Hemocyanin

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Hemocyanin, copper containing domain
Hemocyanin2.jpg
Single Oxygenated Functional Unit from the hemocyanin of an octopus
Identifiers
Symbol Hemocyanin_M
Pfam PF00372
InterPro IPR000896
PROSITE PDOC00184
SCOP 1lla
SUPERFAMILY 1lla
Hemocyanin, all-alpha domain
PDB 1hcy EBI.jpg
Crystal structure of hexameric haemocyanin from Panulirus interruptus refined at 3.2 angstroms resolution
Identifiers
Symbol Hemocyanin_N
Pfam PF03722
InterPro IPR005204
PROSITE PDOC00184
SCOP 1lla
SUPERFAMILY 1lla
Hemocyanin, ig-like domain
PDB 1oxy EBI.jpg
crystallographic analysis of oxygenated and deoxygenated states of arthropod hemocyanin shows unusual differences
Identifiers
Symbol Hemocyanin_C
Pfam PF03723
InterPro IPR005203
PROSITE PDOC00184
SCOP 1lla
SUPERFAMILY 1lla

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.

Species distribution[edit]

Hemocyanins are found only in the Mollusca and Arthropoda: the earliest hemocyanins were found in the snail Helix pomatia (a mollusc) and in the horseshoe crab (an arthropod). They were subsequently found to be common among crustaceans and are utilized by some land arthropods such as the tarantula Eurypelma californicum,[1] the emperor scorpion,[2] and the centipede Scutigera coleoptrata. Also, larval storage proteins in many insects appear to be derived from hemocyanins.

Structure and mechanism[edit]

The underside of the carapace of a Cancer productus crab. The purple coloring is caused by hemocyanin.

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.[3] 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.[4]

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.[5]

Hexamers are characteristic of arthropod hemocyanins.[6] A hemocyanin of the tarantula Eurypelma californicum[1] is made up of 4 hexamers or 24 pepide chains. A hemocyanin from the house centipede Scutigera coleoptrata[7] 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.[8] 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[edit]

Hemocyanin is homologous to the phenol oxidases (e.g. tyrosinase) since both enzymes share 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.[9]

Spectral properties[edit]

Oxygen binding mode with respect to copper centers

Spectroscopy of oxyhemocyanin shows several salient features:[citation needed]

  1. Resonance Raman spectroscopy shows symmetric binding
  2. UV-Vis spectroscopy shows strong absorbances at 350 and 580 nm
  3. OxyHc is EPR-silent indicating the absence of unpaired electrons
  4. 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.[10] (4) shows a considerably weaker O-O bond compared with Karlin's trans-peroxo model.[10]

On the other hand, Nobumasa Kitajima's model shows ν(O-O) of 741 cm−1 and UV-Vis absorbances at 349 and 551 nm, which agree with the experimental observations for oxyHc.[11]

Antitumor effects[edit]

The hemocyanin found in Concholepas concholepas blood has immunotherapeutic effects against bladder cancer in murine models. Researchers in 2006 primed mice 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.[12]

See also[edit]


References[edit]

  1. ^ a b Voit R, Feldmaier-Fuchs G, Schweikardt T, Decker H, Burmester T (December 2000). "Complete sequence of the 24-mer hemocyanin of the tarantula Eurypelma californicum. Structure and intramolecular evolution of the subunits". J. Biol. Chem. 275 (50): 39339–44. doi:10.1074/jbc.M005442200. PMID 10961996. 
  2. ^ Jaenicke E, Pairet B, Hartmann H, Decker H (2012). "Crystallization and preliminary analysis of crystals of the 24-meric hemocyanin of the emperor scorpion (Pandinus imperator)". PLoS ONE 7 (3): e32548. doi:10.1371/journal.pone.0032548. PMC 3293826. PMID 22403673. Lay summaryJohannes Gutenberg-Universität Mainz. 
  3. ^ Strobel A, Hu MY, Gutowska MA, Lieb B, Lucassen M, Melzner F, Pörtner HO, Mark FC (December 2012). "Influence of temperature, hypercapnia, and development on the relative expression of different hemocyanin isoforms in the common cuttlefish Sepia officinalis". J Exp Zool a Ecol Genet Physiol 317 (8): 511–23. doi:10.1002/jez.1743. PMID 22791630. 
  4. ^ Perton FG, Beintema JJ, Decker H (May 1997). "Influence of antibody binding on oxygen binding behavior of Panulirus interruptus hemocyanin". FEBS Lett. 408 (2): 124–6. doi:10.1016/S0014-5793(97)00269-X. PMID 9187351. 
  5. ^ Waxman L (May 1975). "The structure of arthropod and mollusc hemocyanins". J. Biol. Chem. 250 (10): 3796–806. PMID 1126935. 
  6. ^ (van Holde & Miller 1995, p. 9)
  7. ^ Kusche K, Hembach A, Hagner-Holler S, Gebauer W, Burmester T (July 2003). "Complete subunit sequences, structure and evolution of the 6 x 6-mer hemocyanin from the common house centipede, Scutigera coleoptrata". Eur. J. Biochem. 270 (13): 2860–8. doi:10.1046/j.1432-1033.2003.03664.x. PMID 12823556. 
  8. ^ (van Holde & Miller 1995, p. 8)
  9. ^ Decker H, Tuczek F (August 2000). "Tyrosinase/catecholoxidase activity of hemocyanins: structural basis and molecular mechanism". Trends Biochem. Sci. 25 (8): 392–7. doi:10.1016/S0968-0004(00)01602-9. PMID 10916160. 
  10. ^ a b Karlin KD, Cruse RW, Gultneh Y, Farooq A, Hayes JC, and Zubieta J (1987). "Dioxygen-copper reactivity. Reversible binding of O2 and CO to a phenoxo-bridged dicopper(I) complex". J. Am. Chem. Soc. 109 (9): 2668–2679. doi:10.1021/ja00243a019. 
  11. ^ Kitajima N, Fujisawa K, Fujimoto C, Morooka Y, Hashimoto S, Kitagawa T, Toriumi K, Tatsumi K, Nakamura A (1992). "A new model for dioxygen binding in hemocyanin. Synthesis, characterization, and molecular structure of the μ-η22 peroxo dinuclear copper(II) complexes, [Cu(HB(3,5-R2pz)3)]2(O2) (R = isopropyl and Ph)". J. Am. Chem. Soc. 114 (4): 1277–1291. doi:10.1021/ja00030a025. 
  12. ^ Atala A (December 2006). "This Month in Investigative Urology". The Journal of Urology 176 (6): 2335–2336. doi:10.1016/j.juro.2006.09.002. 

Further reading[edit]

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