Structural study of modified fibrinogen microcrystals by electron microscopy

The same molecular shape of proteolytically modified fibrinogen has been identified in two different crystalline forms: orthorhombic P2₁2₁2 with a = 44.7(± 1.7) nm, b = 11.8(± 1.0) nm, c = 3.8 nm and monoclinic P2₁ with a = 17.7(± 0.6) nm, b = 16.2(± 1.0) nm, c = 4.8 nm and β = 95 °. The shape of the molecule is more detailed than has been reported. It has the commonly accepted elongated form 44.5(± 1.5) nm long with a 2-fold axis perpendicular to the major axis of the molecule. Each end domain shows a distinctive substructure and has an asymmetry similar to that reported by Williams (1981) and Mosesson et al. (1981) from single molecule observations. The ends are flattened, having overall dimensions of approximately 12 nm × 9 nm × 4 nm. The major difference between this model and previous models is the absence of a central nodule. There are, however, two small additional protein-dense regions at 5.0 nm from the centre of the rod connecting the two ends. A rough estimate of the relative heights of the different parts of the molecule was obtained using the two different projections of the molecule obtained from the two different crystalline forms. The molecule appears to be slightly bent at the centre, as predicted by Doolittle (1977).