3-DIMENSIONAL RECONSTRUCTION OF MACROBDELLA-DECORA (LEECH) HEMOGLOBINBY CRYOELECTRON MICROSCOPY

Macrobdella decora hemoglobin was observed in vitreous ice by cryoelec tron microscopy and subjected to three-dimensional reconstruction by t he method of random conical tilt series, The refined volume has a reso lution of 40 Angstrom and a D-6 point-group symmetry, Its architecture , with its hexagonal bilayer appearance, resembles those of Lumbricus terrestris (oligochaete) and Eudistylia vancouverii (polychaete). When the reconstruction volume is viewed along its sixfold axis, the verti ces of the upper hexagonal layer are rotated 16 degrees clockwise comp ared to those of the lower layer. In agreement with the ''bracelet'' m odel of Vinogradov et al., a central linker complex is decorated by 12 hollow globular substructures. The linker complex is made up of a cen tral hexagonal toroid linked by 12 c5 connections to two bracelets of c3 connections, which are themselves linked via six c4 connections. Th e portion of the hollow globular substructure corresponding to the dod ecamer of globin chains has a local pseudo threefold symmetry and is c omposed of three elongated structures Visible when the volume is displ ayed at high threshold. The main difference between Macrobdella, Lumbr icus, and Eudistylia hemoglobins is the presence in Macrobdella of a c entral hexagonal toroid instead of a compact flat hexagonal structure.