Crystallographic structures indicate that ␥-chain residue Asn308 participates in D:D interactions and indeed substitutions of ␥Asn308 with lysine or isoleucine have been identified in dysfibrinogens with impaired polymerization. To probe the role of Asn308 in polymerization, we synthesized 3 variant fibrinogens: ␥Asn308 changed to lysine (␥N308K), isoleucine (␥N308I), and alanine (␥N308A). We measured thrombin-catalyzed polymerization by turbidity, fibrinopeptide release by high-performance liquid chromatography, and factor XIIIa-catalyzed crosslinking by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. In the absence of added calcium, polymerization was clearly impaired with all 3 variants. In contrast, at 0.1 mM calcium, only polymerization of ␥N308K remained markedly abnormal. The release of thrombin-catalyzed fibrinopeptide B (FpB) was delayed in the absence of calcium, whereas at 1 mM calcium FpB release was delayed only with ␥N308K. Factor XIIIa-catalyzed ␥-␥ dimer formation was delayed with fibrinogen (in absence of thrombin), whereas with fibrin (in presence of thrombin) ␥-␥ dimer formation of only ␥N308K was delayed. These data corroborate the recognized link between FpB release and polymerization. They show fibrin cross-link formation likely depends on the structure of protofibrils. Together, our results show substitution of Asn308 with a hydrophobic residue altered neither polymer formation nor polymer structure at physiologic calcium concentrations, whereas substitution with lysine altered both.
IntroductionFibrinogen is a 340-kDa glycoprotein that circulates in the blood at 5.88 to 11.76 M (2-4 g/L). It is composed of a pair of 3 polypeptide chains, A␣, B, and ␥, connected by 29 intrachain and interchain disulfide bonds. 1 The 6 chains are arranged into 3 globular nodules. The central E nodule contains the N-termini of all chains and the distal D nodules contain the C-termini of the , ␥, and a short segment of the ␣ chains. Coiled coils composed of all 3 chains link the E and D nodules. 2 The C-terminal residues of A␣ chains form globular nodules that are associated with one another, as well as with the E nodule. 3 Recent crystallographic studies have provided high-resolution structures of the C-terminal region of the ␥ chain, which contains functional sites for fibrin polymerization, calcium binding, platelet aggregation, and factor XIII (FXIII) cross-linking. [4][5][6] During coagulation, thrombin cleaves fibrinogen, releasing fibrinopeptides A (FpA) and B (FpB) from the N-termini of the A␣ and B chains, respectively, and converting fibrinogen to fibrin monomers (for reviews, see Blomback 1 and Doolittle 2 ). Fibrin monomers polymerize spontaneously through a 2-step process, with a second step being calcium-dependent. 7,8 The first step begins with the release of FpA. This exposes the "A" site, which interacts with the "a" site in the D nodule of another molecule, leading to the formation of half-staggered, double-stranded protofibrils. 9,10 Within protofibrils, individual fibrin mol...