Heparin binding to antithrombin III: Variation in binding sites and affinity |
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Authors: | Michael W. Piepkorn David Lagunoff Gottfried Schmer |
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Affiliation: | 1. Department of Pathology, University of Washington, School of Medicine, Seattle, Washington 98195 USA;2. Department of Laboratory Medicine, University of Washington, School of Medicine, Seattle, Washington 98195 USA;3. Department of Biochemistry, University of Washington, School of Medicine, Seattle, Washington 98195 USA |
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Abstract: | Heparin fractions of different molecular weights and anticoagulant activities were prepared by chromatography on protamine-Sepharose, and the association constants and stoichiometry for binding to antithrombin III were determined by measurement of enhancement of tryptophan fluorescence. A 7,900 molecular weight heparin preparation bound to antithrombin III with a stoichiometry of close to 2:1, whereas 14,300 and 21,600 molecular weight fractions bound at approximately 1:1 with the protein. Apparent association constants were 0.66 × 106 M?1 for the low molecular weight preparation and 2.89 × 106 M?1 for the high molecular weight material. Maximal fluorescence enhancement was greater with the higher molecular weight heparin. These results suggest a model of heparin-antithrombin III binding in which two sites on antithrombin III can accommodate one large heparin molecule with high affinity or two smaller molecules with low affinity. |
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