The primary structure of human serum transferrin. The structures of seven cyanogen bromide fragments and the assembly of the complete structure

The amino acid sequences of seven cyanogen bromide fragments of human serum transferrin have been determined, and the primary structure of transferrin established by determining the order of these and three additional fragments (Sutton, M. R., MacGillivray, R. T. A., and Brew, K. (1975) Eur. J. Biochem. 51, 43-48) in the polypeptide chain. The order of the fragments was deduced from peptides that overlap methionyl residues which were obtained by thermolysin digestion of performic acid-oxidized transferrin or by partial peptic hydrolysis of unmodified transferrin, together with other evidence. The polypeptide chain of transferrin contains 679 amino acid residues, which together with the two N-linked oligosaccharide chains gives a calculated molecular weight of 79,570. Transferrin consists of two homologous domains (residues 1-336, 337-679), each associated with a single Fe-binding site, with both sites of glycosylation in the carboxyl-terminal domain at positions 413 and 611. Consideration of the primary structure in relation to previously published results provides information concerning the evolutionary development of transferrins and related proteins, and the locations of metal-binding residues in the transferrin molecule.