Table 1.

Thermodynamic and kinetic parameters for the interaction of the organic activators with plasma antithrombin at pH 7.4, I 0.15, 25 °C.

	KD,OBS(μM)a	ΔFMAX(%)a	ΔG° (kcal/mol)	kACT(M−1s−1)	Accelerationb
IAS5	320±10c	−1.3±0.2	4.8±0.2	69,780±580	30
IES5	805±70	−1.2±0.2	4.2±0.4	6160±240	2.7
IES4	330±20	−1.4±0.2	4.8±0.3	10860±480	4.7
IAS4	>1000d	na	na	5,370e	2.3e

^aMeasured spectrofluorometrically using a saturable decrease in intrinsic tryptophan fluorescence (λ_EM = 340 nm) due to the binding of the organic activator to plasma antithrombin. See ‘Experimental Procedures’ for details.

^bRepresents the ratio of second-order rate constant of antithrombin inhibition of factor Xa in the presence of organic activator (k_ACT) to that in its absence (k_UNCAT). The k_UNCAT value for pH 7.4 buffer used in calculations was 2300 M⁻¹s⁻¹, as reported previously.¹⁰

^cErrors represent ±1 S.E.

^dThis value was not possible to measure accurately because of insufficient saturation of antithrombin.

^eEstimated on the assumption that K_D is 1,000 μM.