Circular dichroism of glycopeptide fractions from α1-acid glycoprotein, thyroglobulin, and ovalbumin

Abstract

Using sequential pronase digestions, glycopeptide fractions were prepared from human α 1-acid glycoprotein, hen egg ovalbumin, and bovine thyroglobulin, two types of glycopeptides being obtained from the latter. The fractions were characterized on the basis of hexose, hexosamine, sialic acid, and peptide content. The glycopeptide fraction from α 1-acid glycoprotein is complex (i.e., the carbohydrate moiety contains mannose, galactose, N-acetylglucosamine, and sialic acid), as is one of the glycopeptide fractions from thyroglobulin (type I). The glycopeptide fraction from ovalbumin and the type II glycopeptide fractions from thyroglobulin are simple (i.e., the carbohydrate moiety contains only mannose and N-acetylglucosamine). The circular dichroic spectra of the two complex glycopeptide fractions and the ovalbumin glycopeptide fraction were similar and were characterized by a negative extremum between 207.5 nm and 211 nm with magnitudes in the range of —6400 deg·cm 2·dmol −1 to —7200 deg·cm 2·dmol −1 (referred to the molar concentration of N-acetylated sugars). The thyroglubulin type II glycopeptide fraction exhibited a circular dichroic spectrum with an extremum of —29 200 deg·cm 2·dmol −1 at 205 nm. Removal of sialic acid from the complex glycopeptide fractions greatly increased the (negative) magnitude of ellipticity at the extremum. The circular dichroic spectra of the complex glycopeptide fractions were reasonably additive using the spectra of monomeric sialic acid and the asialo-derivatives. This demonstrates that the contributions of sialic acid to the circular dichroic spectrum are nearly additive. The implications of this observation are that covalent attachment of these terminal residues to the oligosaccharides does not lead to strong interactions with other chromophores nor to positioning in a particularly asymmetric environment. In contrast, the magnitudes of the observed ellipticity extrema in the circular dichroic spectra of the asialo-derivatives, in which N-acetylglucosamine is the major chromophore, are much greater than can be accounted for on the basis of monomeric contributions (i.e., free N-acetylglucosamine). This finding shows that the optical activity of N-acetylglucosamine is greatly influenced by the formation of the carbohydrate core in glycoproteins and suggests the possible formation of secondary structure in the carbohydrate moiety.