Structure of Lumbricus terrestris hemoglobin at 30 A resolution determined using angular reconstitution

The three-dimensional (3D) structure of the giant hemoglobin of the common earthworm Lumbricus terrestris was determined from cryomicroscopical images of a vitrified molecular solution. From about 5000 molecular images, the best-fit 3D structure was calculated using recently developed analysis techniques. Multivariate statistical analysis data compression and automatic classification were used to find the characteristic projection images of the oligomer. The angular reconstitution approach was then applied to find the Euler angle orientations of these characteristic views. The Lumbricus hemoglobin molecule has an overall D6 (622) point-group symmetry and is found to have a local threefold symmetry axis within the 1/12th subunit. This additional symmetry appears to predict the presence of 36 abcd globin tetramers in the Lumbricus hemoglobin or a total of 144 heme chains for the whole oligomer. A distinct doughnut-shaped structure is elucidated in the center of the molecule. This central subunit, representing around 10% of the protein volume, may consist of non-heme-containing linker chains found generally in giant annelid hemoglobins.