Effects of the N-Linked Glycans on the 3D Structure of the Free α-Subunit of Human Chorionic Gonadotropin

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Abstract
To gain insight into intramolecular carbohydrate−protein interactions at the molecular level, the solution structure of differently deglycosylated variants of the α-subunit of human chorionic gonadotropin have been studied by NMR spectroscopy. Significant differences in chemical shifts and NOE intensities were observed for amino acid residues close to the carbohydrate chain at Asn78 upon deglycosylation beyond Asn78-bound GlcNAc. As no straightforward strategy is available for the calculation of the NMR structure of intact glycoproteins, a suitable computational protocol had to be developed. To this end, the X-PLOR carbohydrate force field designed for structure refinement was extended and modified. Furthermore, a computational strategy was devised to facilitate successful protein folding in the presence of extended glycans during the simulation. The values for φ and ψ dihedral angles of the glycosidic linkages of the oligosaccharide core fragments GlcNAc2(β1−4)GlcNAc1 and Man3(β1−4)GlcNAc2 are restricted to a limited range of the broad conformational energy minima accessible for free glycans. This demonstrates that the protein core affects the dynamic behavior of the glycan at Asn78 by steric hindrance. Reciprocally, the NMR structures indicate that the glycan at Asn78 affects the stability of the protein core. The backbone angular order parameters and displacement data of the generated conformers display especially for the β-turn 20−23 a decreased structural order upon splitting off the glycan beyond the Asn78-bound GlcNAc. In particular, the Asn-bound GlcNAc shields the protein surface from the hydrophilic environment through interaction with predominantly hydrophobic amino acid residues located in both twisted β-hairpins consisting of residues 10−28 and 59−84.