| Factor XIIIa (FXIIIa) (fibrinoligase, plasma transglutaminase, fibrin stabilizing factor, E.C. 2.3.2.13) is a glutaminyl-peptide g-glutamyl transferase functioning in the final stages of the coagulation cascade, stabilizing the fibrin clot by crosslinking the a- and g-chains of fibrin to form homopolymers (1-4). Several unique features of FXIIIa set it apart from proteins with which it is generally associated. Factor XIIIa is the only non-proteolytic enzyme of the coagulation cascade. Unlike other transamidases, it exists totally in zymogenic form and is the only sulfhydrylase enzyme which functions extracellularly. Plasma FXIIIa is a tetramer, (Mr=312,000), composed of 2 pairs of non-identical subunits (A2B2) (4-7). Platelet FXIII is a dimer (Mr=46,000), composed of only a pair of identical A subunits (A2) as is factor XIII from all intracellular sources (7-11). The A subunit contains 6 free sulfhydryl groups, one of which is the active center (12). The conversion of plasma FXIII (A2B2) to the active transglutaminase, FXIIIa (A2) results from the thrombin catalyzed hydrolysis of the Arg36-Gly37 peptide bond at the NH2-terminal of the A subunit (13). The conformational change induced by this cleavage results in exposure of the active site cysteine. Full activity is achieved only after the Ca2+ dependent (Kd=10e-3M) dissociation of the B subunit dimer from the A2 dimer (14-16). Dissociation of the A2B2 complex is enhanced by binding to the region of fibrin(ogen) containing residues 242-424 of the Aa chain (14-16). Thus, at normal plasma concentrations of FXIII (90nM), fibrinogen (8uM) and Ca2+ (2.5mM free), all the FXIIIa generated would be in the fully activated (A2) molecular form. Other physiologically relevant crosslinked complexes generated by FXIIIa include fibrin-fibrinogen (17), fibrin-fibronectin (18), fibrin-vWF (19), fibrin-a2antiplasmin (20), fibrin-actin (21), fibrin-thrombospondin, fibronectin-collagen (19), vWF-collagen (19), actin-myosin (21,22), factor V-factor V (23). In addition to clot stabilization, FXIIIa is thought to function in various processes involving cell proliferation, such as wound healing, tissue remodeling, atherosclerosis, and tumor growth. Stabilization of Fibrin: The conversion of soluble fibrin to an insoluble fibrin clot is catalyzed by the transamidase factor XIIIa. Lysine and glutamine side chains of a- and g-chains of fibrin are crosslinked to form homopolymers. Storage and Stability: May be stored at 4C for short-term only. For long-term storage, aliquot and store at -20C. Aliquots are stable for 6 months after receipt at -20C. For maximum recovery of product, centrifuge the original vial after thawing and prior to removing the cap. Further dilutions can be made in assay buffer. |
