The role of amino-terminal disulfide bonds in the structure and assembly of human fibrinogen.

Human fibrinogen contains two half-molecules, each composed of an alpha, beta, and gamma chain linked by disulfide bonds. The two half-molecules (alpha beta gamma) are held together in the native protein by additional disulfide bonds located in the amino terminus of each chain. Site-directed mutagenesis, in which the amino-terminal Cys residues (alpha-Cys28 and 36, beta-Cys65, and gamma-Cys8 and 9) were converted to Ser, was carried out in order to study the role of the amino-terminal disulfide bonds in the structure and assembly of fibrinogen. An analysis of the fibrinogen synthesized in transfected baby hamster kidney (BHK) cells employing various combinations of these mutations revealed that alpha-Cys36 and beta-Cys65 form disulfide bonds between two alpha beta gamma half-molecules, rather than within the same half-molecule; furthermore, these two disulfide bonds are sufficient to hold the two alpha beta gamma half-molecules together as intact fibrinogen. Disulfide bonds formed by gamma-Cys8 and 9 were also sufficient to hold the two fibrinogen alpha beta gamma half-molecules together, while the disulfide bond between the two alpha-Cys28 residues failed to form in the absence of the disulfide bonds linking the alpha and beta chains and the two gamma chains.