Illustration of a fibrinogen molecule
Hemostasis is a complex series of cell/cell, cell/protein, and protein/protein interactions designed to stem blood loss. Fibrinogen is a tridomainal disulfide linked plasma protein comprised of two symmetrical halves. Each half composed of three polypeptide chains termed Aα (red), Bβ (blue), and γ (green). Human fibrinogen can be separated into two major fractions by DEAE chromatography, fibrinogen 1 (‘peak 1 fibrinogen’) and fibrinogen 2 (‘peak 2 fibrinogen’). The two fibrinogens differ from each other with respect to their γ chain composition. Fibrinogen 1 contains two γA chains which are comprised of 411 amino acids. Heterodimeric fibrinogen 2 molecules contain one γA and one γ´ chain. The variant γ´ chain is longer (427 residues), and has a more anionic, carboxyl terminal sequence than the γA chain beyond position 408. For many years the physiological function of the γ´ chain was unknown. Recently factor XIII has been shown to bind specifically to γ´ chains in fibrinogen 2 and thrombin has been shown to bind to the anionic γ´ extension of fibrin 2. After a cascading series of zymogen activations the plasma protein fibrinogen is converted to fibrin by the thrombin mediated removal of four peptides, two from the Aα chain and two from Bβ. Fibrin assembly commences with formation of double stranded twisting fibrils, subsequently, lateral fibril associations occur, resulting in thick fibers. Concomitant with converting fibrinogen to fibrin, thrombin converts protransglutaminase (factor XIII) to an active transglutaminase (factor XIIIa). Factor XIIIa introduces covalent cross-links between appropriately positioned γ chains to form γ dimers and between α chains forming α polymers.
Illustration of a two stranded fibrin fibril
The interactions between fibrin(ogen) and factor XIII are incompletely understood. To this end I am actively investigating how factor XIII is carried in the plasma by fibrinogen 2, the activation of factor XIII to factor XIIIa by thrombin, the movement of the active enzyme through the fibrin matrix, the effects of γ tetramers and γ trimers on the properties of the fibrin matrix, the intrinsic transglutaminase activity of the zymogen form of factor XIII, and the incorporation of α2-plasmin inhibitor into fibrin(ogen) the the factor XIII zymogen. An in vitro fibrinogen and factor XIII expression system is in the process of being established. Using these systems and site directed mutagenesis to introduce specific changes into the proteins the interactions between these molecules will be probed. Studies designed to elucidate the interactions between fibrin(ogen)/fibrin(ogen) and thrombin/fibrin will also use the in vitro fibrinogen expression system. These latter studies are being performed in collaboration with the Fibrinogen Research Laboratory (Dr. M.W. Mosesson) at the Blood Research Institute of The Blood Center of Southeastern Wisconsin.