Electron microscope structural study of modified fibrin and a related modified fibrinogen aggregate.

The structure of proteolytically modified fibrin and a closely related modified fibrinogen aggregate have been studied by analysis of electron microscope images. For both structures, we propose a model that consists of double-stranded, 2-fold helical protofibrils, which are associated laterally to form ordered fibrils, with a C222 space group: a = 44.0 nm, b = c = 9.4 nm. Each fibril is 80 nm or less in diameter, and twists along its length in a right-handed sense, with a pitch from 7 to 12 times the molecular length. The fibrils associate laterally to form bundles, which tend to twist in a left-handed sense, with a pitch of the order of 40 times the molecular length. The specific volume of modified fibrin calculated from this model is 3.9 A3 per dalton, which is comparable to the specific volume of 3.6 A3 per dalton for modified fibrinogen crystals but is lower than the 6 A3 per dalton determined for fibrin from light-scattering experiments. Comparison of our electron microscope results with X-ray and neutron diffraction data suggest a similar, but less well-ordered, structure for native fibrin, with a smaller fibril, approximately 18.4 nm wide, consisting of eight protofibrils.