Structural Studies of Human Hemopexin

The primary structure of human hemopexin (Hpx) is being deduced from sequence analysis of a series of peptides obtained from chemical and enzymatic digests of the protein. Human Hpx consists of about 440 amino acid residues. It has five sites of attachment of GlcN oligosaccharides at the signal sequence of Asn-X-Thr/Ser. A unique structural feature is the virtual blocking of the N-terminal threonine residue which is O-linked to a GalN oligosaccharide that has not previously been identified in this protein. Two of the five GlcN oligosaccharides are present in a histidine-rich sequence, in which the histidines flank beta-turns presumably at the surface of Hpx. Clusters of tryptophan residues occur in four regions, each of which contains three or four tryptophan residues separated by O to 9 other residues. This clustering is significant because both histidine and tryptophan have been implicated in the binding of heme in Hpx and other heme proteins. Limited tryptic digestion of Hpx to define the heme binding domain split the protein into two fragments with Mr=15,000 and 45,000; these are connected by a disulfide bridge. The heme-binding site is probably in the 45,000-dalton fragment because a histidine-rich sequence in the fragment showed some homology with that of the heme-binding site of cytochrome c.