The Basic Concept of Glycosylation

Glycobiology aims at structure-function correlational analysis of carbohydrates (sugar or glycan). A monosaccharide is the simplest form of carbohydrate that no longer be hydrolyzed. The other forms of carbohydrates are formed by glycosidic linkages of monosaccharides, such as disaccharides, oligosaccharides, and polysaccharides, comprising two, three to ten, and more than ten monosaccharides, respectively. Carbohydrates act as one of the major energy sources (e.g., ATP) and are also involved in cellular protection, stabilization, organization, and barrier functions. In the cellular system, carbohydrates are present in pure and proteinconjugated forms, which are referred to as glycoproteins. Conjugated carbohydrates are also present in the form of glycolipids and proteoglycans. Notably, N- and O-linked glycosylation as major forms occur in the rough surface endoplasmic reticulum (RER) and Golgi apparatus respectively, adding carbohydrates to proteins and thus making glycoproteins. Relatively fewer common types of glycosylation are the C-linked glycosylation, S-linked glycosylation, glypiation, and phosphoglycosylation. A complex interplay of two enzyme groups such as glycosyl transferases (adding carbohydrates to proteins) and glycosidases/glycosyl hydrolases (removing carbohydrates from proteins) control the glycosylation extent. Prominent cellular factors regulating glycosylation are the availability of carbohydrates, proteins, enzymes, movement of proteins from RER to Golgi, and several other environmental factors regulating post-translational modifications. This chapter describes the various aspects of glycobiology including protein glycosylation, purification, and analysis of glycans, and their role in physiology and pathophysiology.