The Size and Shape of Human and Bovine Antithrombin III

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Abstract
Human and bovine antithrombin, purified by affinity chromatography on heparin-agarose, were characterized with regard to chemical composition, size, shape and conformation. Both preparations contained several active components of identical or similar size but different electrical charge. Amino acid and carbohydrate analyses revealed striking similarities between human and bovine antithrombin, while immunological analyses failed to demonstrate any cross-reactivity. The MW were determined by sedimentation equilibrium to be 58,000 for human and 56,000 for bovine antithrombin. The small MW difference suggested by these values was verified by several empirical methods of MW estimation. Hydrodynamic measurements indicated that the 2 proteins have similar molecular shapes, both of which are slightly more extended than that of typical globular proteins. The internal folding of the 2 polypeptide chains is also similar, as evidenced by the identity of the far-UV circular dichroism spectra. Specifically, these analyses suggested a low .alpha.-helix content of both proteins. The marked structural similarity of human and bovine antithrombin indicates that the 2 proteins may also exhibit extensive functional similarities in the binding of heparin and the inhibition of various coagulation factors.