Carbohydrate: Heteropolysacharides

📌 Definition & Classification

• Polysaccharides are polymers of more than 10 monosaccharide units.
• They are non-sweet and poorly soluble in water.
• Classification:
  🔹 By Function → Storage (starch, glycogen) or Structural (cellulose, pectin).
  🔹 By Composition →
• • Homopolysaccharides: one type of monosaccharide.
  • Heteropolysaccharides: more than one type of monosaccharide.

🖇 Heteropolysaccharides

• Formed of repeating disaccharide units containing different monosaccharides.
• Divided into three main groups:
  1️⃣ Glycosaminoglycans (GAGs)
  2️⃣ Proteoglycans
  3️⃣ Glycoproteins

🟣 Glycosaminoglycans (GAGs)
📌 Structure

• Long, unbranched chains of repeating disaccharides.
• Each disaccharide has:
  🔹 One acidic sugar (D-glucuronic or L-iduronic acid).
  🔹 One amino sugar (glucosamine or galactosamine).
• Highly negatively charged because of uronic acid and sulfate groups.

📌 Location

• Found in extracellular matrix (ECM) of connective tissues such as cartilage, bone, tendons, and skin.
• Also present in synovial fluid, vitreous humor, and cornea.

📌 Functions

• Bind cations (mainly Na⁺) and attract water → maintain tissue hydration.
• Form gel-like structure → cushioning, shock absorption, and lubrication.

📌 Types and Main Roles

• Hyaluronic Acid: Lubrication in joints, shock absorption, tissue hydration.
• Chondroitin Sulfate: Maintains shape and structure of tissues.
• Dermatan Sulfate: Structural support of skin and sclera.
• Keratan Sulfate: Maintains corneal transparency.
• Heparin: Acts as a natural anticoagulant, secreted by mast cells in lungs, liver, and blood.
• Heparan Sulfate: Found on cell surfaces, involved in cell signaling.

🚨 Notes on GAGs

• All are sulfated except hyaluronic acid.
• All are extracellular except heparin.
• Most are components of proteoglycans except hyaluronic acid and heparin.

🟣 Proteoglycans
📌 Definition

• Consist of a core protein with one or more GAG chains covalently attached.
  📌 Composition
• About 95% carbohydrate (GAGs) and 5% protein.
  📌 Location
• Major component of the extracellular matrix.
  📌 Functions
• Provide structural support and framework for tissues.
• Help maintain hydration and elasticity.

🟣 Glycoproteins
📌 Definition

• Proteins that contain branched oligosaccharide chains.
  📌 Composition
• About 95% protein and 5% carbohydrate.
  📌 Functions
• Found in mucins of GI, respiratory, and urogenital tracts.
• Components of extracellular matrix and cell membranes.
• Almost all plasma proteins (except albumin) are glycoproteins.
• Include enzymes, hormones, immunoglobulins, interferons, and complement proteins.
• Form blood group antigens (ABO system).
• Responsible for cell recognition and attachment (e.g., glycophorin in RBCs).

🟠 Differences Between Proteoglycans and Glycoproteins

• Proteoglycans are mainly found in the ECM, while glycoproteins are mainly in cell membranes.
• Proteoglycans contain mostly carbohydrates (GAGs), whereas glycoproteins contain mostly protein.
• GAG chains in proteoglycans are long, linear, and negatively charged; oligosaccharides in glycoproteins are short and branched.
• Proteoglycans contain uronic acids and sulfate groups; glycoproteins usually do not.
• Proteoglycans have structural and supportive functions; glycoproteins are involved in recognition, signaling, and immunity.