The structures of the carbohydrate moieties of the alpha subunit of human chorionic gonadotrophin

Abstract

The alpha subunit of human chorionic gonadotrophin was reduced with dithiothreitol followed by carboxymethylation with iodoacetic acid. The modified glycoprotein was hydrolysed with trypsin to give various peptides, the identities of which were established, and glycopeptides. The glycopeptides were separated by gel filtration and ion-exchange chromatography; they were subjected to component analysis and were found to represent the two carbohydrate moieties in the parent glycoprotein. Sequential removal with glycoside hydrolases of monosaccharide units from the glycopeptides demonstrated (1) that galactose, mannose, glucosamine (2-amino-2-deoxyglucose) and neuraminic acid (5-amino-3,5-dideoxy-glycero-galacto-2-nonulosonic acid) residues possess the D configurations, (2) that the glucosamine units are N-acetylated and (3) the order of the monosaccharide units in the chain, the neuraminic acid units being furthest from the peptide backbone of the subunit and substituting the D-galactose units. Methylation analysis of the glycopeptides by adaptation of the Hakomori technique demonstrated that: (4) D-galactose, D-mannose and N-acetylglucosamine (2-acetamido-2-deoxy-D-glucose) units exist in the pyranose forms; (5) the D-galactopyranose units are linked in the 1 and 6 positions; (6) the D-mannopyranose units exist in several forms, one in a terminal non-reducing position, one as 1,2-linked residues and some as 1,6-linked branch points; (7) the N-acetylglucosamine units are 1,6-linked. On the basis of the results of methylation and enzymic analysis, structures are proposed for the carbohydrate moieties and the assignments are compared with other data previously obtained by periodate-oxidation studies [Kennedy et al. (1974) Carbohydr. Res. 36, 369-377].