Hemocyanins are large multi-subunit copper proteins that transport oxygen in many arthropods and molluscs. The amino acid sequence of subunit a of Panulirus interruptus hemocyanin (657 residues) has been completed and fitted to the electron-density map (3.2 Å resolution). Comparison of amino acid sequence data for several different hemocyanin subunits of arthropod species indicated that the general features of the polypeptide architecture as found in spiny lobster hemocyanin occur in all arthropods. This structure must therefore be at least as old as the estimated time of divergence of crustaceans and chelicerates, 540–600 million years ago.
