7.3 Glycoconjugates: Proteoglycans, glycoproteins, and Glycolipids

1. 7.3 Glycoconjugates: Proteoglycans, glycoproteins, and Glycolipids

2. Glycoconjugate • Roles of glycoconjugate (information carriers) • Signal transduction (by recognition extracellular signal or parasites) • Cell-cell & cell-ECM communications • Protein labeling for translocation and degradation • Proteoglycan • Sulfated glycosaminoglycan-attached membrane proteins (cell surface) or secreted protein (ECM) • Glycosaminoglycan is the major part • Binding sites for other proteins (electrostatic interactions) • Major components of connective tissue providing strength and resilience • Glycoproteins • Oligosaccharide-attached proteins • Outer face of plasma membrane, ECM, blood • Inside cells; golgi, secretory granules, lysosomes • Specific sites for recognition (information-rich oligosaccharide) • Glycolipids • Membrane sphingolipids; oligosaccharide on hydrophilic head groups • Specific site for recognition

3. Proteoglycan • Roles of proteoglycan • 40 types in mammalian cells • Tissue organizer • Development of specialized tissues • Mediating growth factor activity • Regulating extracellular assembly of collagen fibrils • Structure • Core protein-tetrasaccharide bridge-glycosaminoglycan • Secretion to ECM or integration to the membrane • e.g. Syndecans and glypicans

4. Proteoglycan Domain structure of heparan sulfate • Modulation of ligand – receptor interactions at cell surface • Highly sulfated domains (NS) vs. unmodified domains (NA) • Variation of sulfation pattern in NS domain among proteoglycan

5. Heparan Sulfate • Protein interactions with NS domains of heparan sulfate

6. Proteoglycan Aggregates • Structure • Many aggrecans + single hyaluronate  Proteoglycan aggregates • Mr> 2 X 108 • Function • Strong interaction with fibrous matrix proteins (collagen, elastin, fibronectin) in ECM • Tensile strength, resilience • Anchoring cell to ECM • Cell-cell interaction, migration

7. Glycoproteins • Glycosylation to proteins • O-linked glycosylation : Ser, Thr • N-linked glycosylation : Asn • Types of glycoproteins • Membrane proteins • Secreted proteins • Antibody, hormones, milk proteins (lactalbumin), proteins released by the pancreas, lysosomal proteins • Advantages of oligosaccharide attachment • Hydrophilicity (polarity & solubility) • Destination label • Label for protein quality control • Protein folding • Protection from proteolytic attacks • Informational roles (recognition & communication)

8. Glycolipids and Lipopolysaccharides • Gangliosides • Eukaryotic membrane lipid with oligosaccharide containing a sialic acid and monosaccharides in the lipid head group • E.g. blood typing • Lipopolysaccharides • Structure in the outer surface of gram(-) bacteria (Escherichia coli, Salmonella typhimrium) • Prime immunological target against bacterial infections • Some are toxic to animals

9. 7.4 Carbohydrates as Informational Molecules : The Sugar Code

10. Lectins • Carbohydrate-binding proteins with high affinity and specificity • Biological functions • Cell-cell recognition/ Signaling/ Adhesion/ Intracellular targeting of newly synthesized proteins • Detection and separation of glycoproteins

11. Half-Life regulation of hormones • Luteinizing hormone and thyrotropin • Peptide hormones produced in the pituitary • N-linked glycosylation ending with GalNA4S (b14)GlcNAc • Recognized by a lectin of hepatocytes • Uptake and degradation • Periodic rise and fall • Glycoproteins with sialic acid at the end • Protected from degradation in the liver (e.g. ceruloplasmin, erythrocytes) • Removal of sialic acid by sialidase (neuraminidase) from old proteins • Recognition of unprotected Gal by asialoglycoprotein receptor in the liver • Endocytosis and degradation

12. Lectins in development of diseases • Selectins (plasma membrane lectins) mediating cell-cell recognition and adhesion • Movement of neutrophils through the capillary wall to tissues at sites of infection or inflammation • P-selectin on capillary endothelial cell surface binds to glycoprotein of neutrophils  slow-down neutrophils movement along the capillary • Interaction between integrin on capillary endothelial cell surface and ligand on neutrophils  Stops neutrophils rolling, and begins extravasation into infected tissue • Mediation of inflammatory responses • - rheumatoid arthritis, asthma, psoriasis, transplantation rejection

13. Lectins in development of diseases • Hemagglutinin (lectin of influenza virus) • Essential for viral entry and infection • Interaction with sialic acid residue from host cell’s oligosaccharides entry & bud-out • Sialidase (neuramidase) • Cleavage of sialic acid  releasing viral particles • Target for antiviral drugs • e.g) Tamiflu, Relenza • Lectins of herpes simplex viruses • Interaction with heparan sulfate on host cell surface • Sulfation pattern dependency •  target for drug development

14. Bacterial Adhesion by Lectins • Helicobacter pylori • Responsible for gastric ulcers • Bacterial membrane lectins • Interaction with oligosaccharide Lewis b of gastric epithelial cells • Part of the type O blood group determinant • Greater incidence of gastric ulcer in people of blood type O • Leb analogues as potential drugs • Toxins (lectins) • Vibriocholeraetoxin (cholera toxin) • Attach to oligosaccharide of ganglioside GM1 on the surface of host epithelial cell • Bordetellapertussis toxin • Enters target cells after interacting with host oligosaccharide with a terminal sialic acid • Toxin analogs without carbohydrate binding site as potential vaccines

15. Intracellular Lectins • Mannose 6-P receptor • Membrane-associated lectin on the lumenal side of golgi complex • Interaction with mannose 6-P-containing proteins •  transport vesicle to fuse lysosome • Targeting mechanism of most enzyme in lysosome • ER lectins • Calnexin (membrane) & calreticulin (soluble) •  Induce native folding of new proteins • Protein quality control • Glycosylation as quality control signals • ERGIC53; transport folded proteins to golgi complex (maturation) • EDEM; transport abnormally folded proteins to cytosol (degradation)

16. Lectin-Carbohydrate Interactions • Strong and specific interactions • Hydrogen bonding • Van der Waals interactions • Salt bridge • General interactions • Hydrophobic interactions Mannose-6-P with receptor Sialic acid - Sialoadhesin

17. Roles of Oligosaccharide Recognition and Adhesion at the Cell Surface