Title : Posttranslational modifications of human inter-alpha-inhibitor: identification of
glycans and disulfide bridges in heavy chains 1 and 2
Abstract :
- Inter-alpha-inhibitor (IalphaI) is a serine proteinase inhibitor found in high concentrations in human plasma
- The protein is composed of a light inhibitory chain called bikunin and two heavy chains of unknown function
- The three polypeptide chains are covalently assembled via a carbohydrate cross-link [Enghild , J. J., Salvesen, G., Hefta, S. A., Thogersen, I. B., Rutherfurd, S., & Pizzo, S. V. (1991) J. Biol
- Chem
- 266, 747-751]
- The aim of this study was to complete the primary structure by characterizing additional covalent posttranslational modifications of the heavy chains
- Analysis revealed three N-linked oligosaccharides located on Asn251 and Asn554 of heavy chain 1 and on Asn64 of heavy chain 2: all these were complex biantennary structures composed of (Asn)-GlcNAc2-Man-(Man-GlcNAc-Gal-SA)2. Asn )-GlcNAc2-Man-(Man-GlcNAc-Gal-SA)2
- In addition, the IalphaI heavy chains carried several O-linked glycans located on Thr619 of heavy chain 1 and a cluster of four O-linked oligosaccharides on Thr612, Ser619, Thr621, and Thr637 of heavy chain 2
- The oligosaccharides were short (Ser/Thr)-GalNAc-Gal-SA trisaccharides.
- The IalphaI heavy chains contain nine Cys residues , of which eight are involved in disulfide bridges
- The unpaired Cys residue residing on heavy chain 1, Cys26 , appears to be modified by dihexosylation
- The other Cys residues exclusively form intrachain disulfide bridges
- In heavy chain 1 the two disulfide bonds are formed between Cys210 and Cys213 and between Cys234 and Cys506 , and in heavy chain 2, between Cys207 and Cys210 and between Cys596 and Cys597
- Interestingly, three of these four disulfides are formed between Cys residues that are either adjacent or only two amino acid residues apart