Dependence of the pseudo-first-order rate constants (k_obs) for the inhibition of APC-fXa^143-154 and thrombin-fXa^143-154 on the AT concentration in the absence and presence of pentasaccharide (H₅). (A) The k_obs values for the mutant APC were determined from the time-dependent inhibition of the protease (2 nM) at different concentrations of AT in complex with saturating concentrations of pentasaccharide (1-2 μM) 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 for the inhibition reaction were determined for the mutant thrombin (1 nM) at different concentrations of AT in either the absence (•) or presence (○) of a saturating concentration of pentasaccharide (1 μM). Solid lines in both panels are the best fitof kinetic data to a linear equation.