Molecular symmetry and arrangement of subunits in extracellular hemoglobin from the nematode Ascaris suum

The arrangement of subunits and molecular symmetry of extracellular hemoglobin from the nematode Ascaris suum, an 11.7S molecule mass 332 kDa and composed of eight identical subunits, was studied. Dissociation of the molecule at alkaline and acid pH yielded 4.6S and 2.7S components, identified as polypeptide‐chain dimers and monomers, respectively. Cross‐linking with glutardialdehyde followed by SDS/PAGE resulted in a maximum number of eight bands identified in order of decreasing mobility as monomeric and 2–8 cross‐linked‐polypeptide‐chain species. Comparison with values predicted from theory shows that the distribution of protein among the various cross‐linked species, obtained after different extents of exposure to cross‐linker, is consistent with a two‐layered arrangement of subunits involving one type of interaction between subunits from different layers and another between subunits within the same layer. Electron micrographs of the molecule showed two profiles, a square and a rectangle. We propose a model for the molecule which is eight subunits arranged in two layers, stacked in an eclipsed orientation. The proposed model is consistent with the results from sedimentation, cross‐linking and electron microscopy. Taken together, our findings indicate D₄ symmetry for Ascaris hemoglobin.