Disulfide bridges in the middle part of human fibrinogen.

Human fibrinogen contains 29 disulfide bridges per molecule. The amino acid sequences around all half-cystine residues are known. When fibrinogen is cleaved by cyanogen bromide five disulfide-containing fragments are formed. The second-largest of them is derived from the middle part of all three peptide chains, it is monomeric and contains 345 amino acid residues, 12 of which are half-cystines. The arrangement of the six disulfide bonds was determined by analysing sequences and amino acid compositions of subfragments isolated after cleavage with trypsin, thermolysin and staphylococcal protease and after clearage of the disulfide bonds. All half-cystine residues were found to be linked in unique pairs. Six half-cystine residues, two in each of the three peptide chains and forming the -Cys-X-X-X-Cys- sequences, were shown to connect the chains in a ring-like structure, similar to the one in the N-terminal part of the molecule. The remaining six half-cystine residues were found to connect two sections of the gamma-chain in a loop-like structure and four 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.