The molecular architecture of the extracellular hemoglobin of Ophelia bicornis: Analysis of two-dimensional crystalline arrays

Abstract The double-layered hexagonal disks of the extracellular hemoglobin of the annelid worm Ophelia bicornis form two types of two-dimensional crystalline arrays. The hexagonal type exhibited a typical honeycomb pattern of top views with a center-to-center distance of 26.2 nm. Laterally oriented molecules formed rectangular crystals with lattice constants a = 26.7 run and b = 19.8 nm. The three-dimensional structure was determined from both crystal forms by reconstruction from images of tilt series. At the resolutions obtained, 1.8 nm for the hexagonal form and 2.5 nm for the rectangular form, flattening of the hemoglobin molecules against the support was observed. Nevertheless the two independent reconstructions provided information about the mass distribution within the main subunit and the connectivity between different parts of the molecule.