Fibrinogen Molecule

Fibrinogen occurs in normal human plasma at a concentration of about 0.25%, and constitutes about 4% of the plasma protein. In some animal plasmas the proportion is somewhat greater. It can be separated from the other proteins by various fractionation procedures, and preparations of as high as 98% purity have been made (Seegers and Smith, 1941 Cohn and others, 1946 Morrison, Edsall, and Miller, 1947). In analyses for purity of such preparations, fibrinogen is defined as the protein recoverable as fibrin after clotting, under conditions in which the conversion to fibrin is known to be complete and the occlusion of other protein by the fibrin is reduced to a minimum (Morrison, 1947). 

From studies of double refraction of flow (Edsall, Foster, and Schein-berg, 1947), and sedimentation and viscosity (Oncley, Scatchard, and Brown, 1947), the dimensions of the fibrinogen molecule considered as an elongated ellipsoid of revolution have been estimated to be 35 A. X 700 A., with a molecular weight of about 500,000 assumption of the shape of a cylindrical rod would probably not alter these figures greatly. 

The amino acid composition of fibrinogen is characterized by rather high proportions of residues containing sulfur and also hydroxyl groups (Brand, 1944,1946). On the basis of a molecular weight of 500,000, the number of residues per molecule of human fibrinogen is 113 of cysteine and half-cystine, 85 of methionine, 395 of serine, 277 of threonine, and 

160 of tyrosine. Similar figures have been foimd for human and bovine fibrin. 

---

Fibrin is an elastic filamentous protein elaborated from its precursor, fibrinogen, which is present in plasma at high concentration. Fibrin is formed in response to the actions of thrombin. Thrombin cleaves small peptides from the fibrinogen molecule, forming fibrin monomers that will begin to polymerize and become crosslinked. 

Fig. 3. Fibrinogen molecule with its two A a, two B 3, and two -y chains. Thrombin (Ha) acts on the A a chain to generate fibrinopeptide A (FPA) and the a-chain. It also cleaves the B 3 chain to generate fibrinopeptide B (FPB) and the p-chain. S-S represents disulfide bonds. Altogether, 29 disulfide bonds hold together the six polypeptide chains that make up the fibrinogen molecule.

Figure 9.7. Diagrammatic representation of the fibrinogen molecule and its conversion to the soft clot of fibrin. Reproduced (in modified form) by permission from Textbook of Biochemistry with Clinical Correlations (3rd Ed.) Devlin (1992). This material is used by permission of John Wiley Sons, Inc.

The adsorption experiments were carried out by quantifying each of proteins adsorbed on the material from mono-component protein solutions, from four-component protein solutions, and from plasma and diluted plasma. Adsorption profiles of protein were largely different, depending on the aforementioned experimental conditions. For instance, the behavior of any particular protein from diluted plasma varied in response to the extent of plasma dilution. Cooper s results are illustrated in Fig. 3, on fibrinogen adsorption onto five polymer surfaces. It is seen that the adsorption profiles are different one another, being influenced by the different nature of the polymer surfaces. The surface concentrations of adsorbed protein are mostly time-dependent, and maxima in the adsorption profiles were observed. This is interpreted in terms of replacement of adsorbed fibrinogen molecules by other proteins later in time (Vroman effect). Corresponding profiles were also presented for FN and VN. 

View of a modified bovine fibrinogen molecule. The 45-nm-long disulfide-linked dimer is composed of three nonidentical polypeptide chains. The N termini of the six chains from the two halves come together in the center in a small globular "disulfide knot." The C termini form globular domains at the ends. The 340-kDa molecule has been treated with a lysine-specific protease which has removed portions of two chains to give the 285-kDa molecule whose crystal structure is shown. Arrows point to attached oligosaccharides. From Brown et at. (2000) Proc. Natl. Acad. Sci. U. S. A. 97, 85-90. Courtesy of Carolyn Cohen. 

