Carbohydrates and Glycobiology

Overview

This study guide covers the structure, function, and biological significance of glycoconjugates, including peptidoglycans, proteoglycans, glycoproteins, and glycolipids. It also explores their roles in the extracellular matrix, bacterial cell walls, and molecular recognition processes.

Glycoconjugates

General Properties

• Glycoconjugates are molecules in which carbohydrates are covalently linked to proteins or lipids.

• They play key roles in cell structure, signaling, and recognition.

• Main types include peptidoglycans, proteoglycans, glycoproteins, and glycolipids.

Types of Glycoconjugates

• Peptidoglycans: Polysaccharides linked by peptides; major component of bacterial cell walls.

• Proteoglycans: Proteins associated with one or more glycosaminoglycan chains; polysaccharide portion is the major weight.

• Glycoproteins: Proteins with one or more complex oligosaccharides; serve as information-rich recognition molecules.

• Glycolipids: Membrane lipids with hydrophilic oligosaccharide head groups; recognized by specific proteins.

Peptidoglycans and Bacterial Cell Walls

Structure and Composition

• Bacterial cell walls contain heteropolysaccharides of alternating modified glucose units.

• Main components:

  • N-acetylglucosamine (GlcNAc or NAG)

  • N-acetylmuramic acid (Mur2Ac or NAM)

• These units alternate and are joined by β(1→4) glycosidic linkages.

• Peptide cross-links (e.g., pentaglycine) provide structural integrity.

Lysozyme Action

• Lysozyme is an enzyme found in tears and bacterial viruses.

• It cleaves the β(1→4) glycosidic bond between GlcNAc and Mur2Ac, compromising cell wall integrity.

Penicillin and Antibiotic Resistance

• Penicillin is a β-lactam antibiotic that blocks cell wall formation by inhibiting transpeptidases responsible for forming pentaglycine cross-links.

• Mechanism: Penicillin mimics the substrate and forms a stable, inactive complex with the transpeptidase enzyme.

• Bacteria can develop resistance by producing β-lactamase enzymes or altering transpeptidase structure.

• Augmentin™ combines amoxicillin with clavulanate to inhibit β-lactamase and overcome resistance.

Proteoglycans and the Extracellular Matrix

Structure and Function

• Proteoglycans act as tissue organizers and mediate growth factor activities.

• They consist of a core protein covalently linked to one or more glycosaminoglycan (GAG) chains.

• GAGs are linear, complex heteropolysaccharides composed of repeating disaccharide units.

Glycosaminoglycans (GAGs)

• Definition: Linear, complex heteropolysaccharides.

• Composition: Repeating disaccharide units, typically one N-acetylglucosamine or N-acetylgalactosamine (sometimes sulfated) and one uronic acid.

• Examples: Hyaluronate, chondroitin sulfate, keratan sulfate, heparin.

• Function: Components of the extracellular matrix, forming a porous gel between cells and interacting with fibrous proteins (collagen, elastin, fibronectin).

Major GAGs and Their Structures

Extracellular Matrix

• Composition: Meshwork of glycosaminoglycans and proteoglycans embedded with fibrous proteins.

• Cartilage: Proteoglycans provide flexibility; collagen provides tensile strength.

• Intercellular Layer: Proteoglycans organize extracellular receptors and direct cell/neural migration.

Glycoproteins

Structure and Linkage

• Glycoproteins are proteins with one or more complex oligosaccharides.

• Oligosaccharides are linked to proteins via:

  • O-linked: Attached to Serine (Ser) or Threonine (Thr) side chains via GalNAc.

  • N-linked: Attached to Asparagine (Asn) side chains via GlcNAc.

The Sugar Code: Carbohydrates as Information

• Carbohydrates can encode vast information due to:

  • 20 different monosaccharides

  • Multiple linkage points (e.g., 1→2, 1→3, 1→4)

  • Linear and branched structures

• Number of possible hexameric oligosaccharides:

• Comparison: Only hexapeptides and 4,096 hexanucleotides.

Functions of Glycoproteins

• Mediate recognition by other proteins

• Facilitate protein targeting and localization

• Influence protein folding and stability

• Cell-cell recognition, signaling, and adhesion

• Lectins: Proteins that bind carbohydrates with high specificity and affinity; often found on cell surfaces.

• Examples of glycoproteins: Antibodies, milk proteins, hormones, interferons, cell surface receptors.

Specificity of Glycoprotein Recognition

• Recognition depends on the structure and linkage of oligosaccharides.

• Example: Avian influenza virus binds only to avian cell receptors with specific glycan structures, not human cells.

• P-selectin: A cell surface lectin that targets lymphocytes to sites of infection by binding specific glycoproteins.

Lectin-Carbohydrate Interactions

• Lectins recognize specific carbohydrate motifs (e.g., mannose-6-phosphate).

• Interactions include hydrogen bonding, hydrophobic contacts, and aromatic stacking (e.g., indolyl moiety of tryptophan).

• Dissociation constant () for lectin binding is often M or less, indicating high affinity.

Glycolipids

Structure and Biological Roles

• Glycolipids are membrane lipids with oligosaccharide head groups.

• Lipopolysaccharides coat the outer membrane of gram-negative bacteria and can be toxic to humans (cause shock).

• Gangliosides: Membrane lipids with oligosaccharide polar head groups containing sialic acid (N-acetylneuraminic acid) and other sugars.

• Defects in ganglioside metabolism (e.g., Tay Sachs disease) lead to neurological degeneration due to hexosaminidase A deficiency.

Blood Group Antigens

• Differences in blood group antigens (O, A, B) are due to variations in oligosaccharides on cell surface sphingoglycolipids.

• Key sugars: Fucose (Fuc), Galactose (Gal), N-acetylglucosamine (GlcNAc), N-acetylgalactosamine (GalNAc).

Oligosaccharides in Adhesion and Recognition

Role in Cell-Cell Interactions

• Oligosaccharides on cell surfaces mediate adhesion, recognition, and signaling.

• Examples include viral entry, bacterial toxins, and immune cell targeting.

• Glycans (oligosaccharide chains) can act as specific recognition molecules due to their diversity and branching.

Glycans in Viral Evasion and Pathology

SARS-CoV-2 and Glycan Shields

• Glycans on viral proteins (e.g., SARS-CoV-2 spike) help viruses evade immune detection by mimicking host glycans.

• Site-specific glycan analysis reveals roles in protein folding and immune evasion.

• Glycan shields vary in density among viruses, influencing their ability to evade immune responses.

• Some glycans hinder opening of viral proteins, while others promote it, affecting infectivity.

Discussion and Review

Key Properties of Glycans as Recognition Molecules

• Structural diversity (monosaccharide composition, linkage types, branching)

• Stereochemistry of glycosidic linkages

• Ability to form specific, high-affinity interactions with proteins (lectins)

• Dynamic roles in cell signaling, adhesion, and immune evasion

Quiz Question

• What does N-linked mean? It refers to oligosaccharides attached to the side chain amide nitrogen of asparagine residues in proteins.

Additional info: The notes include advanced examples such as glycan shields in SARS-CoV-2, illustrating the relevance of glycobiology in current biomedical research.