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Electron microscopy of fibers and discs of hemoglobin S having sixfold symmetry
Abstract:
Aggregated forms of deoxyhemoglobin S were examined with a field emission transmission electron microscope. Images of isolated helical fibers were obtained from sickled cell lysates stained directly on the electron microscope grid. Optical and digital analyses of the electron micrographs showed that the fibers are similar to those characterized by J. T. Finch, M. F. Perutz, J. F. Bertles, and J. Döbler [(1973) Proc. Natl. Acad. Sci. USA 70, 718-722] in that they consist of stacked discs each composed of six hemoglobin molecules. The fibers exhibit an outer diameter of 160-170 A and an inner diameter of about 60 A with an axial spacing of 58 A per disc. The fiber can be described as a helix consisting of 56 discs per helical turn. We observed discs of six hemoglobin molecules, which may be stable substructural components of the fibers. They were observed in preparations of hemoglobin fibers and exhibited 6-fold symmetry by power spectrum analysis. A reconstructed image of a disc digitally filtered for 6-fold symmetry has a maximum external diameter of approximately 170 A and a central hole of 60 A diameter and is similar to the axial projection of a single disc from a low-resolution, three-dimensional reconstructed model of a fiber.
Insights
Electron microscopy revealed that aggregated deoxyhemoglobin S forms helical fibers. These fibers consist of stacked discs, each with six hemoglobin molecules, providing insights into sickle cell disease pathology.
Area of Science:
- Biophysics
- Structural Biology
- Hematology
Background:
- Sickle cell disease is caused by abnormal hemoglobin aggregation.
- Understanding the structure of these aggregates is crucial for developing treatments.
Purpose of the Study:
- To investigate the ultrastructure of aggregated deoxyhemoglobin S fibers.
- To characterize the molecular organization of these fibers using advanced imaging techniques.
Main Methods:
- Field emission transmission electron microscopy was used to image isolated helical fibers.
- Optical and digital analyses were performed on electron micrographs.
- Power spectrum analysis was employed to determine symmetry.
Main Results:
- Deoxyhemoglobin S forms helical fibers with an outer diameter of 160-170 Å and inner diameter of ~60 Å.
- Fibers consist of stacked discs, each composed of six hemoglobin molecules, arranged with 6-fold symmetry.
- A helical turn comprises 56 discs, with an axial spacing of 58 Å per disc.
Conclusions:
- The observed helical fibers of deoxyhemoglobin S are consistent with previous findings.
- Discs of six hemoglobin molecules appear to be stable substructural units within the fibers.
- These findings contribute to a detailed understanding of the molecular basis of sickle cell disease.