Three-Dimensional Reconstruction of Sepia officinalis Hemocyanin from Frozen-Hydrated Specimens

The three-dimensional reconstruction of Sepia hemocyanin from randomly oriented native molecules was carried out by the method of the random conical tilt series on a frozen-hydrated specimen. As other molluscan hemocyanins, the molecule resembles a hollow cylinder or pentahedron composed of five dimeric subunits. Each dimeric subunit, composed of 16 functional units, comprises one wall oblique unit made up of 12 functional units and one arch made up of four functional units. The five wall oblique units are separated from each other by five oblique clefts bridged by the five arches, formerly termed collar structures. Each arch is composed of two types of functional units that are probably Soe, a functional unit absent in Octopus hemocyanin, and Soh, the C-terminal functional unit of the polypeptide chain. The architecture of the arches and their intramolecular location in front of the edges of the pentahedron are strongly reminiscent of the arches of Octopus hemocyanin. The D5 point-group symmetry of the molecule suggests that the orientation of the polypeptide chains is antiparallel as in Octopus hemocyanin. Several models of architecture compatible with these results are designed.