Dependence of the pseudo-first-order rate constants (k_obs) for the inhibition of wild-type and mutant APC on the AT-heparin complex concentrations. (A) The k_obs values were determined from the time-dependent inhibition of wild-type APC (2 nM) at different concentrations of the AT-heparin (H₇₀) complex (x-axis) in TBS/Ca²⁺ at room temperature as described in “Materials and methods.” (B) The same as in panel A, except that the k_obs values were determined for the APC-fXa^143-154 mutant. Solid lines in both panels are best fit of kinetic data to a hyperbolic equation.