Structural analysis of human and bovine alpha-fetoprotein by electron microscopy, image processing, and circular dichroism.

The images of human and bovine alpha-fetoprotein molecules have been enhanced by combining dark-field electron microscopy with a laser-assisted optical system. This system filters out random background noise while permitting true averaged signal reconstruction of the molecule. A single averaged molecular image was digitized into a matrix, each pixel being assigned a gray scale level to produce a relative mass map for each molecule. These maps were interpreted from the alpha-helix, beta-form, and random coil of the purified proteins as determined by circular dichroism. Results showed that both molecules are "U shaped", apparently monomeric, with outside dimensions of approximately 80 A. Both molecules have asymmetrical structural features, notably three mass dense regions at both extremities and at the vertex of the molecules. Circular dichroism data suggest a high degree of similar stabilized alpha-helix and extensive beta-form in these regions. Mass map analysis of hAFP correlates with the subdomains organized by disulfide bridges.