Characterization of the B-chain of human plasma α2HS-glycoprotein. The complete amino acid sequence and primary structure of its heteroglycan

Abstract

α2HS-Glycoprotein, a normal human plasma protein, was recently shown to consist of two polypeptide chains. In the present study, we have separated these two chains from one another and have elucidated the complete primary structure of the B-chain. Employing automated Edman degradation, the polypeptide moiety of this chain was shown to consist of 27 amino acid residues with an unequal distribution of the neutral and charged amino acid residues. The first 20 residues are uncharged, whereas the carboxyl-terminal heptapeptide contains all charged residues. Utilizing 500-MHz 1H-NMR spectroscopy, the carbohydrate unit proved to be a trisaccharide consisting of sialic acid, galactose, and N-acetylgalactosamine O-glycosidically linked to serine (residue 6). The structure of the B-chain was found to be as follows. (formula; see text) Thus, the molecular weight of the B-chain is 3386. Evaluation of the polypeptide chain by the procedure of Chou and Fasman (Chou, P.Y., and Fasman, G.D. (1979) Adv. Enzymol. 47, 45- 148) predicts that the B-chain has two beta-turns. Thereby, the carbohydrate unit which is linked to the Ser residue located in the first β-turn appears to be directed away from the protein. The second β-turn probably includes the Cys residue which links the B- to the A-chain. In agreement with the CD analysis, the B-chain lacks beta-conformation but possesses a short α-helical region.