7.3 Glycoconjugates: Proteoglycans, glycoproteins, and Glycolipids

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

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

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

Heparan Sulfate Protein interactions with NS domains of heparan sulfate

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

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)

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

7.4 Carbohydrates as Informational Molecules : The Sugar Code

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

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

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

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

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

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)

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

Roles of Oligosaccharide Recognition and Adhesion at the Cell Surface