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Introduction

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High-Yield Terms

  • Sugar code: refers to the total complement of sugars in an organism and how it relates to normal and abnormal physiology and pathology, also called the glycome

  • N-glycans: glycoprotein family with carbohydrate attachment to asparagine residues

  • O-glycans: glycoprotein family with carbohydrate attachment to serine, threonine, or hydroxylysine hydroxyl groups

  • Lipid-linked oligosaccharide (LLO): refers to the core glycan structure attached to the lipid dolichol pyrophosphate prior to addition to an asparagine residue of an N-linked glycoprotein; the core glycan structure is also referred to as the en bloc oligosaccharide

  • Hexosamine biosynthesis pathway (HBP): pathway for conversion of glucose to O-GlcNAc, which is important for the carbohydrate modification of numerous cytoplasmic and nuclear proteins

  • Lectin: any of a family of proteins that contain a carbohydrate-binding domain; lectin is derived from the Latin word meaning “to select”

  • Mucin: any of a family of high molecular weight, heavily glycosylated proteins with the ability to form gels that lubricate and form chemical barriers

  • Congenital disorders of glycosylation (CDG): a family of diseases resulting from inherited defects in the synthesis, processing, and modification of both N-linked and O-linked glycoproteins and related glycan-modified molecules

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High-Yield Concept

This significance underscores the relatively new field of study called glycomics, which analogous to genomics and proteomics, involves the analysis of the glycome (the total complement of sugars in an organism) and how it relates to normal and abnormal physiology and pathology. The glycome can be considered the sugar code of an organism.

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Membrane-associated carbohydrate is exclusively in the form of oligosaccharides covalently attached to proteins, forming glycoproteins and to a lesser extent covalently attached to lipid, forming the glycolipids. Most proteins that are secreted, or bound to the plasma membrane, are modified by carbohydrate attachment. The posttranslational attachment of carbohydrate to proteins plays a critical role in overall biochemical complexity in humans. The importance of this protein modification can be emphasized by the fact that approximately 50% of all human proteins are known to be glycosylated and at least 1% of the human genome is represented by genes involved in glycan synthesis, processing, and homeostasis.

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In addition to their critical roles in normal biochemical and physiological processes, the carbohydrate structures of many glycoproteins have been hijacked by pathogens and used as attachment sites for entry into host cells. HIV, poxviruses, parvoviruses, rhinoviruses, and herpes virus 6 all gain entry into cells by attaching to cell-surface glycoproteins. In addition, malarial parasite infection into erythrocytes involves recognition of cell-surface glycoproteins. Even bacteria utilize glycoproteins for attachment to host cells. Helicobacter pylori, the causative agent for one of the most common forms of cancer in humans, adenocarcinoma, adheres to gastric mucosal cells via glycoprotein recognition.

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Glycoproteins

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The predominant sugars found in glycoproteins are glucose (Glc), galactose (Gal), mannose (Man), fucose (Fuc), N-acetylgalactosamine (GalNAc), N-acetylglucosamine (GlcNAc), and N...

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