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Glycoproteins and their formations, including the two common types of glycoproteins and their site and modes of synthesis with associated diseass

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  1. 1. Glycoproteins: Biological and Clinical Significance Presented By: Dr. Nilesh Chandra
  2. 2. OBJECTIVES: • Classes Of Glycoproteins • Biosynthesis Of Different Classes Of Glycoproteins • Biological Roles Of Glycoproteins • Glycoproteins And Diseases
  3. 3. Classes of Glycoproteins: O-Linked Glycoproteins GalNAcSer(Thr) linkage GlcNAc-Ser[Thr] linkage N-Linked Glycoproteins Complex Hybrid High- mannose GPI-Linked Glycoproteins Other minor groups
  4. 4. N-linkage (N -acetylglucosamine to asparagine)
  5. 5. Dolichol-P-P-Oligosaccharide: Structure of Dolichol (polyisoprenol) Structure of dolichol-P-P-oligosaccharide Dol-P-P-GlcNAc2Man9Glc3.
  6. 6. Biosynthesis of DolicholP-P- oligosaccharide: • Dolichol Dolichol phosphate (Dol-P) • Dol-P + UDP-GlcNac Dol-P-P-GlcNac +UMP dolichol kinase ATP ADP
  7. 7. Biosynthesis of DolicholP-P- oligosaccharide (contd.)
  8. 8. 1= oligosaccharide: protein transferase 2= -glucosidase I 3= -glucosidase II 4= endoplasmic reticulum 1,2-mannosidase 5=Golgi apparatus -mannosidase I I= N-acetylglucosaminylphosphotransferase II= N-acetylglucosamine-1-phosphodiester α-N-acetylglucosaminidase
  9. 9. 6= N-acetylglucosaminyltransferase I 7= Golgi apparatus -mannosidase II 8= N-acetylglucosaminyltransferase II 9= fucosyltransferase 10= galactosyltransferase 11= sialyltransferase.
  10. 10. Major Classes of N-Linked Oligosaccharides: • Complex • Hybrid • High-mannose Share a common pentasaccharide, Man3GlcNAc2
  11. 11. The boxed area encloses the pentasaccharide core common to all N-linked glycoproteins
  12. 12. O-linkage (N -acetylgalactosamine to serine)
  13. 13. Biosynthesis of O-Linked Glycoproteins: • Stepwise donation of sugars from nucleotide sugars such as: – UDP-GalNAc – UDPGal – CMP-NeuAc • Enzymes involved: membrane-bound glycoprotein glycosyltransferases. • Enzymes located in various subcompartments of Golgi apparatus.
  14. 14. Subclasses of O-glycosidic linkages : 1. The GalNAcSer(Thr) linkage - (A)submaxillary mucins (B)fetuin
  15. 15. Subclasses of O-glycosidic linkages (contd.) 2. Gal-Gal-XylSer trisaccharide (link trisaccharide)- Transfer of a Xyl residue to Ser from UDP-xylose Two residues of Gal then added to the Xyl residue Further chain growth on the terminal Gal • O-glycosidic bond between xylose (Xyl) and Ser is unique to proteoglycans
  16. 16. Subclasses of O-glycosidic linkages (contd.) 3. Gal-hydroxylysine (Hyl) linkage- – Hydroxylysine in collagen modified by the addition of galactose or galactosyl-glucose through an O- glycosidic linkage. • Hydroxylysine - a glycosylation site that is unique to collagen.
  17. 17. Subclasses of O-glycosidic linkages (contd.) 4. GlcNAc-Ser[Thr] linkage – – On many nuclear proteins and cytosolic proteins. – a single GlcNAc attached to a serine or threonine residue. – formed by donation to Ser (or Thr) of a GalNAc residue, employing UDP-GalNAc as its donor.
