Evidence for a central substructure in a Lumbricus terrestris hemoglobin obtained with STEM low-dose and digital processing techniques.

Applications of scanning transmission electron microscope (STEM) low-dose and digital image processing techniques to the annelid extracellular hemoglobin of Lumbricus terrestris have shown the existence of a central substructure. The techniques involve low-dose STEM images of less than 2 e/A2 irradiation. Many such low-dose images were then aligned in their symmetry, N-fold averaged, and summed. The central substructure appears to be supported by spokes which are connected to a surrounding hexagonal bilayer of 12 subunits. In comparison to the summed image of Nephtys hombergii hemoglobin which is known to possess a central substructure, the morphology of both central mass and spokes were very similar. This finding of a central substructure is consistent with the report of a small angle X-ray scattering study (Pilz et al., 1980, Int. J. Biol. Macromol. 2, 279-283) which predicted the possible existence of substructure in the center of the molecule.