CRYSTAL-STRUCTURE OF A 30 KDA C-TERMINAL FRAGMENT FROM THE GAMMA-CHAIN OF HUMAN FIBRINOGEN

Background: Blood coagulation occurs by a cascade of zymogen activatio n resulting from minor proteolysis, The final stage of coagulation inv olves thrombin generation and limited proteolysis of fibrinogen to giv e spontaneously polymerizing fibrin, The resulting fibrin network is c ovalently crosslinked by factor XIIIa to yield a stable blood clot, Fi brinogen is a 340 kDa glycoprotein composed of six polypeptide chains, (alpha beta gamma)(2), held together by 29 disulfide bonds. The globu lar C terminus of the gamma chain contains a fibrin-polymerization sur face, the principal factor XIIIa crosslinking site, the platelet recep tor recognition site. and a calcium-binding site, Structural informati on on this domain should thus prove helpful in understanding clot form ation. Results: The X-ray crystallographic structure of the 30 kDa glo bular C terminus of the gamma chain of human fibrinogen has been deter mined in one crystal form using multiple isomorphous replacement metho ds. The refined coordinates were used to solve the structure in two mo re crystal forms by molecular replacement; the crystal structures have been refined against diffraction data to either 2.5 Angstrom or 2.1 A ngstrom resolution. Three domains were identified in the structure, in cluding a C-terminal fibrin-polymerization domain (P), which contains a single calcium-binding site and a deep binding pocket that provides the polymerization surface. The overall structure has a pronounced dip ole moment, and the C-terminal residues appear highly flexible. Conclu sions: The polymerization domain in the gamma chain is the most variab le among a family of fibrinogen-related proteins and contains many aci dic residues. These residues contribute to the molecular dipole moment in the structure, which may allow electrostatic steering to guide the alignment of fibrin monomers during the polymerization process. The f lexibility of the C-terminal residues, which contain one of the factor XIIIa crosslinking sites and the platelet receptor recognition site, may be important in the function of this domain.