Glycosaminoglycans (GAGs)

Overview

Glycosaminoglycans (GAGs) are large, negatively charged complexes composed of unbranched polysaccharide chains. They are a major component of the extracellular matrix (ECM), where they interact with proteins and water to form a gel-like substance that provides structural support and regulates the movement of molecules.

• Definition: GAGs are long, unbranched heteropolysaccharides consisting of repeating disaccharide units.

• Composition: Typically up to 95% carbohydrate, with a small core protein.

• Function: Provide flexible support for the ECM, influence movement of materials, and contribute to the lubricating properties of mucous secretions.

• Water Binding: GAGs bind large amounts of water, producing a gel-like matrix.

Structure

GAGs are composed of repeating disaccharide units, each containing an acidic sugar and an amino sugar. Their structure imparts a strong negative charge, which is essential for their biological functions.

• Amino Sugar: Either D-glucosamine or D-galactosamine, often N-acetylated or sulfated, eliminating positive charge.

• Acidic Sugar: Either D-glucuronic acid or its C-5 epimer, L-iduronic acid.

• Negative Charge: Carboxyl and sulfate groups confer strong negative charge at physiological pH.

Structure–Function Relationship

The extended, negatively charged chains of GAGs repel each other and attract water, resulting in unique physical properties.

• Extended Chains: High negative charge causes chains to be extended in solution.

• Water Shell: Chains are surrounded by water, allowing them to slide past each other.

• Compression and Resilience: When compressed, water is expelled; upon release, GAGs spring back due to charge repulsion.

• Biological Examples: Contributes to the resilience of synovial fluid and the vitreous humor of the eye.

Classification of GAGs

GAGs are classified based on their monomeric composition, glycosidic linkages, and degree/location of sulfation.

Proteoglycans

Structure and Function

Proteoglycans are found in the ECM and on cell surfaces. They consist of a core protein to which multiple GAG chains are covalently attached, forming large, complex aggregates.

• Core Protein: Central protein to which up to 100 GAG chains are attached.

• GAG Attachment: Chains extend outward and repel each other due to negative charge.

• Main GAGs: Chondroitin sulfate and keratan sulfate are common in proteoglycans.

GAG–Protein Linkage

GAGs are covalently linked to the core protein, most commonly through a trihexoside linker (galactose-galactose-xylose) attached to a serine residue.

• Linkage: O-glycosidic bond between xylose and the hydroxyl group of serine.

• Aggregate Formation: Proteoglycan monomers can associate with hyaluronic acid to form large aggregates, stabilized by link proteins.

Synthesis of GAGs and Proteoglycans

Polysaccharide Chain Elongation

GAG chains are elongated by sequential addition of acidic and amino sugars, donated by their UDP derivatives, catalyzed by glycosyltransferases in the Golgi apparatus.

• UDP-Sugar Donors: UDP-GlcNAc, UDP-GalNAc, UDP-GlcA, etc.

• Enzymes: Glycosyltransferases catalyze addition of sugars.

Sulfation

Sulfation of GAGs occurs after monosaccharide incorporation, using 3'-phosphoadenosyl-5'-phosphosulfate (PAPS) as the sulfate donor, catalyzed by sulfotransferases.

• Sulfate Source: PAPS

• Enzyme: Sulfotransferases

Degradation of GAGs

Lysosomal Degradation

GAGs are degraded in lysosomes by acid hydrolases. The process involves sequential removal of sugars from the nonreducing end.

• Enzymes: Acid hydrolases, exoglycosidases, and sulfatases

• Phagocytosis: Extracellular GAGs are engulfed before degradation.

• Genetic Disorders: Deficiency of hydrolases leads to mucopolysaccharidoses (e.g., Hurler syndrome), characterized by GAG accumulation and clinical symptoms.

Glycoproteins

Structure and Function

Glycoproteins are proteins with covalently attached oligosaccharide chains. They have shorter, often branched carbohydrate chains compared to proteoglycans and do not contain repeating disaccharide units.

• Carbohydrate Content: Typically 2–10 sugar residues, may be longer.

• Branching: Oligosaccharide chains are often branched.

• Functions: Cell recognition, cell-surface antigens, ECM components, mucins, plasma proteins (except albumin).

Oligosaccharide Structure

Glycoprotein oligosaccharides are branched heteropolymers composed mainly of hexoses, with possible addition of N-acetylneuraminic acid (NANA) and fucose.

• Attachment: O-glycosidic link to serine/threonine or N-glycosidic link to asparagine.

• Types: Complex oligosaccharides (diverse sugars) and high-mannose oligosaccharides (mainly mannose).

Synthesis of Glycoproteins

Glycoproteins are synthesized in the rough endoplasmic reticulum (RER) and Golgi apparatus. The carbohydrate chains are added via nucleotide sugar donors.

• O-Linked Glycoproteins: Sequential addition of sugars to serine/threonine residues, mainly in the Golgi.

• N-Linked Glycoproteins: Preformed oligosaccharide is transferred from dolichol pyrophosphate to asparagine residue in the RER, then processed in the Golgi.

• Nucleotide Sugar Donors: UDP-Galactose, UDP-GlcNAc, GDP-mannose, CMP-NANA, etc.

Glycoprotein Degradation

Glycoproteins are degraded in lysosomes by acid hydrolases. Deficiency of these enzymes leads to lysosomal storage diseases (oligosaccharidoses).

• Enzymes: Acid hydrolases, exoglycosidases

• Genetic Disorders: Oligosaccharidoses result from enzyme deficiencies, causing accumulation of partially degraded glycoproteins.

Summary Table: Comparison of GAGs, Proteoglycans, and Glycoproteins

Key Equations and Biochemical Pathways

• UDP-Sugar Formation:

• Sulfation Reaction:

• N-Linked Glycosylation:

Clinical Relevance

• Mucopolysaccharidoses: Genetic disorders due to defective GAG degradation, leading to tissue accumulation and symptoms such as skeletal abnormalities and intellectual disability.

• Oligosaccharidoses: Lysosomal storage diseases caused by defective glycoprotein degradation.

Additional info: The study notes include inferred details about the classification and synthesis pathways of GAGs and glycoproteins, as well as clinical relevance, to provide a comprehensive, self-contained guide for biochemistry students.