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Christoph Kuehne, Frank Depoix, Jürgen Markl (Institute of Zoology, Johannes Gutenberg University, Mainz, Germany)The blue copper-containing oxygen transporter hemocyanin in the hemolymph of many arthropods is based on ~75 kDa subunits assembled as hexamers (1x6mers) or oligo-hexamers (2x6mers, 4x6mers, 6x6mers, 8x6mers), depending on the animal species. Highly cooperative oxygen binding characteristics make them viable targets for analyzing the underlying chemomechanical processes. The crystal structure of subunits and 1x6mer hemocyanins are available in the databases. From oligo-hexamers, 3D-electron microscopy (3D-EM) has been performed, yielding 3D reconstructions at ca. 10 Å resolution [1,2]. Rigid-body fitting of homology-modeled subunits into these cryoEM structures defined the molecular interfaces between the hexamers which is required for understanding the mechanisms of inter-hexamer force transfer during cooperative oxygen binding [1,2]. New 3D data will be shown from the 8x6mer hemocyanin of a living fossil, the horseshoe crab Limulus polyphemus (Xiphosura).  Martin et al., 2007. J. Mol. Biol. 366: 1332-1350.  Markl et al. 2009. J. Mol. Biol. 392: 362-380.