Human Fibrinogen Fragment D
The thrombin (IIa) catalyzed cleavage of soluble fibrinogen (Fbg) to form fibrin (Fbn) is the terminal proteolytic event in the coagulation cascade. These soluble Fbn monomers spontaneously polymerize to form an insoluble Fbn network which is stabilized by the factor XIIIa catalyzed cross linking of lys and glu residues of and chains. This Fbn network is the major protein component of the hemostatic plug.
Plasma fibrinogen is large glycoprotein (Mr=340,000) synthesized in the liver and circulating at a concentration of 2.6 mg/ml. It is a disulfide linked dimer composed of 3 pairs of disulfide linked non-identical polypeptide chains (A, B and C). Notable features of the A chain are the N-terminal peptide (fibrinopeptide A (FPA, 1-16)), factor XIIIa cross linking sites and 2 phosphorylation sites. When synthesized, Fbg is fully phosphorylated, but circulates at only 20-30% phosphorylation. The B chain contains fibrinopeptide B (FPB, 1-14), one of the 3 N-linked carbohydrate moieties (Mr=2500) and an N-terminal pyroglutamic acid. The chain contains the other N-linked glycosylation site and a factor XIIIa crosslinking sites. The 2 elongated subunits ((AB)2) are aligned in an antiparallel manner forming a trinodular arrangement of the six chains. The nodes are formed by disulfide rings between the 3 parallel chains. The central node (n-disulfide knot, E domain) is formed by the N-termini of all six chains held together by 11 disulfide bonds. This region contains the 2 IIa-sensitive sites. The release of FPA by cleavage at R16-G17 generates Fbn I, exposing a polymerization site (17-20) on the A chain. These regions bind to complimentary regions on the D domain of Fbn to form protofibrils. Subsequent IIa cleavage of FPB (R14-G15) from the B chain exposes additional polymerization sites and promotes lateral growth of the Fbn network.
Each of the 2 domains between the central node (E domain) and the C-terminal nodes (D domain) is composed of parallel -helical regions of the A, B and chains coiled around each other to form a "coiled coil" with polar residues directed outward and nonpolar residues forming a hydrophobic core. In this region, all 3 chains possess a protease (plasmin) sensitive site. The other major plasmin sensitive site is in the hydrophilic preturbance of the -chain from the C-terminal node. Controlled plasmin degradation at these sites converts Fbg into fragment D and fragment E. The individual fragments are isolated by salt fractionation, gel filtration and ion exchange chromatography. The fragments are supplied lyophilized for storage at 4C. Highly purified research grade fibrinogen (>95% clottable) is prepared by a combination of conventional and affinity techniques. It is supplied as a frozen solution in ctirate-phosphate for storage at -80C.
This product is for in vitro research use only, is not to be consumed by animals or humans, and should be handled with general precaution as if capable of spreading infectious disease.