Differential scanning microcalorimetry has been used together with limited proteolysis to identify cooperative domains in fibrinogen. It was found that each terminal part of the fibrinogen molecule is made up of six cooperative domains, with three domains formed by the C-terminal part of the /3-chain and three other domains formed by the C-terminal part of the 7-chain (Medved, 1990 Medved et at, 1986, 1997 Privalov and Medved, 1982). The amino acid sequence of the oC domains suggested that each is made up of a globular and an extended portion. Microcalorimetry... 

The assembly of polypeptide chains into functional fibrinogen molecules has been studied in several expression systems. The order in which the polypeptide chains are joined together by disulfide bonds has been determined and specific structural features that are important for assembly, such as the coiled-coil, have been identified (Xu et al., 1996). Substitution of the cysteines with serine showed that the interchain disulfide ring at the proximal end of the coiled-coil, in addition to the disulfides between the two halves of the molecule, are necessary for assembly of the two half molecules (Zhang and Redman, 1996). The distal interchain disulfide ring is not necessary for assembly of the two half molecules but is necessary for secretion. Disruption of intrachain disulfide bonds and deletions of portions of the polypeptide chains have revealed which... 

In homozygous forms of the dysfibrinogenemias, the mutations occur in all fibrinogen molecules, so only abnormal homodimers will be present. However, homozygous mutations are very rare, so most dysfibrinogenemias are heterozygous. As a consequence, fibrinogen molecules in most subjects consist of various proportions of mutant homodimers, normal homodimers, and heterodimers. 

Medved, L. V. (1990). Relationship between exons and domains in the fibrinogen molecule. Blood Coag. Fibrinolysis 1, 439—142. 

Mosesson, M. W., Hainfeld, J., Wall, J., and Haschemeyer, R. H. (1981). Identification and mass analysis of human fibrinogen molecules and their domains by scanning transmission electron microscopy. / Mol. Biol. 153, 695-718. 

Siebenlist, K. R., Meh, D. A., and Mosesson, M. W. (1996). Plasma factor XIII binds specifically to fibrinogen molecules containing gamma chains. Biochem. 35, 10448-10453. 

Both the intrinsic and extrinsic systems ultimately lead to the conversion of prothrombin to thrombin.92 Thrombin is an enzyme that quickly converts the inactive fibrinogen molecule to fibrin. Individual strands of fibrin bind together to form a meshlike structure, which ultimately forms the framework for the blood clot. Other cellular components, especially platelets, help reinforce the clot by sticking to the fibrin mesh. 

Schwinte P, Voegel J-C, Picart C et al (2001) Stabilizing effects of various polyelectrolyte multilayer films on the structure of adsorbed/embedded fibrinogen molecules an ATR-FTIR study. J Phys Chem B 15 11906-11916... 

Figure 2.30. Domain structure of fibrinogen. The fibrinogen molecule is composed of globular end regions separated from a central domain by helical threads. The central domain has cross-links that hold all the polypeptide chains together. Fibrinopeptides A and B (FPA and FPB) are found in the central domain.

Fibrinogen is a plasma protein, formed in the liver that is the basis for the formation of a blood clot. The blood concentration of this protein is about 3% and in the presence of other blood proteins pieces of the fibrinogen molecule, fibrinopeptides A and B, are cleaved and fibrinogen is polymerized to form a fibrin clot (Figure 2.30). 

Fibrinogen is a rather asymmetric molecule that consists of three pairs of identical polypeptide chains, called the a, /3, and y chains (MW 63,500, 56,000, and 47,000, respectively). Blood coagulation is initiated by the action of thrombin on fibrinogen, whereby two small peptides are removed from the fibrinogen. This permits fibrinogen molecules to aggregate via physical interactions to form... 

Platelet glycoprotein Ilb-IIIa receptor binding is given as fmol 125I-fibrinogen/108 platelets or 125I-fibrinogen molecules bound per platelet. 

---