The Role of γA/γ′ Fibrinogen in Plasma Factor XIII Activation

Abstract

Factor XIII zymogen activation is a complex series of events that involve fibrinogen acting in several different roles. This report focuses on the role of fibrinogen as a cofactor in factor XIII activation by thrombin. We demonstrate that fibrinogen has two distinct activities that lead to an increased rate of factor XIII activation. First, the thrombin proteolytic activity is increased by fibrin. The cleavage rates of both a small chromogenic substrate and the factor XIII activation peptide are increased in the presence of either the major fibrin isoform, ␥A/␥A fibrin, or a minor variant form, ␥A/␥ fibrin. This enhancement of thrombin activity by fibrin is independent of fibrin polymerization and requires only cleavage of the fibrinopeptides. Subsequently, ␥A/␥ fibrinogen accelerates plasma factor XIII activation by a non-proteolytic mechanism. This increased rate of activation results in a slightly more rapid cross-linking of fibrin ␥A and ␥ chains and a significantly more rapid cross-linking of fibrin ␣ chain multimers. Together, these results show that although both forms of fibrin increase the rate of activation peptide cleavage by thrombin, ␥A/␥ fibrinogen also increases the rate of factor XIII activation in a non-proteolytic manner. A revised model of factor XIII activation is presented below.Plasma coagulation factor XIII (EC 2.3.2.13) is the zymogen precursor to the active transglutaminase factor XIIIa. Plasma factor XIII consists of two a subunits (M r ϳ83,000) and two b subunits (M r ϳ80,000) with the stoichiometry a 2 b 2 , whereas a form of factor XIII stored in platelets lacks the b subunits (1-4). In the final stages of blood coagulation, thrombin activates plasma factor XIII by cleaving an activation peptide (M r ϳ4000) from the amino terminus of each a subunit (3). In the presence of calcium, active factor XIII (factor XIIIa) then catalyzes the formation of isopeptide bonds in polymerized fibrin strands between adjacent glutamine/lysine side chains (5). Cross-linking occurs in the ␣ and ␥ chains of fibrin but not in the ␤ chains, and it results in the formation of ␥-␥ dimers, ␥ chain multimers, ␣ chain multimers, and complexes between ␣ and ␥ chains (6). Cross-linked fibrin shows increased stability (7) and increased proteolytic resistance to fibrinolytic enzymes (8).Factor XIII activation by thrombin is a multistep process that is modulated by calcium and by the presence of fibrinogen. An explicit model of factor XIII activation has been proposed (9) in which the initial activation step is thrombin cleavage of the activation peptide from the a subunit, which is then referred to as the aЈ subunit. However, the a 2 Јb 2 complex is not catalytically active. In the presence of high concentrations of calcium, i.e. Ͼ10 mM, the a 2 Јb 2 complex dissociates into the a 2 Ј and b 2 subunits. Fibrin facilitates this dissociation step such that it occurs at physiologic calcium concentrations (ϳ1.5 mM), thereby increasing the rate of factor XIII activation under physiologic conditions. Finally, a conformati...

The pleiotropic role of the fibrinogen γ′ chain in hemostasis

“…[64][65][66] The activation kinetics of FXIII by monitoring the FXIII activity to cross-link biotinlabeled pentylamine into a casein substrate was enhanced by both ␥A/␥A and ␥A/␥Ј fibrinogen, and the effect observed for ␥A/␥Ј fibrinogen was greater than that for ␥A/␥A fibrinogen. 67 This suggests that ␥A/␥Ј fibrinogen serves to enhance FXIII activation more than ␥A/␥A fibrinogen. However, when kinetics of FXIII activation were assessed by monitoring cleaved FXIII-A subunits on sodium dodecyl sulfate-polyacrylamide gel electrophoresis, there appeared to be no distinction between the enhancement effect of ␥A/␥A and ␥A/␥Ј fibrin.…”

“…However, when kinetics of FXIII activation were assessed by monitoring cleaved FXIII-A subunits on sodium dodecyl sulfate-polyacrylamide gel electrophoresis, there appeared to be no distinction between the enhancement effect of ␥A/␥A and ␥A/␥Ј fibrin. 67 These authors suggested that ␥A/␥Ј fibrinogen may facilitate FXIII activation by enhancing the dissociation of the A and B subunits of FXIII, rather than the cleavage of the activation peptide by thrombin. In addition, more rapid and increased cross-linking of ␥A/␥Ј fibrin by FXIIIa compared with ␥A/␥A fibrin was found.…”

Studies on the basis for the properties of fibrin produced from fibrinogen-containing γ′ chains

“…23,24 In recent years several reports have appeared comparing various structural and functional features of fibrin(ogen)s 1 and 2. [25][26][27][28][29][30][31] Factor XIII has been shown to bind to the fibrinogen ␥Ј chains 26 and thrombin also binds to the ␥Ј extension in fibrin 25 through its exosite 2. 32,33 Fibrin 2 reportedly becomes more "extensively" cross-linked by factor XIIIa than does fibrin 1.…”

Coming full circle with factor XIII

“…7 First, the difference in affinity between the 2 fibrinogen forms was only 20-fold; the heterodimer bound more tightly than the ␥A/␥A homodimer. Because the concentrations of the 2 chains in fibrinogen differ by ϳ 15-fold, one would anticipate FXIII-A2B2 would distribute between the 2 forms and not be exclusively bound to peak 2 fibrinogen.…”

“…Lastly, the studies of Farrell's group measured interactions by equilibrium analytical ultracentrifugation. 7 This approach requires that FXIII-A2B2 and fibrinogen remain together at high concentrations for more than 24 hours. Under these conditions, Mosesson has found and we have confirmed, FXIII-A2B2 will catalyze the formation of ␥ dimers in fibrinogen.…”