PMID: 7287752Nov 10, 1981

Binding of high affinity heparin to antithrombin III. Stopped flow kinetic studies of the binding interaction

The Journal of Biological Chemistry
S T OlsonJ D Shore


The kinetics of high affinity heparin binding to human antithrombin III has been studied by stopped flow fluorimetry, using the 40% antithrombin fluorescence enhancement resulting from this interaction. At mu 0.15, pH 7.4, and 25 degrees C, the observed pseudo-first order rate constant varies hyperbolically with heparin concentration with a limiting rate constant of 440 +/- 90 s-1, demonstrating that heparin binding is a two-step process involving a conformational change in antithrombin III. An identical dependence is produced when antithrombin is varied, consistent with a symmetrical mechanism in which heparin binding induces a conformational change in antithrombin rather than perturbing an equilibrium between two conformational states of the protein. The rate constant for dissociation of the antithrombin-heparin complex is 1.1-1.5 s-1 at mu 0.15, as determined from the ordinate intercept at low heparin concentrations or by dissociation of the antithrombin-heparin complex with iodide. Observation of a single pseudo-first order binding rates over a 400-fold heparin concentration range with no detectable lags is compatible with the initial binding step being in rapid equilibrium with a KD of 4.3 +/- 1.3 X 10(-5) M at mu 0.15. Va...Continue Reading

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