Molecular symmetry of the dodecamer subunit of Lumbricus terrestris hemoglobin.

Abstract The principal functional subunit of the ∼3500 kDa extracellular Lumbricus terrestris hemoglobin is a 213 kDa dodecamer of four chemically distinct globin chains, consisting of a non-covalent complex of three trimer submits (disulfide-bonded chains a, b and c ) and three monomer subunits (chain d ). X-ray diffraction of crystals of the dodecamer grown at neutral pH, were found to be monoclinic, with the unit cell dimensions: a =112.3 A, b =190.0 A, c =69.6 A, β=102.0° with h + k + l =2 n + 1 absent, character istic of space group I 121. In addition, these crystals exhibit a pseudo trigonal P 321 symmetry with unit cell dimensions a =190.5 A, b =190.5 A, c =69.5 A, γ=120.0°. Assuming that the assymetric unit contains an entire dodecamer, a model of the latter was constructed that satisfies the symmetry of the trigonal pseudo cell and is consistent with the symmetry of the I 121 crystallographic cell. The resulting model has strong implications concerning the hexagonal bilayer structure of the native hemoglobin.