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Information about Haemocyanin

Enlarge picture
Single Oxygenated Functional Unit from the hemocyanin of an octopus.


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 blood of most molluscs, and some arthropods such as the horseshoe crab. They are second only to hemoglobin in biological popularity of use in oxygen transport.

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 between 1.6-3. Hill constants vary depending on species and laboratory measurement settings. Hemoglobin for comparison has a Hill coefficient of usually 2.8-3. 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.

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 in weights exceeding 1500 kDa. The subunits are usually , 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, which must be contained in cells because its small size would lead it to clog and damage blood-filtering organs such as the kidneys. This free-floating nature can allow for increased hemocyanin density over hemoglobin and increased oxygen carrying capacity. On the other hand, free-floating hemocyanin can increase viscosity and increase the energy expenditure needed to pump blood.

Structure

Enlarge picture
Oxygen binding mode with respect to copper centers
Spectroscopy of oxyhemocyanin shows several salient features:
  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.[1] (4) shows a considerably weaker O-O bond compared with Karlin's trans-peroxo model.[1]

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.[2]

The weak O-O bond of oxyhemocyanin is because of metal-ligand backdonation into the σ* orbitals. The donation of electrons into the O-O antibonding orbitals weakens the O-O bond, giving a lower than expected infrared stretching frequency.

See also

References

1. ^ K. D. Karlin, R. W. Cruse, Y. Gultneh, A. Farooq, J. C. Hayes and J. Zubieta (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. 
2. ^ N. Kitajima, K. Fujisawa, C. Fujimoto, Y. Morooka, S. Hashimoto, T. Kitagawa, K. Toriumi, K. Tatsumi and A. Nakamura (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(HB(3,5-R2pz)3)]2(O2) (R = isopropyl and Ph)". J. Am. Chem. Soc. 114 (4): 1277-1291. DOI:10.1021/ja00030a025. 
Proteins are large organic compounds made of amino acids arranged in a linear chain and joined together by peptide bonds between the carboxyl and amino groups of adjacent amino acid residues.
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In biochemistry, a metalloprotein is a generic term for a protein that contains a metal cofactor. The metal may be an isolated ion or may be coordinated with a nonprotein organic compound, such as the porphyrin found in hemoproteins.
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2, 1
(mildly basic oxide)
Electronegativity 1.90 (Pauling scale)
Ionization energies
(more) 1st: 745.5 kJmol−1
2nd: 1957.9 kJmol−1
3rd: 3666 kJmol−1

Atomic radius 135 pm
Atomic radius (calc.
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2, −1
(neutral oxide)
Electronegativity 3.44 (Pauling scale)
Ionization energies
(more) 1st: 1313.9 kJmol−1
2nd: 3388.3 kJmol−1
3rd: 5300.5 kJmol−1

Atomic radius 60 pm
Atomic radius (calc.
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Color or colour[1] (see spelling differences) is the visual perceptual property corresponding in humans to the categories called red, yellow, blue, black, etc.
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The term blue may refer to any of a number of similar colours. The sensation of blue is made by light having a spectrum dominated by energy in the wavelength range of about 440–490 nm.
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Mollusca
Linnaeus, 1758

Classes

Caudofoveata
Aplacophora
Polyplacophora
Monoplacophora
Bivalvia
Scaphopoda
Gastropoda
Cephalopoda
† Rostroconchia
† Helcionelloida
† ?Bellerophontida
The molluscs
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Arthropoda
Latreille, 1829

Subphyla and Classes
  • Subphylum Trilobitomorpha
  • Trilobita - trilobites (extinct)
  • Subphylum Chelicerata

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Limulus

Species: L. polyphemus

Binomial name
Limulus polyphemus
Linnaeus, 1758

The horseshoe crab, horsefoot, king crab
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Hemoglobin, also spelled haemoglobin and abbreviated Hb, is the iron-containing oxygen-transport metalloprotein in the red blood cells of the blood in vertebrates and other animals.
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3, 4, 6
(amphoteric oxide)
Electronegativity 1.83 (Pauling scale)
Ionization energies
(more) 1st: 762.5 kJmol−1
2nd: 1561.9 kJmol−1
3rd: 2957 kJmol−1

