Disulfide Bridges in the Middle Part of Human Fibrinogen

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Abstract
Human fibrinogen contains 29 disulfide bridges/molecule. The amino acid sequences around all half-cystine residues are known. When fibrinogen is cleaved by cyanogen bromide, 5 disulfide-containing fragments are formed. The 2nd-largest of them is derived from the middle part of all 3 peptide chains, it is monomeric and containd 345 amino acid residues, 12 of which are half-cystines. The arrangement of the 6 disulfide bonds was determined by analyzing sequences and amino acid compositions of subfragments isolated after cleavage with trypsin, thermolysin and staphylococcoal protease and after cleavage of the disulfide bonds. All half-cystine residues were linked in unique pairs. Six half-cystine residues, 2 in each of the 3 peptide chains and forming the -Cys-X-X-X-Cys-sequences, connected the chains in a ring-like structure, similar to the one in the N-terminal part of the molecule. The remaining 6 half-cystine residues connected 2 sections of the .gamma.-chain in a loop-like structure and 4 sections of the .beta.-chain in a loop-inside-a-loop-like structure, the inner .beta.-chain loop being homologous to the .gamma.-chain loop.

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