Anticoagulants and the Propagation Phase of Thrombin Generation

The view that clot time-based assays do not provide a sufficient assessment of an individual's hemostatic competence, especially in the context of anticoagulant therapy, has provoked a search for new metrics, with significant focus directed at techniques that define the propagation phase of thrombin generation. Here we use our deterministic mathematical model of tissue-factor initiated thrombin generation in combination with reconstructions using purified protein components to characterize how the interplay between anticoagulant mechanisms and variable composition of the coagulation proteome result in differential regulation of the propagation phase of thrombin generation. Thrombin parameters were extracted from computationally derived thrombin generation profiles generated using coagulation proteome factor data from warfarin-treated individuals (N = 54) and matching groups of control individuals (N = 37). A computational clot time prolongation value (cINR) was devised that correlated with their actual International Normalized Ratio (INR) values, with differences between individual INR and cINR values shown to derive from the insensitivity of the INR to tissue factor pathway inhibitor (TFPI). The analysis suggests that normal range variation in TFPI levels could be an important contributor to the failure of the INR to adequately reflect the anticoagulated state in some individuals. Warfarin-induced changes in thrombin propagation phase parameters were then compared to those induced by unfractionated heparin, fondaparinux, rivaroxaban, and a reversible thrombin inhibitor. Anticoagulants were assessed at concentrations yielding equivalent cINR values, with each anticoagulant evaluated using 32 unique coagulation proteome compositions. The analyses showed that no anticoagulant recapitulated all features of warfarin propagation phase dynamics; differences in propagation phase effects suggest that anticoagulants that selectively target fXa or thrombin may provoke fewer bleeding episodes. More generally, the study shows that computational modeling of the response of core elements of the coagulation proteome to a physiologically relevant tissue factor stimulus may improve the monitoring of a broad range of anticoagulants

Specificity and reactive loop length requirements for crmA inhibition of serine proteases

The viral serpin, crmA, is distinguished by its small size and ability to inhibit both serine and cysteine proteases utilizing a reactive loop shorter than most other serpins. Here, we characterize the mechanism of crmA inhibition of serine proteases and probe the reactive loop length requirements for inhibition with two crmA reactive loop variants. P1 Arg crmA inhibited the trypsin-like proteases, thrombin, and factor Xa, with moderate efficiencies (~102–104 M−1sec−1), near equimolar inhibition stoichiometries, and formation of SDS-stable complexes which were resistant to dissociation (kdiss ~10−7 sec−1), consistent with a serpin-type inhibition mechanism. Trypsin was not inhibited, but efficiently cleaved the variant crmA as a substrate (kcat/KM of ~106 M−1 sec−1). N-terminal sequencing confirmed that the P1 Arg–P1′Cys bond was the site of cleavage. Altering the placement of the Arg in a double mutant P1 Gly-P1′Arg crmA resulted in minimal ability to inhibit any of the trypsin family proteases. This variant was cleaved by the proteases ~10-fold less efficiently than P1 Arg crmA. Surprisingly, pancreatic elastase was rapidly inhibited by wild-type and P1 Arg crmAs (105–106 M−1sec−1), although with elevated inhibition stoichiometries and higher rates of complex dissociation. N-terminal sequencing showed that elastase attacked the P1′Cys–P2′Ala bond, indicating that crmA can inhibit proteases using a reactive loop length similar to that used by other serpins, but with variations in this inhibition arising from different effective P2 residues. These results indicate that crmA inhibits serine proteases by the established serpin conformational trapping mechanism, but is unusual in inhibiting through either of two adjacent reactive sites

The Signature 3-O-Sulfo Group of the Anticoagulant Heparin Sequence Is Critical for Heparin Binding to Antithrombin but Is Not Required for Allosteric Activation*

Heparin and heparan sulfate glycosaminoglycans allosterically activate the serpin, antithrombin, by binding through a specific pentasaccharide sequence containing a critical 3-O-sulfo group. To elucidate the role of the 3-O-sulfo group in the activation mechanism, we compared the effects of deleting the 3-O-sulfo group or mutating the Lys114 binding partner of this group on antithrombin-pentasaccharide interactions by equilibrium binding and rapid kinetic analyses. Binding studies over a wide range of ionic strength and pH showed that loss of the 3-O-sulfo group caused a massive ∼60% loss in binding energy for the antithrombin-pentasaccharide interaction due to the disruption of a cooperative network of ionic and nonionic interactions. Despite this affinity loss, the 3-O-desulfonated pentasaccharide retained the ability to induce tryptophan fluorescence changes and to enhance factor Xa reactivity in antithrombin, indicative of normal conformational activation. Rapid kinetic studies showed that loss of the 3-O-sulfo group affected both the ability of the pentasaccharide to recognize native antithrombin and its ability to preferentially bind and stabilize activated antithrombin. By contrast, mutation of Lys114 solely affected the preferential interaction of the pentasaccharide with activated antithrombin. These findings demonstrate that the 3-O-sulfo group functions as a key determinant of heparin pentasaccharide activation of antithrombin both by contributing to the Lys114-independent recognition of native antithrombin and by triggering a Lys114-dependent induced fit interaction with activated antithrombin that locks the serpin in the activated state