Distribution of O-linked sugar units on hCG and its free alpha-subunit.
hCG, a glycoprotein hormone produced by the trophoblast in pregnancy, is composed of two dissimilar subunits, alpha and beta, joined non-covalently. hCG has four O-linked sugar units, all attached to the beta-subunit. The trophoblast also produces a free form of the alpha-subunit, which unlike the alpha-component of hCG, can contain an O-linked sugar unit. The structures of the O-linked sugar units were examined. Four structures were identified on urinary hCG. A hexasaccharide, NeuAc alpha 2-3Gal beta 1-3(NeuAc alpha 2-3Gal beta 1-4GlcNAc beta 1-6)GalNAc- accounting for 13%, a tetrasaccharide, NeuAc alpha 2-3Gal beta 1-3(NeuAc alpha 2-6)GalNAc-, for 34%, a trisaccharide, NeuAc alpha 2-3Gal beta 1-3GalNAc-, for 43% and a disaccharide, NeuAc alpha 2-6GalNAc- for 10% of the total O-linked sugar structures. Similar mixtures were found on peptides containing one, three or four sugar units suggesting a random distribution among attachment sites. The distribution of O-linked sugar structures on hCG and free alpha-subunit from trophoblast explant cultures was compared. The mixture of structures attached at the single site on the free alpha-subunit paralleled that at the four sites on the hCG.
D002236:Carbohydrate Conformation; D006063:Chorionic Gonadotropin / Q000032:analysis* / Q000652:urine; D015292:Glycoprotein Hormones, alpha Subunit; D006031:Glycosylation; D006801:Humans; D009844:Oligosaccharides / Q000032:analysis*; D010446:Peptide Fragments / Q000032:analysis* / Q000652:urine; D010908:Pituitary Hormones, Anterior / Q000032:analysis* / Q000652:urine; D014327:Trophoblasts / Q000032:analysis
D006063:Chorionic Gonadotropin; D015292:Glycoprotein Hormones, alpha Subunit; D009844:Oligosaccharides; D010446:Peptide Fragments; D010908:Pituitary Hormones, Anterior