Primary Structure of Human Fibrinogen and Fibrin

Chemical cleavage of methionyl bonds in human fibrinogen with cyanogen bromide produces several large fragments. One of these (N-DSK) contains NH2-terminal portions of the three chains of the parent molecule. The NH2-terminal portion of the γ chain of fibrinogen has been isolated. The γ chain fragment has tyrosine as NH2-terminal residue and homoserine as COOH-terminal residue and therefore evidently consists of one single polypeptide chain. It has a molecular weight of 11,500 as determined by ultracentrifugal analysis. molecular weight of 12,700 was found by gel electrophoresis in sodium dodecyl sulfate. The charge heterogeneity of the γ chain fragment observed upon electrophoresis in polyacrylamide gels is probably due to microheterogeneity of its carbohydrate moiety. The NH2-terminal γ chain fragment consists of 78 amino acid residues. The amino acid sequence has been determined. The fragment contains about half of the half-cystines of the whole γ chain. They occur in a cluster, which indicates an uneven distribution of half-cystines within the γ chain of fibrinogen. One of the half-cystine-containing segments shows similarities in sequence with segments in the A α and B β chains. The carbohydrate moiety in the γ chain consists of 2 residues of glucosamine, 6 residues of galactose and mannose, and 1 to 2 residues of sialic acid. The carbohydrate moiety is most likely linked to residue 52. Some differences in specificity of trypsin, plasmin, and thrombin when acting on the γ chain are discussed.