Why is structural characterization of biological glycans on glycoconjugates so challenging, and what two biochemical approaches have been developed to address these challenges?

To understand the challenges of structural characterization of glycans, it is important to recognize that glycans (complex carbohydrates) are a diverse group of molecules found on glycoconjugates. Glycoconjugates are glycoproteins, glycolipids, or proteoglycans with one or more glycans linked to the protein or lipid component. The structural characterization of a glycan refers to its specific sequence and branching configurations.

One of the main challenges is the complexity of glycan structures. Glycans are composed of monosaccharide residues that can be linked in a variety of ways. The complexity arises from the fact that numerous monosaccharides can be combined in various configurations, including different stereochemistries of the glycosidic bond, branching, and linkage position. This variety results in a great diversity of glycan structures, making their characterization a considerable challenge.

Another challenge in characterizing glycans is the heterogeneity in glycan populations. Glycan biosynthesis is not template-driven, unlike DNA, RNA, and proteins. Therefore, glycans synthesized by the same cell can be similar, but not identical. This heterogeneity further complicates the characterization process, as it becomes necessary to distinguish subtle variations in the glycan structure.

Mass spectrometry (MS) is a powerful analytical technique for characterizing glycans because it can provide information on the molecular mass and sequence of the glycan. MS can be used to analyze glycans obtained by enzymatic digestion of glycoconjugates or chemical cleavage. By comparing the observed mass with predicted masses of known glycans, the candidates for a specific glycan structure can be narrowed down. Additionally, tandem mass spectrometry (MS/MS) can provide more detailed structural information, including linkage position and branching patterns.

Nuclear Magnetic Resonance (NMR) spectroscopy is another biochemical approach used for the structural characterization of glycans. NMR can provide detailed structural information about glycan structure, such as the monosaccharide composition, anomeric configuration, and linkage position. It uses the magnetic properties of atomic nuclei in the presence of an external magnetic field. The NMR signals will provide information to help establish the glycan's structural characteristics. It is particularly useful for studying larger, more complex glycans and offers complementary information to mass spectrometry. In conclusion, structural characterization of biological glycans is challenging due to their complexity and heterogeneity. Two biochemical approaches, mass spectrometry and nuclear magnetic resonance, have been developed to address these challenges and provide detailed information about glycan structures.