Function and 3D Structure of the N-Glycans on Glycoproteins

Abstract

Glycosylation is one of the most common post-translational modifications in eukaryotic cells and plays important roles in many biological processes, such as the immune response and protein quality control systems. It has been notoriously difficult to study glycoproteins by X-ray crystallography since the glycan moieties usually have a heterogeneous chemical structure and conformation, and are often mobile. Nonetheless, recent technical advances in glycoprotein crystallography have accelerated the accumulation of 3D structural information. Statistical analysis of “snapshots” of glycoproteins can provide clues to understanding their structural and dynamic aspects. In this review, we provide an overview of crystallographic analyses of glycoproteins, in which electron density of the glycan moiety is clearly observed. These well-defined N -glycan structures are in most cases attributed to carbohydrate-protein and/or carbohydrate-carbohydrate interactions and may function as “molecular glue” to help stabilize inter- and intra-molecular interactions. However, the more mobile N -glycans on cell surface receptors, the electron density of which is usually missing on X-ray crystallography, seem to guide the partner ligand to its binding site and prevent irregular protein aggregation by covering oligomerization sites away from the ligand-binding site.

Expression and glycoengineering of functionally active heteromultimeric IgM in plants

“…To date, the biological relevance as well as the processes that drive this shift in glycosylation are poorly understood (33). The glycosylated model of pentameric IgM presented here explains altered glycosylation patterns as a consequence of different accessibility of the glycosylation sites.…”

Sweetened kallikrein-related peptidases (KLKs): glycan trees as potential regulators of activation and activity

“…N-glycosylation at Asn159 in KLK4 could serve as such stabilizer for the tetrameric and octameric building blocks observed in the crystal structures, as it fits well in the gap between subunits (2BDI) (Mekdes Debela et al, 2006a). More generally, mobile or disordered glycan trees enable or support protein interaction or ligand docking, as observed for membrane receptors and particularly large binding partners (Nagae and Yamaguchi, 2012). Thus, glyco-KLK interaction with substrates and regulators, which might themselves represent carriers of glycans, may depend to a great extent on the carbohydrates involved.…”

“…In the case of multiple glycosylation sites within one molecule, e.g., KLK1 with three N-glycans and three O-glycans surrounding Brought to you by | MIT Libraries Authenticated Download Date | 5/11/18 6:39 PM the active site cleft, their interplay may strongly affect substrate access, possibly like a filter, while additional conformational changes of the 99-and 148-loop may considerably influence the substrate turnover. Also, single glycan trees at certain positions of monomer subunits can serve as guides and stabilizers upon oligomerization, as in the hexameric insect storage protein arylphorin (Nagae and Yamaguchi, 2012). N-glycosylation at Asn159 in KLK4 could serve as such stabilizer for the tetrameric and octameric building blocks observed in the crystal structures, as it fits well in the gap between subunits (2BDI) (Mekdes Debela et al, 2006a).…”

“…Frequently, glycan trees serve as distinct and specific modules in molecular recognition, either in intramolecular interaction, as seen in various antibody Fc fragments, or in intermolecular interactions, as observed in the neonatal Fc receptor complex with heterodimeric Fc (hdFc) (Nagae and Yamaguchi, 2012). In the case of multiple glycosylation sites within one molecule, e.g., KLK1 with three N-glycans and three O-glycans surrounding Brought to you by | MIT Libraries Authenticated Download Date | 5/11/18 6:39 PM the active site cleft, their interplay may strongly affect substrate access, possibly like a filter, while additional conformational changes of the 99-and 148-loop may considerably influence the substrate turnover.…”

Mannoside recognition and degradation by bacteria

“…1A). Mammalian N-glycans found on mature proteins are rarely of the HMNG class, but are rather hybrid or complex, HMNG being mostly restricted to immature proteins (Nagae & Yamaguchi, 2012).…”