Structure of human hemopexin: O-glycosyl and N-glycosyl sites and unusual clustering of tryptophan residues.

The primary structure of human hemopexin is being deduced from sequence analysis of a series of peptides obtained from chemical and enzymatic digests of the protein. Human hemopexin consists of about 440 amino acid residues. It has five sites of attachment of glucosamine oligosaccharides at the signal sequence of Asn-X-Thr/Ser. A unique structural feature is the virtual blocking of the amino-terminal threonine residue, which is O-linked to a galactosamine oligosaccharide that has not previously been identified in this protein. The galactosamine oligosaccharide and one glucosamine oligosaccharide are located in the amino-terminal region, three of the glucosamine oligosaccharides are in the middle region, and one glucosamine oligosaccharide is in the carboxyl-terminal region of the protein. Two of the five glucosamine oligosaccharides are present in a histidine-rich sequence of the middle region of the protein, in which the histidines flank beta-turns presumably at the surface of hemopexin. Clusters of tryptophan residues occur in four regions, each of which contains three or four tryptophan residues separated by 0-12 other residues. This clustering is significant because both histidine and tryptophan have been implicated in the binding of heme. A computer analysis did not identify significant matches of human hemopexin to any protein, including cytochromes and other heme-binding proteins, which suggests that the human hemopexin gene evolved from a unique primordial gene differing from those of other heme-binding proteins.