Hemocyanin



 

Hemocyanins (also spelled haemocyanins) are respiratory 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 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 pH.

Hemocyanin is made of many individual subunit proteins, each of which contains two 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 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, 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

  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.[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. ^ a b 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.
 
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