Three-dimensional reconstruction of native and reassembled Lumbricus terrestris extracellular hemoglobin. Localization of the monomeric globin chains.

The approximately 3.5 MDa hexagonal bilayer (HBL) hemoglobin (Hb) of the earthworm Lumbricus terrestris is composed of monomers and disulfide-bonded trimers (T) of globin chains and of four types of heme-deficient linker chains (L). Cryoelectron microscopic images of native Hb and of HBL reassembled from the constituent subunits depleted in monomer subunit (HBL[T+L]) were subjected to three-dimensional reconstructions by the random conical tilt series method. Native Hb has an architecture very similar to those of other annelid and vestimentiferan Hbs, consisting of 12 hollow globular substructures (HGS). Each HGS is comprised of six dense masses, has a 3-fold symmetry, and is organized in two hexagonally symmetric layers, with the vertices of the upper layer rotated 16 degrees clockwise relative to those of the lower layer. The layers are tethered to a central linker complex, consisting of two bracelets of connections perpendicular to the 6-fold axis and a set of six vertical connections linked to a flat hexagonal mass. HBL[T+L] shared all these features with the native Hb, except for a large hole around the 3-fold symmetry axis in each HGS, indicating the probable location of the missing monomer subunit.