title: By increasing the affinity of heparin for fibrin, Zn(2+) promotes the formation of a ternary heparin-thrombin-fibrin complex that protects thrombin from inhibition by antithrombin.
abstract: 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.
authors.affiliation: Department of Medicine, McMaster University, Hamilton, Ontario, Canada.
authors.forename: Howard H
authors.initials: HH
authors.lastname: Chan
authors.forename: Beverly A
authors.initials: BA
authors.lastname: Leslie
authors.forename: Alan R
authors.initials: AR
authors.lastname: Stafford
authors.forename: Robin S
authors.initials: RS
authors.lastname: Roberts
authors.forename: Nadine N
authors.initials: NN
authors.lastname: Al-Aswad
authors.forename: James C
authors.initials: JC
authors.lastname: Fredenburgh
authors.forename: Jeffrey I
authors.initials: JI
authors.lastname: Weitz
mesh_terms: D000991:Antithrombins / Q000378:metabolism*; D001777:Blood Coagulation; D005337:Fibrin / Q000737:chemistry / Q000378:metabolism*; D006493:Heparin / Q000737:chemistry / Q000378:metabolism*; D006801:Humans; D008958:Models, Molecular; D010949:Plasma / Q000737:chemistry; D011485:Protein Binding; D011487:Protein Conformation; D055550:Protein Stability; D013917:Thrombin / Q000037:antagonists & inhibitors* / Q000737:chemistry / Q000378:metabolism*; D013997:Time Factors; D015032:Zinc / Q000737:chemistry / Q000378:metabolism*
chemical_list: D000991:Antithrombins; D005337:Fibrin; D006493:Heparin; D013917:Thrombin; D015032:Zinc