Blood Coagulation Factor XIII

Mechanical stability in many biological materials is provided by the crosslinking of large structural proteins with gamma-glutamyl-epsilon-lysyl amide bonds. The three-dimensional structure of human recombinant factor XIII (EC 2.3.2.13 zymogen; protein-glutamine:amine gamma-glutamyltransferase a chain), a transglutaminase zymogen, has been solved at 2.8-A resolution by x-ray crystallography. This structure shows that each chain of the homodimeric protein is folded into four sequential domains. A catalytic triad reminiscent of that observed in cysteine proteases has been identified in the core domain. The amino-terminal activation peptide of each subunit crosses the dimer interface and partially occludes the opening of the catalytic cavity in the second subunit, preventing substrate binding to the zymogen. A proposal for the mechanism of activation by thrombin and calcium is made that details the structural events leading to active factor XIIIa'.

The concentration of alpha 2-plasmin inhibitor in blood plasma is higher than that in serum obtained from the blood clotted in the presence of calcium ions, but is the same as that in serum obtained in the absence of calcium ions. Radiolabeled alpha2-plasmin inhibitor was covalently bound to fibrin only when calcium ions were present at the time of clotting of plasma or fibrinogen. Whereas, when batroxobin, a snake venom enzyme that lacks the ability to activate fibrin-stabilizing factor, was used for clotting fibrinogen, the binding was not observed. When fibrin-stablizing, factor-deficient plasma was clotted, the specific binding of alpha 2-plasmin inhibitor to fibrin did not occur even in the presence of calcium ions and the concentration of alpha 2-plasmin inhibitor in serum was the same as that in plasma. Monodansyl cadaverine, a fluorescent substrate of the fibrin-stablizing factor, was incorporated into alpha 2-plasmin inhibitor by activated fibrin-stablizing factor. All these findings indicate that alpha 2-plasmin inhibitor is cross-linked to fibrin by activated fibrin-stabilizing factor when blood is clotted. Analysis of alpha 2-plasmin inhibitor-incorporated fibrin by sodium dodecyl sulfate gel electrophoresis showed that the inhibitor was mainly cross-linked to polymerized alpha-chains of cross-linked fibrin. Cross-linking of alpha 2-plasmin inhibitor to fibrin renders fibrin clot less susceptible to fibrinolysis by plasmin.