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)
Vibrio cholerae toxin (cholera toxin)
Attach to oligosaccharide of ganglioside GM1 on the surface of host epithelial cell
Bordetella pertussis 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