi:10.1007/s00360-001-0247-7. PMID11916114. S2CID26023927.
^Rannulu NS, Rodgers MT (March 2005). "Solvation of copper ions by imidazole: structures and sequential binding energies of Cu+(imidazole)x, x = 1-4. Competition between ion solvation and hydrogen bonding". Physical Chemistry Chemical Physics. 7 (5): 1014–1025. Bibcode:2005PCCP....7.1014R. doi:10.1039/b418141g. PMID19791394.
^ abDecker H, Tuczek F (August 2000). "Tyrosinase/catecholoxidase activity of hemocyanins: structural basis and molecular mechanism". Trends in Biochemical Sciences. 25 (8): 392–397. doi:10.1016/S0968-0004(00)01602-9. PMID10916160.
^Decker H, Schweikardt T, Nillius D, Salzbrunn U, Jaenicke E, Tuczek F (August 2007). "Similar enzyme activation and catalysis in hemocyanins and tyrosinases". Gene. 398 (1–2): 183–191. doi:10.1016/j.gene.2007.02.051. PMID17566671.
^ abKitajima N, Fujisawa K, Fujimoto C, Morooka Y, Hashimoto S, Kitagawa T, et al. (1992). "A new model for dioxygen binding in hemocyanin. Synthesis, characterization, and molecular structure of the μ-η2:η2 peroxo dinuclear copper(II) complexes, [Cu(BH(3,5-R2pz)3)]2(O2) (R = i-Pr and Ph)". Journal of the American Chemical Society. 114 (4): 1277–91. doi:10.1021/ja00030a025.
^Gaykema WP, Hol WG, Vereijken JM, Soeter NM, Bak HJ, Beintema JJ (1984). "3.2 Å structure of the copper-containing, oxygen-carrying protein Panulirus interruptus haemocyanin". Nature. 309 (5963): 23–9. Bibcode:1984Natur.309...23G. doi:10.1038/309023a0. S2CID4260701.
^Kodera M, Katayama K, Tachi Y, Kano K, Hirota S, Fujinami S, et al. (1999). "Crystal Structure and Reversible O2-Binding of a Room Temperature Stable μ-η2:η2-Peroxodicopper(II) Complex of a Sterically Hindered Hexapyridine Dinucleating Ligand". Journal of the American Chemical Society. 121 (47): 11006–7. doi:10.1021/ja992295q.
^Atala A (2006). "This Month in Investigative Urology". The Journal of Urology. 176 (6): 2335–6. doi:10.1016/j.juro.2006.09.002.
^McFadden DW, Riggs DR, Jackson BJ, Vona-Davis L (November 2003). "Keyhole limpet hemocyanin, a novel immune stimulant with promising anticancer activity in Barrett's esophageal adenocarcinoma". American Journal of Surgery. 186 (5): 552–555. doi:10.1016/j.amjsurg.2003.08.002. PMID14599624.
^Pascual C, Gaxiola G, Rosas C (2003). "Blood metabolites and hemocyanin of the white shrimp, Litopenaeus vannamei: The effect of culture conditions and a comparison with other crustacean species". Marine Biology. 142 (4): 735–745. doi:10.1007/s00227-002-0995-2. S2CID82961592.
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