Hemerythrin & Hemocyanin
This page shows some interactive JSmol views of hemerythrin and hemocyanin. Explore the structures for yourself:
The inital view for hemerythrin shows an assembly of 8 subunits (2 each of 4 independent subunits), each contains one
dinuclear iron site.
The second view shows a single subunit, composed of 4 α-helices.
This basic structure is typical of the class of "non-heme iron proteins".
Note that ligands for the iron (green) come from each of the helices.
The third view shows the active site. Both iron ions are 6-coordinate, bridged by an OH- ion and by two carboxylates (one Glu and one Asp).
One of the iron also has 3 His ligands, while the second has 2 His ligands and carries the dioxygen molecule.
The dioxygen is bound end-on and the binding is stabilised by
a hydrogen bond (2.64 Å) to the bridging OH-. In fact, it appears the proton is transferred to the bound oxygen,
generating hydroperoxide (HOO-) H-bonded to a bridging oxo ion.
The deoxyhemerythrin active site is very similar. As might be expected, one iron is effectively 5-coordinate, leaving a vacant site
for dioxygen to bind. There is a water molecule close to this iron - but actually closer to (and hydrogen bonded to) the
OH- bridge.
The first hemocyanin figure shows one subunit, which is considerably larger then the hemerythrin equivalent.
The subunit contains one dinuclear copper active site.
The copper active site views show that the copper ions (green) are coordinated to three His residues and bridgen by the dioxygen.
The dioxygen binds in the unusual μ-η2:η2 geometry (O-O 1.41 Å).
Overall the geometry at the copper ions approaches square pyramidal.
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Hemocyanin data from: Crystallographic analysis of oxygenated and deoxygenated states of arthropod hemocyanin shows unusual differences. Magnus, K.A., Hazes, B., Ton-That, H., Bonaventura, C., Bonaventura, J., Hol, W.G. (1994) Proteins 19: 302-309. (PDB code 1OXY).
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