Essentials of Glycobiology Chapter 1: Historical Background and Overview

1. Essentials of GlycobiologyChapter 1: Historical Background and Overview Alexander Hsieh

2. Overview • Historical Background of the Field of Glycobiology • General Terms and Definitions • Common Monosaccharide Units of Glycoconjugates • Nomenclature and Symbols in Structural Depictions • General Oligosaccharide Classes • Biological Implications Related to Synthesis and Function

3. Historical Background What is Glycobiology? • Central Dogma of molecular biology: DNA ->RNA ->Protein • Common incorrect (why?) extrapolation: DNA -> RNA -> Protein -> cell -> organism • All cells in nature carry Oligosaccharides and Glycans in a dense and complex way • Corrected “New Dogma”:

4. Historical Background cont’d • Why was there a need for a new dogma? • Study of glycans lagged behind other major classes: • Inherent structural complexity • Difficulty of sequence determination • Biosynthesis cannot be predicted directly from DNA template However… • In 1988, Rademacher, Parekh, and Dwek recognized the coming together of several traditional disciplines and coined the term “Glycobiology” Formal Definition: • Glycobiology: the study of the structure, biosynthesis, and biology of saccharides (sugar chains or glycans) that are widely distributed in nature and the proteins that recognize them • Universal Principles of Glycobiology

5. General Terms and Definitions • Carbohydrate: Polyhydroxyaldehydes or polyhydroxyketones or larger compounds that can be hydrolyzed into such units • Monosaccharide: Carbohydrates that cannot be hydrolyzed into simpler units • Oligosaccharides: Branched or linear chains of monosaccharides attached by glycosidic linkages • Glycosidic Linkage: Attachment of monosaccharides to one another through the hydroxyl group at the chiral center • Glycoconjugate: A compound in which one or more monosaccharides or oligosaccharides (“glycones”) are covalently linked to a noncarbohydrate functional group (“aglycone”)

6. Common Monosaccharide Units of Glycoconjugates • Pentoses: 5-carbon sugars • Ex: D-xylose (Xyl) • Hexoses: 6-carbon neutral sugars • Ex: D-glucose (Glc), D-galactose (Gal), or D-mannose (Man) • Hexosamines: Hexoses with an amino group at the 2-position • Ex: N-acetyl-D-glucosamine (GlcNAc), N-acetyl-D-galactosamine (GalNAc) • Deoxyhexoses: 6-carbon neutral sugars with a hydroxyl group at the 6-position • Ex: L-fucose (Fuc) • Uronic Acids: Hexoses with negatively charged a carboxylate at the 6-position • Ex: D-gluronic Acid (GlcA) or L-iduronic Acid (IdA) • Sialic Acids: Family of 9-carbon acidic sugars • Ex: N-acetylneuraminicacid (Neu5Ac)

7. Nomenclature and Symbols in Structural Depictions Isomers differentiated by color/black/white/shading Modifications to monosaccharide units are listed before the name Numbers represent linkages Same shading/color used for different monosaccharides with same stereochemistry

8. General Oligosaccharide Classes • Glycoproteins • Defined as glycoconjugates in which a protein carries one or more glycans covalently attached to a polypeptide backbone, usually by N- or O- Linkages • N-Glycans(N-linked oligosaccharides or N-(Asn)-linked) • Defined as a sugar chain covalently linked to an asparagine residue of a polypeptide chain; follows the sequence: • Asn – X – Ser/Thr • O-Glycans(O-linked oligosaccharides or O-(Ser/Thr)-linked) • Typically linked to the polypeptide via N-acetylgalactosamine (GalNAc) to a hydroxyl group of a serine or threonine residue • Proteoglycan • Defined as a glycoconjugate that has one or more glycosaminoglycan (GAG) chains attached to a core protein through a region ending in a xylose residue linked to the hydroxyl group of a serine residue

9. General Oligosaccharide Classes cont’d • Glycophosphatidylinositol Anchor • Defined as a glycan bridge between phosphatidyliniositol and a phosphoethanolamine that is in amide linkage to the carbxyl end of a protein; constitutes the only anchor to the lipid bilayer membrane for the proteins • Glycosphingolipid (GSL, or glycolipid for short) • Consist of a glycan attached via glucose or galactose to the terminal primary hydroxyl group of the lipid functional group ceramide (composed of a long chain base (sphingosine) and a fatty acid)

10. Biological Implications related to Synthesis and Function • Most biosynthesis of glycans confined within ER and Golgi compartments, much synthesis takes place internally • Most newly synthesized proteins originating from the ER are modified by glycosyltransferases at various stages while traveling to their final destination, allows for much diversity • Like all components of living cells, glycans are constantly being synthesized, used, then degraded (the turnover); circle of life • Glycan function falls into 2 categories: • Structural and Modulatory functions • (involve the glycans themselves or or their movulation of the molecules to which they are attached) • Specific recognition of Glycans by Glycan-Binding Proteins • (can lead to glycan-protein interactions central to the processes of the organism) • The field of Glycobiology is constantly evolving • Elucidation of additional glycan function/synthesis leads to important discoveries relevant to areas such as Genetics, Evolutionary Biology, Medicine, and Biotechnology.