During clotting, an end-to-end association ensues' (Fig. 1b), followed by a side-to-side aggregation of the filaments (Fig. 1c), producing a half-staggered overlap of individual molecules with a characteristic cross-striation of about 22.5 nm. In this array, the central nodule of one molecule interacts with the two end- to-end associated, terminal globules in the neighboring filament.
Composition
As far as chemical composition is concerned, human fibrinogen, with a molecular weight of 337,897, comprises two sets of  three open polypeptide chains (Aa, Bb, and Y) held together by disulfide bonds; thus, the composition can be expressed as(Aa2 Bb2 Y2  Only the Bb and rchains carry carbohydrate residues.
Each of the three polypeptides is encoded by a distinct messenger ribonucleic acid, which indicates that the of fibrinogen is the result of the concerted expression of three genes. The Aa chain is the largest (mol wt 66,026), followed by Bb  (mol wt 54,358) and Y chain (mol wt48,529). Though the exact mode of assembly of these constituent chains in the model given in Fig 1 is not yet known, they are arranged such known, they arranged such that all N termini are located in the central globular region and all C termini are found on the outermost structures. The long, flexible appendages protruding from each end of the molecule comprise the C-terminal regions of the Aa chains and the short appendages the C termini ofY chains. The complete amino acid sequences of all chains of human fibrinogen are known. Considerable evolutionary variations exist in vertebrates with regard to the sequences of N-terminal peptides in the Aa Bb chains (called fibrinopeptides A and B) and in the exposed C-terminal half of the Aa  chain. Clotting mechanism
It is now possible to reconstruct with reasonable certainty the sequence ofevents surrounding the clotting offibrinogen in normal plasma, and each of the reaction steps in the scheme in Fig. 2 can be studied separately with the use of purified components. Clotting is regulated by two enzymes, thrombin and factor XIIIa (fibrinoligase, activated fibrin-stabilizing factor, transglutaminase). Thrombin is a protease similar to trypsin but ofmuch greater specificity. It exerts a dual control by regulating the rate of fibrin formation as well as producing factor XIIIa by processes of limited proteolysis. In the fibrinogen-fibrin conversion, about 97% ofthe parent protein fibrin and 3% appears as fibrinopeptides A and B; in the reaction of thrombin with factor XIII, activation fragments are released from the N termini of the a subunits of factor XIII.The presence of fibrinogen greatly accelerates the latter reaction so that the production of fibrin and the activation of factor XIII are harmonized. In the plasma milieu, the fibrin molecules readily aggregate into a clot. In order to obtain a clot structure of a strength sufficient to stem bleeding, it is necessary for the  thrombin-modified factor XIII (indicated as XIII in ) to be activated to XIIIa This is promoted by fibrinogen and is accomplished by the action of Ca2+ ions which dissociate the catalytic subunits (a*2) from the inert, carrier subunits (b2)

Factor XIIIa acts as a transamidating enzyme which strengthens the fibrin clot by creating a few r-glutamyl-e-Iysine cross-links (Fig. 3). Without such cross-links, a clot structure would be like a brick wall without mortar.
Individuals with the hereditary absence of factor XIII often suffer from severe bleeding, even though their clotting times are in the normal range. In addition, various molecular disorders of fibrin stabilization are known in which some acquired inhibitor suddenly makes its appearance in the circulation, directed either against the activation of factor XIII or against the cross linking of fibrin by XIIIa. Several such inhibitors were found to be autoimmune IgG antibodies. Primary amines also specifically inhibit cross-linking without interfering with fibrin formation.