  18. 18. Differences between O- & N- linked Glycoproteins: N-Linked Glycoproteins Synthesis:- • Cotranslationally • En-bloc transfer of Oligosachharide chain on the protein and further modification. • Enzymes not membrane bound. • Dolichol P-P- Oligosaccharide involved. • Inhibited by Tunicamycin O-Linked Glycoproteins Synthesis:- • Post- translationally • Oligosaccharide chain synthesized on the protein. • Enzymes membrane bound. • Dolichol not involved. • Not inhibited by Tunicamycin e.g. Fetuine.g. Calnexin
  19. 19. GPI linkage Acetylcholinesterase linkage to the plasma membrane of the human red blood cell.
  20. 20. Biosynthesis of GPI anchors: Glypiation • Begins in the endoplasmic reticulum. • GPI anchor is assembled independently. • The hydrophobic end cleaved off. • Replaced by the GPI-anchor. • GPI-linked proteins thought to be preferentially located in lipid rafts.
  21. 21. Some GPI-Linked Proteins: TABLE Acetylcholinesterase (red cell membrane) Alkaline phosphatase (intestinal, placental) Decay-accelerating factor (red cell membrane) 5'-Nucleotidase (T lymphocytes, other cells) Thy-1 antigen (brain, T lymphocytes) Variable surface glycoprotein (Trypanosoma brucei)
  22. 22. Functions Served by Glycoproteins: 1. Structural molecule 2. Lubricant and protective agent 3. Transport molecule 4. Immunologic molecule 5. Hormone 6. Enzyme 7. Cell attachment-recognition site
  23. 23. Functions Served by Glycoproteins (contd.) 8. Interact with specific carbohydrates 9. Antifreeze 10.Receptor 11.Affect folding of certain proteins 12.Regulation of development 13.Hemostasis (and thrombosis)
  24. 24. Role of Oligosaccharide Chains of Glycoproteins: • Modulate physicochemical properties, eg,: – Solubility – Viscosity – Charge – Conformation – Denaturation – binding sites for various molecules, bacteria viruses and some parasites • Protect against proteolysis, from inside and outside of cell
  25. 25. Schematic diagram of a mucin
  26. 26. Role of Oligosaccharide Chains of Glycoproteins (contd.) • Affect proteolytic processing of precursor proteins to smaller products • Are involved in biological activity, eg, of human chorionic gonadotropin (hCG) • Affect insertion into membranes, intracellular migration, sorting and secretion • Affect embryonic development and differentiation • May affect sites of metastases selected by cancer cells
  27. 27. Some Diseases Due to Glycoproteins: Disease HEMPAS Leukocyte adhesion deficiency, type II Paroxysmal nocturnal hemoglobinuria I-cell disease Congenital disorders of glycosylation
  28. 28. HEMPAS: • Hereditary Erythroblastic Multinuclearity With A Positive Acidified Lysis Test . • Also known as Congenital Dyserythropoietic Anemia Type II. • Claimed to be due to defects in alpha–mannosidase II • Characterized by – Ineffective erythropoiesis – Hemolysis & erythroblast morphological abnormalities – Hypoglycosylation of some red blood cell (RBC) membrane proteins. • Treatment consists of frequent blood transfusions and chelation therapy
  29. 29. Leukocyte adhesion deficiency (LAD) II: • A congenital disorder of glycosylation • Mutations affecting the activity of a Golgi-located GDP-fucose transporter • Marked decrease in neutrophil rolling • Subjects suffer – Life-threatening, recurrent bacterial infections – Psychomotor and mental retardation • The condition appears to respond to oral fucose
  30. 30. Paroxysmal Nocturnal Hemoglobinuria: Scheme of causation of paroxysmal nocturnal hemoglobinuria
  31. 31. I-Cell Disease: Summary of the causation of I-cell disease
  32. 32. Congenital Disorder of Glycosylation • Previously called carbohydrate-deficient glycoprotein syndrome. • Glycosylation of a variety of tissue proteins and/or lipids is deficient or defective. • CDG-I mainly in ER & related to steps prior processing of glycan chains and transfer of oligosac chain to protein. • CDG-II includes all defects localized in the processing of N- glycans on the glycosylated protein.These are situated mainly in the Golgi compartment. • Often cause serious, sometimes fatal, malfunction of several different organ systems (especially the nervous system, muscles, and intestines) in affected infants.
