Carbohydrates and Glycobiology Study Notes

Carbohydrates and Glycobiology

Overview of Carbohydrates

  • Definition: Carbohydrates are organic compounds composed of carbon (C), hydrogen (H), and oxygen (O), typically in the form of Cn(H2O)n.

  • Production: Synthesized in plants through photosynthesis, using carbon dioxide (CO2) and water (H2O).

  • Molecular Weight Range: From small molecules like glyceraldehyde (molecular weight = 90 g/mol) to large polysaccharides like amylopectin (molecular weight = 200,000,000 g/mol).

  • Key Functions:

    • Energy Source: Carbohydrates serve as a primary energy source and storage (e.g., starch and glycogen).

    • Structural Components: Components of cell walls and exoskeletons (e.g., cellulose).

    • Information Molecules: Involved in cell signaling and recognition.

  • Glycosylation: Carbohydrates can be covalently attached to proteins, forming glycoproteins and proteoglycans.

Classification of Carbohydrates

  • Monosaccharides: The simplest form of carbohydrates.

    • Examples: Glucose, fructose, galactose.

  • Disaccharides: Composed of two monosaccharides.

    • Examples: Sucrose (glucose + fructose), lactose (glucose + galactose).

  • Polysaccharides: Large molecules consisting of long chains of monosaccharide units.

    • Description: Can be branched or unbranched and are the primary form of carbohydrate storage.

  • Glycoconjugates: Include glycoproteins and glycolipids, where carbohydrates are covalently bonded to proteins or lipids.

Structures of Monosaccharides

  • Aldoses and Ketoses:

    • Aldoses contain an aldehyde group.

    • Ketoses contain a ketone group.

  • Examples:

    • Glyceraldehyde (aldotriose).

    • Dihydroxyacetone (ketotriose).

Stereochemistry of Sugars

Stereoisomers

  • Enantiomers: Stereoisomers that are non-superimposable mirror images. Designated as D or L based on the chiral center furthest from the carbonyl group:

    • Example: D-Glucose and L-Glucose.

    • Most hexoses in nature are D-isomers, such as D-glucose.

  • Diastereomers: Stereoisomers that are not mirror images and have different physical properties.

    • Example: Erythrose and threose differ in solubility.

  • Epimers: A subtype of diastereomers that differ at a single carbon atom's configuration.

    • Example: D-Mannose (epimer at C-2 of glucose).

Drawing Monosaccharides

  • Fischer Projections: Commonly used representation for chiral compounds.

    • Horizontal bonds extend toward the viewer and vertical bonds extend away.

Ring Structures of Monosaccharides

Cyclization Process

  • Pentoses and hexoses readily undergo intramolecular cyclization, forming rings (pyranoses or furanoses).

Anomeric Carbon

  • The carbon that becomes a chiral center during cyclization is known as the anomeric carbon.

  • The configuration of the anomer (α or β) is determined by the position of the hydroxyl group relative to the CH2OH group:

    • α-Anomer: Hydroxyl group on the opposite side of the CH2OH moiety (trans).

    • β-Anomer: Hydroxyl group on the same side as the CH2OH moiety (cis).

Reducing and Nonreducing Sugars

  • Reducing Sugars: Sugars that can reduce metal ions and have a free aldehyde or ketone group.

    • Examples: Glucose, maltose.

  • Nonreducing Sugars: Sugars that do not have a free anomeric carbon.

    • Example: Trehalose.

Polysaccharides

Types of Polysaccharides

  • Homopolysaccharides: Polymers made of the same monosaccharide.

  • Heteropolysaccharides: Polymers made from two or more different monosaccharides.

  • Structure: Polysaccharides do not have a defined molecular weight unlike proteins since they are not synthesized using a template.

Examples of Polysaccharides

  • Glycogen: A branched homopolysaccharide of glucose, serving as storage in animals.

    • Structure: Glucose units linked by α(1→4) bonds with branch points every 8–12 residues by α(1→6) bonds.

  • Starch: A mixture of amylose (unbranched α(1→4) linked) and amylopectin (branched with α(1→6) links every 24-30 residues).

  • Cellulose: A linear homopolysaccharide of glucose with β(1→4) linkages, resistant to hydrolysis.

  • Chitin: A linear homopolysaccharide of N-acetylglucosamine, forming exoskeletons of arthropods.

Glycosaminoglycans and Proteoglycans

  • Glycosaminoglycans (GAGs): Linear polymers of disaccharide units, often negatively charged and involved in ECM structure and lubrication.

    • Major types include hyaluronate, chondroitin sulfate, keratan sulfate, and heparin.

  • Proteoglycans: GAGs attached to a core protein, which interact with other cells and regulate growth and repair processes.

Glycoconjugates

Glycoproteins

  • Proteins with carbohydrates attached that play roles in cell recognition and signaling.

    • Types: O-linked (connected through hydroxyl of Ser/Thr) or N-linked (connected to Asn).

Glycolipids

  • Lipids with oligosaccharides attached, involved in cell membrane structure and function.

Proteoglycans' Functions

  • Interact with receptors, form aggregates for tissue integrity, and provide lubrication.

Extracellular Matrix (ECM)

  • Structure: Composed of proteoglycans, collagen, and elastin, providing strength and elasticity to tissues.

  • Interaction with Cells: Involves membrane proteins such as integrins, linking cytoskeletons and ECM, regulating cell behavior and signaling.

Summary of Key Concepts

  • Recognized structures of important monosaccharides, disaccharides, and polysaccharides.

  • Discussed biological roles and functions of glycoconjugates, including glycoproteins and proteoglycans in various cellular processes.

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