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The nonspecific binding of heparin to plasma proteins compromises its anticoagulant activity by reducing the amount of heparin available to bind antithrombin. In addition, interaction of heparin with fibrin promotes formation of a ternary heparin-thrombin-fibrin complex that protects fibrin-bound thrombin from inhibition by the heparin-antithrombin complex. Previous studies have shown that heparin binds the E domain of fibrinogen. The current investigation examines the role of Zn(2+) in this interaction because Zn(2+) is released locally by platelets and both heparin and fibrinogen bind the cation, resulting in greater protection from inhibition by antithrombin. Zn(2+) promotes heparin binding to fibrinogen, as determined by chromatography, fluorescence, and surface plasmon resonance. Compared with intact fibrinogen, there is reduced heparin binding to fragment X, a clottable plasmin degradation product of fibrinogen. A monoclonal antibody directed against a portion of the fibrinogen αC domain removed by plasmin attenuates binding of heparin to fibrinogen and a peptide analog of this region binds heparin in a Zn(2+)-dependent fashion. These results indicate that the αC domain of fibrinogen harbors a Zn(2+)-dependent heparin binding site. As a consequence, heparin-catalyzed inhibition of factor Xa by antithrombin is compromised by fibrinogen to a greater extent when Zn(2+) is present. These results reveal the mechanism by which Zn(2+) augments the capacity of fibrinogen to impair the anticoagulant activity of heparin.

Heparin binds fibrin and, by bridging thrombin onto fibrin, promotes the formation of a ternary heparin-thrombin-fibrin complex that protects thrombin from inhibition by antithrombin. Because thrombin binds γ(A)/γ'-fibrin, a variant with an extended γ-chain, with higher affinity than the bulk γ(A)/γ(A)-fibrin, γ(A)/γ'-fibrin affords bound thrombin more protection from inhibition by antithrombin-heparin. We examined the effect of Zn(2+) on heparin-thrombin-fibrin complex formation because Zn(2+) modulates heparin-protein interactions. Zn(2+) increased the affinity of heparin for γ(A)/γ(A)- and γ(A)/γ'-fibrin by 4.3- and 3.7-fold, respectively, but had no effect on the affinity of thrombin for either form of fibrin. In contrast, in the presence of heparin, Zn(2+) increased the affinity of thrombin for γ(A)/γ(A)-fibrin 4-fold (from a K(d) value of 0.8 to 0.2 μM) and slowed the rate of thrombin dissociation from γ(A)/γ(A)-fibrin clots. These findings suggest that Zn(2+) enhances the formation of ternary heparin-thrombin-fibrin complexes with γ(A)/γ(A)-fibrin but does not influence the already high affinity interaction of thrombin with γ(A)/γ'-fibrin. Consistent with this concept, in the presence of Zn(2+), γ(A)/γ(A)-fibrin protected thrombin from inhibition by antithrombin-heparin to a similar extent as γ(A)/γ'-fibrin. Therefore, by enhancing the binding of heparin to fibrin, physiological concentrations of Zn(2+) render fibrin-bound thrombin more protected from inhibition by antithrombin. Because fibrin-bound thrombin can trigger thrombus expansion, these findings help to explain why recurrent thrombosis can occur despite heparin treatment.

Thrombin exosite 1 binds the predominant gamma(A)/gamma(A)-fibrin form with low affinity. A subpopulation of fibrin molecules, gamma(A)/gamma'-fibrin, has an extended COOH terminus gamma'-chain that binds exosite 2 of thrombin. Bivalent binding to gamma(A)/gamma'-fibrin increases the affinity of thrombin 10-fold, as determined by surface plasmon resonance. Because of its higher affinity, thrombin dissociates 7-fold more slowly from gamma(A)/gamma'-fibrin clots than from gamma(A)/gamma(A)-fibrin clots. After 24 h of washing, however, both gamma(A)/gamma'- and gamma(A)/gamma(A)-fibrin clots generate fibrinopeptide A when incubated with fibrinogen, indicating the retention of active thrombin. Previous studies demonstrated that heparin heightens the affinity of thrombin for fibrin by simultaneously binding to fibrin and exosite 2 on thrombin to generate a ternary heparin-thrombin-fibrin complex that protects thrombin from inhibition by antithrombin and heparin cofactor II. In contrast, dermatan sulfate does not promote ternary complex formation because it does not bind to fibrin. Heparin-catalyzed rates of thrombin inhibition by antithrombin were 5-fold slower in gamma(A)/gamma'-fibrin clots than they were in gamma(A)/gamma(A)-fibrin clots. This difference reflects bivalent binding of thrombin to gamma(A)/gamma'-fibrin because (a) it is abolished by addition of a gamma'-chain-directed antibody that blocks exosite 2-mediated binding of thrombin to the gamma'-chain and (b) the dermatan sulfate-catalyzed rate of thrombin inhibition by heparin cofactor II also is lower with gamma(A)/gamma'-fibrin than with gamma(A)/gamma(A)-fibrin clots. Thus, bivalent binding of thrombin to gamma(A)/gamma'-fibrin protects thrombin from inhibition, raising the possibility that gamma(A)/gamma'-fibrin serves as a reservoir of active thrombin that renders thrombi thrombogenic.

Although exosites 1 and 2 regulate thrombin activity by binding substrates and cofactors and by allosterically modulating the active site, it is unclear whether there is direct allosteric linkage between the two exosites. To begin to address this, we first titrated a thrombin variant fluorescently labeled at exosite 1 with exosite 2 ligands, HD22 (a DNA aptamer), γ'-peptide (an analog of the COOH terminus of the γ'-chain of fibrinogen) or heparin. Concentration-dependent and saturable changes in fluorescence were elicited, supporting inter-exosite linkage. To explore the functional consequences of this phenomenon, we evaluated the capacity of exosite 2 ligands to inhibit thrombin binding to γA/γA-fibrin, an interaction mediated solely by exosite 1. When γA/γA-fibrinogen was clotted with thrombin in the presence of HD22, γ'-peptide, or prothrombin fragment 2 there was a dose-dependent and saturable decrease in thrombin binding to the resultant fibrin clots. Furthermore, HD22 reduced the affinity of thrombin for γA/γA-fibrin 6-fold and accelerated the dissociation of thrombin from preformed γA/γA-fibrin clots. Similar responses were obtained when surface plasmon resonance was used to monitor the interaction of thrombin with γA/γA-fibrinogen or fibrin. There is bidirectional communication between the exosites, because exosite 1 ligands, HD1 (a DNA aptamer) or hirudin-(54-65) (an analog of the COOH terminus of hirudin), inhibited the exosite 2-mediated interaction of thrombin with immobilized γ'-peptide. These findings provide evidence for long range allosteric linkage between exosites 1 and 2 on thrombin, revealing further complexity to the mechanisms of thrombin regulation. [ABSTRACT FROM AUTHOR]