Amino acid sequence studies on the .alpha. chain of human fibrinogen. Complete sequence of the largest cyanogen bromide fragment

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Abstract
The largest fragment produced by complete cyanogen bromide digestion of the .alpha. chain of human fibrinogen contains 236 residues and has a calculated MW of 23,949. The complete amino acid sequence of the fragment was determined by the isolation of peptides generated by plasmin, trypsin (including digestion of citraconylated material), staphylococcal protease and chymotrypsin. Some key subfragmentation was achieved by selective chemical cleavage at tryptophan residues. The fragment had unusual amino acid composition, more than half of its residues being glycine, serine, threonine and proline. There were few nonpolar residues, although 7 of the .alpha.-chain''s 10 tryptophans occur in this fragment. The fragment contains 2 cysteine residues located 30 residues apart, connected by an intrachain disulfide bond in the native molecule. The tryptophans occurred with a definite periodicity that highlighted a series of 13-residue homology repeats. The fragment also contained the 2 principal .alpha.-chain cross-linking sites.