Atomic radius 140 pm
Atomic radius (calc.
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porphyrin is a heterocyclic macrocycle derived from four pyrrole-like subunits interconnected via their α carbon atoms via methine bridges (=CH-). The macrocycle, therefore, is a highly conjugated system, and is consequently deeply coloured—the name porphyrin
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A heme or haem is a prosthetic group that consists of an iron atom contained in the center of a large heterocyclic organic ring called a porphyrin. Not all porphyrins contain iron, but a substantial fraction of porphyrin-containing metalloproteins have heme as
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2, 1
(mildly basic oxide)
Electronegativity 1.90 (Pauling scale)
Ionization energies
(more) 1st: 745.5 kJmol−1
2nd: 1957.9 kJmol−1
3rd: 3666 kJmol−1

Atomic radius 135 pm
Atomic radius (calc.
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A prosthetic group is a non-protein (non-amino acid) component of a conjugated protein. The prosthetic group may be organic (such as a vitamin, sugar, or lipid) or inorganic (such as a metal ion). Prosthetic groups usually bond covalently to their protein.
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Histidine (abbreviated as His or H)[1] is one of the 20 most common natural amino acids present in proteins. In the nutritional sense, in humans, histidine is considered an essential amino acid, but mostly only in children.
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crustaceans (Crustacea) are a large group of arthropods, comprising approximately 52,000 described species [1], and are usually treated as a subphylum [2].
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Protein folding is the physical process by which a polypeptide folds into its characteristic three-dimensional structure.[1] Each protein begins as a polypeptide, translated from a sequence of mRNA as a linear chain of amino acids.
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protein complex is a group of two or more associated proteins formed by protein-protein interaction that is stable over time. Protein complexes are a form of quaternary structure.
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Limulus

Species: L. polyphemus

Binomial name
Limulus polyphemus
Linnaeus, 1758

The horseshoe crab, horsefoot, king crab
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Arthropoda
Latreille, 1829

Subphyla and Classes
  • Subphylum Trilobitomorpha
  • Trilobita - trilobites (extinct)
  • Subphylum Chelicerata

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The Hill equation is an equation used in enzyme characterization, which should not be confused with the Hill differential equation that is also sometimes referred to as simply the Hill equation.
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In biochemistry, a macromolecule exhibits cooperative binding if its affinity for its ligand changes with the amount of ligand already bound.

Cooperative binding is a special case of allostery.
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    pH is a measure of the acidity or alkalinity of a solution. Aqueous solutions at 25 ℃ with a pH less than seven are considered acidic, while those with a pH greater than seven are considered basic (alkaline). The pH of 7.
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    2, 1
    (mildly basic oxide)
    Electronegativity 1.90 (Pauling scale)
    Ionization energies
    (more) 1st: 745.5 kJmol−1
    2nd: 1957.9 kJmol−1
    3rd: 3666 kJmol−1

    Atomic radius 135 pm
    Atomic radius (calc.
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    The unified atomic mass unit (u), or dalton (Da), is a small unit of mass used to express atomic and molecular masses. It is defined to be one twelfth of the mass of an unbound atom of the carbon-12 nuclide, at rest and in its ground state.
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    dimer refers to a molecule composed of two identical subunits or monomers linked together.

    Chemistry

    The molecules in a dimer are connected by covalent bonds or weaker interactions such as hydrogen bonds.
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    In chemistry, an oligomer consists of a finite number of monomer units (ολιγος, or oligos, is Greek for "a few"), in contrast to a polymer which, at least in principle, consists of an unbounded number of monomers.
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    Heterogeneous (American English)) means that something (an object or system) consists of a diverse range of different items. It is the antonym of , which means that an object or system consists of many identical items.
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    The kidneys are organs that filter wastes (such as urea) from the blood and excrete them, along with water, as urine. The medical field that studies the kidneys and diseases of the kidney is called nephrology[1].
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