  33. 33. Defects in CDG
  34. 34. Diseases due to O-linked Glycoproteins: • Inborn errors of O-glycan metabolism can lead to – severe autosomal recessive multisystem syndrome with neurologic involvement, – some defects, for example, those in persons with the Bombay blood group do not produce a clinical phenotype.
  35. 35. Defects of O glycosylation Name Defect Clinical features Defects in mucin-type o-glycan biosynthesis (familial tumoral calcinosis (FTC) UDP-GalNAc transferase 3 (ppGalNAc T3) abnormal cleavage and secretion of the phosphaturic factor FGF23. massive calcium deposits in skin and subcutaneous tissues and unresponsiveness to parathyroid hormone Defects in o-galactosyl glycan biosynthesisLysyl hydroxylase-1 deficiency Ehlers–Danlos syndrome type Via Deficiency or mutation in Lysyl hydroxylase-1 neonatal kyphoscoliosis, generalized joint laxity, skin fragility, and severe muscle hypotonia at birth Defects in o-mannosyl glycan biosynthesis Walker–Warburg syndrome (WWS) and limb-girdle muscular dystrophy type 2K Muscle–eye–braindisease (MEB)
  36. 36. IEF pattern for transferrin (Type 2)
  37. 37. Some Diseases Involving Glycoproteins: • Glycans Involved in the Binding of Micro- organisms: – Influenza virus A – Human immunodeficiency virus type 1 (HIV-1) – Helicobacter pylori – Plasmodium falciparum • Cancer: Increased branching of cell surface glycans or presentation of selectin ligands may be important in metastasis.
  38. 38. Some Diseases Involving Glycoproteins (contd.) Selectins in Inflammation & in Lymphocyte Homing: • Three types of selectins: Present on – L-selectin : Polymorphonuclear leukocytes, Lymphocytes – P-selectin : Endothelial cell, Platelets – E-selectin : Endothelial cell • All three bind sialyl-Lewisx, a structure present on both glycoproteins and glycolipids
  39. 39. Schematic representation of the structure of sialyl-Lewisx
  40. 40. SUMMARY: • Glycoproteins are of various types, based on their linkages. • Different types of linkages follow different biosynthetic pathways. • Glycoproteins play diverse roles in various important functions of the body. • A great many of the properties of the glycoproteins are due to the oligosachharide component.
  41. 41. SUMMARY (contd.) • Glycoproteins in Disease: – Pathophysiology of : • HEMPAS • LAD II • PNH • I-Cell Disease • CDGs – Role in adhesion of micro-organisms – Role in Chemotaxis and migration of leukocytes – Cancer
  42. 42. References: • Harper's Illustrated Biochemistry, Twenty-Eighth Edition. • Lehninger’s Principles of Biochemistry, Fifth Edition. • Stryer’s Biochemistry, Sixth Edition. • Dennis JW, Nabi IR, Demetriou M. Metabolism, cell surface organization, and disease. Cell. 2009 Dec 24;139(7):1229-41. Review. • Vagin O, Kraut JA, Sachs G. Role of N-glycosylation in trafficking of apical membrane proteins in epithelia. Am J Physiol Renal Physiol. 2009 Mar;296(3):F459-69. Epub 2008 Oct 29. • Luther KB, Haltiwanger RS. Role of unusual O-glycans in intercellular signaling. Int J Biochem Cell Biol. 2009 May;41(5):1011-24. Epub 2008 Oct 8. Review.
  43. 43. Advanced Glycation End-Products: Maillard reaction • AGEs cause tissue damage in diabetes mellitus.
  44. 44. Some consequences of the formation of AGEs
  45. 45. O- mannosyl glycan • O-Mannosyl glycans are a less common type of protein modification • present on a limited number of glycoproteins in the brain, nerves and skeletal ms. • Eg α-Dystroglycan – It is a laminin receptor – 2 subunits – α-DG interacts with laminin-2 in the basal lamina and β-DG binds to dystrophin – induces Ach receptor clustering at neuromuscular junctions