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