Amino acid sequence studies on the alpha chain of human fibrinogen. Covalent structure of the alpha-chain portion of fragment D.

The alpha-chain portion of fragment D has been purified from an exhaustive plasmic digest of human fibrinogen. The major polypeptide species has 91 amino acid residues, although a small amount of a 97-residue chain representing an earlier digestion stage remains. The amino acid sequence of the first 44 residues was determined by stepwise degradation with an automatic solid-phase sequencer. Another large stretch of sequence was revealed by the finding that the alpha chain of fragment D overlaps the cyanogen bromide fragments alphaCNIVA and alphaCNIII (Doolittle, R. F. Cassman, K. G., Cottrell, B. A., Friezner, S. J. Hucko, J. T., and Takagi, T. (1977), Biochemistry 16 (preceding paper in this issue)). The automatic sequencer results were confirmed and extended by the isolation and characterization of 18 of 19 expected tryptic peptides from the fragment D alpha chain. As a result, almost the entire sequence has been obtained. The overlap with key cyanogen bromide fragments has also allowed us to propose an order for the first 198 residues of the fibrinogen alpha chain. A striking homology with the gamma chain and beta chain is apparent which has interesting structural implications.