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Hemocyanin is a complex oxygen carrier protein. It is found in the blue blood
of some molluscs (e.g. octopus) and arthropods (e.g. crab). Unlike hemerythrin, which is
found in cells, hemocyanin is an extracellular protein. It is put into the
blood stream as a large aggregate of monomers held together by calcium or
magnesium ions. The structure of one deoxy-hemocyanin monomer from the giant octopus is
shown on the left. It consists of 413 residues.
2 molecule coordinates to the active site, copper ions are oxidised to copper(II) and the protein changes from colourless to purple. This change of oxidation state is accompanied by a change of geometry (from trigonal pyramidal to distorted tetrahedral) and by a change in ion size.
These significant structural modifications of the active site are passed on to the surrounding alpha helices, in part via the thioether histidine-cysteine bridge. In this way the whole pocket in which the active site is located “cooperates” during the O2 binding.
Before the high-resolution structures of hemocyanin became available, a
lot of its properties and atypical spectroscopic properties (EPR and unusual UV
spectra) have been understood through model compounds.
. This deep purple compound has two copper(II) ions bridged by a side-on bonded
peroxide group. The additional ligand on both Cu centres is hydridotris[(3,5-di-isopropyl)-1-pyrazolyl]borate, [HB(C9H16N2)3]-.
The above dinuclear Cu(II,II) complex is prepared from a colourless
mononuclear copper(I) complex Cu[HB(C9H16N2)3]
in acetone under 1 atm of O2 at –78oC. The structure of
this starting compound is not available yet.
Its structure is shown in the left frame.
If you would like to learn more about hemocyanin models see: