Table 1.

Kinetic and equilibrium parameters for heparin pentasaccharide binding to native and latent antithrombins at I 0.15, pH 7.4, 10°C^a

Antithrombin form	K1 (μM)	k+2 (s−1)	kon = k+2/K1 (μM−1 s−1)b	k−2 = koff (s−1)b	KD (nM)
Native	6±0.4	200±5	33±1	0.3±0.9	4±2
Latent	9±1	200±15	30±4	3±1	150±50

^aKinetic parameters, taken from Schedin-Weiss et al. [28], were measured by rapid kinetic analyses of pentasaccharide binding to native and latent antithrombins monitored by tryptophan or TNS fluorescence changes. Parameters for the two-step mechanism of scheme 1 were obtained by fitting the dependence of the observed pseudo-first order binding rate constant on pentasaccharide concentration to a hyperbolic saturation function. Dissociation constants were obtained by equilibrium binding titrations of antithrombin with pentasaccharide monitored by tryptophan or TNS fluorescence changes.

^bValues measured from the slope and intercept of the initial linear dependence of k_obs versus pentasaccharide concentration. Deviations between the k_on obtained from the slope and that calculated from the ratio, k₊₂/K₁, reflect the greater experimental error in the latter.