QUATERNARY STRUCTURE OF MULTIHEXAMERIC ARTHROPOD HEMOCYANINS

Arthropod hemocyanins are large oligomeric oxygen-transporting protein s with molecular weight ranging from 450 kDa in the spiny lobster (Pan ulirus interruptus) up to more than 3.6 mDa in the horseshoe crab (Lim ulus polyphemus). Hemocyanins from different species consist of one or multiple copies of a hexameric building block (of 450 kDa) and are su fficiently large to be easily visualized in the electron microscope. A rthropod hemocyanins were among the first macromolecules studied by mu ltivariate statistical image analysis techniques. We present an overvi ew of the different characteristic molecular images of various multihe xameric (1 x 6, 2 x 6, 4 x 6, and 8 x 6) assemblies as these occur in electron-microscopical preparations. We also model the different assem blies in three dimensions by merging multiple copies of the X-ray-diff raction electron density of the single hexameric hemocyanin of Panulir us interruptus. By making correct enantiomeric decisions while merging the densities at the various levels of assembly and by fine-tuning th e assembly parameters used, a good match can be obtained between the m icroscopical images and two-dimensional projections calculated from th e three-dimensional (3D) model densities. Knowledge of the quaternary structures of this intricate hierarchical family of oligomers is essen tial for understanding the allosteric interactions associated with the ir strong oxygen-binding cooperativity.