Interactions of Human Fibrinogens with Factor XIII:  Roles of Calcium and the γ‘ Peptide

Abstract

Plasma factor XIII is the zymogen of the transglutaminase factor XIIIa. This enzyme catalyzes the formation of isopeptide cross-links between fibrin molecules in nascent blood clots that greatly increase the mechanical stability of clots and their resistance to thrombolytic enzymes. We have characterized the solution interactions of factor XIII with two variants of fibrinogen, the soluble precursor of fibrin. Both the predominant fibrinogen gamma(A)/gamma(A) and the major variant gamma(A)/gamma' form complexes with a 2 fibrinogen:1 factor XIII ratio. The absence of detectable concentrations of 1:1 complexes in equilibrium mixtures containing free factor XIII and 2:1 complexes suggests that this interaction is cooperative. Factor XIII binds fibrinogen gamma(A)/gamma' approximately 20-fold more tightly than fibrinogen gamma(A)/gamma(A), and the interaction with fibrinogen gamma(A)/gamma' (but not fibrinogen gamma(A)/gamma(A)) is accompanied by a significant release of Ca(2+). Taken together, these results suggest that the strikingly anionic gamma' C-terminal sequence contains features that are important for factor XIII binding. Consistent with this notion, a synthetic 20-residue polypeptide containing the gamma' sequence was found to associate with factor XIII in a 2:1 molar ratio and act as an efficient competitor for fibrinogen gamma(A)/gamma' binding.

Genetic variation in the fibrinogen gamma gene increases the risk for deep venous thrombosis by reducing plasma fibrinogen γ′ levels

“…48,49 Several studies have reported that fibrinogen ␥A/␥Ј heterodimers behave differently from fibrinogen ␥A/␥A homodimers during the early stages of polymerization, ultimately leading to an altered fibrin structure that is more extensively cross-linked by activated factor XIII than ␥A/␥A fibrin and also more resistant to fibrinolysis. [50][51][52] This has been explained by the presence of a unique binding site for factor XIII B in the carboxy terminus of the ␥Ј chain. 53,54 These findings predict that an increase in fibrinogen ␥Ј level or in the ␥Ј/␥ ratio would result in more stable and lysis-resistant clots and, therefore, would represent a prothrombotic state.…”

“…A possible explanation for this finding might be that the alternative carboxy terminus of the ␥Ј chain defines not only a binding site for factor XIII but also a high-affinity, nonsubstrate binding site for thrombin. 51,52,55 Studies on ␥A/␥Ј fibrin(ogen) indicated that this binding site functioned as a thrombin inhibitor 56,57 and, importantly, contributed to the antithrombin activity that develops during fibrin formation (antithrombin 1). 58,59 Thrombin interacts with the carboxy terminal sequence of the ␥Ј chain through its exosite 2, 26,27,57 which prevents the participation of this exosite in the binding of thrombin to heparin 60 and which recently was found to be responsible for the observation that thrombin-induced FPA generation was slower with ␥A/␥Ј fibrinogen than with ␥A/␥A fibrinogen.…”

Fibrinogen gamma-chain splice variant γ′ alters fibrin formation and structure

“…Functional studies of the fibrinogen ␥Ј splice variant have been conducted in the past, such as FXIII-induced cross-linking, 23 thrombin 19 and platelet 21 binding, and clot lysis experiments, 24 but this is the first report on the direct effect of the ␥A/␥Ј variant on the formation and structure of fibrin clots. Both turbidity and fibrinopeptide release experiments showed that heterodimers consisting of ␥Ј and ␥A chains differed strongly during the early events of polymerization from fibrinogen homodimers containing ␥A chains only, ultimately leading to altered fibrin structure.…”

The pleiotropic role of the fibrinogen γ′ chain in hemostasis

“…61 Furthermore, FXIII binds to ␥A/␥Ј fibrinogen with an approximate 20-fold higher affinity than ␥A/␥A fibrinogen, and the interaction with ␥A/␥Ј fibrinogen (but not ␥A/␥A fibrinogen) is accompanied by a significant release of calcium. In addition, a ␥Ј peptide bound FXIII with the same molar ratio as full-length ␥A/␥Ј fibrinogen and served as a competitive inhibitor of FXIII binding to ␥A/␥Ј fibrinogen.…”

“…In addition, a ␥Ј peptide bound FXIII with the same molar ratio as full-length ␥A/␥Ј fibrinogen and served as a competitive inhibitor of FXIII binding to ␥A/␥Ј fibrinogen. 61 In addition, a very recent study showed that FXIII binding to activated platelets is mediated via the ␥Ј chain of aIIb␤3-bound fibrinogen, suggesting a bridging function of the ␥Ј chain between FXIII and activated platelets. 62 However, binding of FXIII to the ␥Ј chain has been put into question by a report from Gersh and Lord who used an enzymelinked immunosorbent assay-based system with recombinant fibrinogens and found no difference in K d for the binding of FXIII to ␥A/␥A, ␥A/␥Ј, or ␥Ј/␥Ј fibrinogen.…”