Chemical Structure
Glycosaminoglycan molecules are long, unbranched (linear) chains based on strictly alternating, repeating disaccharide units.
Each disaccharide includes two mandatory components:
- Hexuronic acid. Most commonly glucuronic or iduronic acid.
- Amino sugar derivative. Either glucosamine or galactosamine.
Due to the presence of acid residues (as well as sulfate groups in most representatives), these heteropolysaccharides carry a pronounced negative charge.
Biological Functions
Specific structure and physicochemical properties (primarily high negative charge density) determine the key roles of glycosaminoglycans in the body:
- Structural function. GAGs actively interact with all other extracellular matrix components to form a unified scaffold.
- Turgor maintenance. Molecules strongly bind water and cations, providing tissues with necessary resilience.
- Shock-absorbing function. In cartilage tissue, glycosaminoglycans act as amortizers, mitigating mechanical stress.
- Protective function (molecular sieve). A dense network of polysaccharides prevents the free spread of pathogens.
- Filtration function. GAGs directly participate in creating filtration barriers (e.g., in renal glomeruli).
Classification and Composition Features
Different types of glycosaminoglycans differ in monomer composition and sulfation status.
| Glycosaminoglycan | Hexuronic Acid | Hexosamine | Structural Features |
|---|---|---|---|
| Hyaluronic acid | D-Glucuronic acid | N-Acetyl-D-glucosamine | Nonsulfated. Contains $\beta(1\to3)$ linkage within the disaccharide. |
| Chondroitin sulfate | D-Glucuronic acid | N-Acetyl-D-galactosamine | Sulfated (typically at C4 or C6 of hexosamine). |
| Keratan sulfate | Absent (replaced by D-galactose) | N-Acetyl-D-glucosamine | Sulfated (at C6). The only GAG without uronic acid. |
| Dermatan sulfate | L-Iduronic (predominant) or D-glucuronic | N-Acetyl-D-galactosamine | Sulfated (at C4). Epimer of chondroitin sulfate. |
| Heparan sulfate | D-Glucuronic or L-iduronic acid | N-Acetyl-D-glucosamine (or N-sulfoglucosamine) | Contains sulfate groups (N- and O-sulfation). |
Catabolism and Clinical Microbiology
The breakdown of glycosaminoglycans in tissues is catalyzed by specific enzymes. Major catabolic enzymes include hyaluronidase, glucuronidase, galactosidase, and sulfatase.
Hyaluronidase is of particular clinical importance in infectious diseases. Several pathogenic microorganisms can secrete this enzyme themselves. These include:
- Pyogenic infection agents (staphylococci, streptococci);
- Gas gangrene agents.
By destroying the extracellular matrix, bacterial hyaluronidase acts as a spreading factor, facilitating the rapid advance of infection deep into tissues.