Hemocyanin structure

I was curious to see whether there were any obvious similarities in the subunit interactions of hemocyanin and hemoglobin. I downloaded 1NOL.pdb of Hazes et al. (1993). They reported it as a homohexamer and so I tried to dock the structure as a compact hexamer. In the crystal the hexamer has a large central cavity, which is unsatisfactory on physical grounds (see the first post of this Blog). Docking of a hexamer was unsatisfactory but a tetramer docked very well.
I have added the tetramer to the Gallery. The implication is that the tetramer has a physiological role.
Like hemoglobin, hemocyanin has an extensive aromatic network linked to the oxygen binding sites. Unlike hemoglobin the network does not appear to link oxygen binding sites on adjacent subunits. It is also unclear how the reported reverse Bohr effect could work.