**3. Non–fiber-forming ECM molecules**

## **3.1. Proteoglycans**

Proteoglycans are heavily glycosylated proteins. They contain glycosaminoglycan (GAG) chain, which is linked with a protein core covalently. They are classified by their size (large and small) or the nature of their GAG chains. The main classes of proteoglycans are heparan sulfates, keratan sulfates and chondroitin sulfates. Their extreme hydrophilic nature and ability to adapt extended conformations make them easier to form hydrogel, and structures formed with these molecules can withstand high compressive forces.

Proteoglycans and their most common locations:

Heparan sulfate: basement membranes and components of cell surfaces.

Chondroitin sulfate: cartilage, heart valves and bone.

Keratan sulfate: cornea and bone.

### **3.2. Hyaluronic acid**

Hyaluronic acid is a polysaccharide, which does not contain protein core and it consists of alternating residues of D-glucuronic acid and N-acetylglucosamine. Hyaluronic acid forms a coat around chondrocytes and they provide resilience of articular cartilage tissue. In extracel‐ lular space, hyaluronic acid also provides the ability to resist compression by absorbing significant amounts of water and providing a counteracting swelling force.
