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Essentials of Glycobiology May 1st, 2008 Ajit Varki Lecture 11 Chapter 12 : Sialic Acids Chapter 32 : I-type Lectins.

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Presentation on theme: "Essentials of Glycobiology May 1st, 2008 Ajit Varki Lecture 11 Chapter 12 : Sialic Acids Chapter 32 : I-type Lectins."— Presentation transcript:

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2 Essentials of Glycobiology May 1st, 2008 Ajit Varki Lecture 11 Chapter 12 : Sialic Acids Chapter 32 : I-type Lectins

3 Major Glycan Classes in Vertebrate Cells

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5 General Questions for Lecture 11 1.Compare and contrast the structure of sialic acids with other vertebrate monosaccharides 2.What advantages does sialic acid diversity provide in vertebrate systems? 3.What are the unique features of the sialic acid biosynthetic pathways in comparison to those of other vertebrate monosaccharides? 4.How would you determine if a previously unstudied organism contains sialic acids? 5.Contrast the addition of  lpha2-6-linked sialic acids to O-GalNAc glycans and N-linked glycans and their recognition by sialic acid-binding lectins. 6.Why do plants and invertebrates that do not express sialic acids have sialic acid binding proteins? 7.There are now more than a dozen human Siglecs known. Why were these and other sialic acid binding proteins not discovered until very recently? 8.Compare the potential function of Siglecs with inhibitory motifs in their cytosolic tails with those that can recruit activatory motifs 9.Why are Siglec homologs found primarily in "higher" animals 10.Why are some Siglecs evolving rapidly? 11.What changes in sialic acid biology occurred during human evolution?

6 Two common “primary” sialic acids. 1. Compare and contrast the structure of sialic acids with other vertebrate monosaccharides

7 Biological Roles of Sialic Acids EXTRINSIC RECEPTOR M Ligands for Intrinsic Receptors Siglecs Factor H Selectins L1CAM Laminins? SIALYLATED GLYCAN = M = Micro-organism/Toxin Influenza Malaria Cholera Helicobacter Mycoplasma Rotavirus SV40 virus Coronavirus Pertussis Tetanus etc. Ligands for Extrinsic Receptors Structural/Physical Roles Molecular Mimicry Meningococcus E.Coli K1 Gonococcus Campylobacter Trypanosoma Group B Streptococcus Etc. SELF INTRINSIC RECEPTOR SELF

8 Natural Diversity in the Sialic Acids

9 O COO O O O O O R1 R2 R4 R7 R8 R9 R5 R1 = H, dissociation at physiological pH gives negative charge; lactones with -OH groups on same molecule or other glycans; lactams with a free amino group at C5; or tauryl group. R2 = H in free Sia; alpha linkage to Gal(3/4/6), GalNAc(6), GlcNAc(4/6), Sia (8/9) or 5-O-Neu5Gc; oxygen linked to C7 in 2,7-anhydro molecule; anomeric hydroxyl eliminated in Neu2en5Ac (double bond to C3) R4 = H, O-acetyl, anhydro to C8, Fuc, Gal R5 = Amino, N-acetyl, N-glycolyl, hydroxyl, N-acetimidoyl, N-glycolyl-O-acetyl, N-glycolyl-O-methyl, N-glycolyl-5-O-2-Neu5Gc R7 = H, O-acetyl, anhydro to C2; substituted by amino and N-acetyl in Leg R8 = H, O-acetyl, anhydro to C4, O-methyl, O-sulfate, Sia, Glc R9 = OH, O-acetyl, O-lactyl, O-phosphate, O-sulfate, Sia: OH substituted by H in Leg. 2. What advantages does sialic acid diversity provide in vertebrate systems?

10 Terminal, oligo-, and poly-sialic acids, and the enzymes that can degrade them

11 Genes and pathways involved in the biology of sialic acids. 3. What are the unique features of the sialic acid biosynthetic pathways in comparison to those of other vertebrate monosaccharides?

12 Examples of terminal glycan sequences recognized by some sialic-acid-binding proteins. 5. Contrast the addition of  lpha2-6-linked sialic acids to O-GalNAc glycans and N-linked glycans and their recognition by sialic acid-binding lectins. 6. Why do plants and invertebrates that do not express sialic acids have sialic acid binding proteins?

13 Biological Roles of Sialic Acids EXTRINSIC RECEPTOR M Ligands for Intrinsic Receptors Siglecs Factor H Selectins L1CAM Laminins? SIALYLATED GLYCAN = M = Micro-organism/Toxin Influenza Malaria Cholera Helicobacter Mycoplasma Rotavirus SV40 virus Coronavirus Pertussis Tetanus etc. Ligands for Extrinsic Receptors Structural/Physical Roles Molecular Mimicry Meningococcus E.Coli K1 Gonococcus Campylobacter Trypanosoma Group B Streptococcus Etc. SELF INTRINSIC RECEPTOR SELF

14 Domain structures of the known Siglecs in humans and mice. 7. There are now more than a dozen human Siglecs known. Why were these and other sialic acid binding proteins not discovered until very recently?

15 From: Crocker P, Paulson J. & Varki, A. Nature Reviews Immunol. 7: , Biological Interactions Involving Siglecs

16 From: Crocker P, Paulson J. & Varki, A. Nature Reviews Immunol. 7: , Signaling Responses Mediated by Siglecs 8. Compare the potential function of Siglecs with inhibitory motifs in their cytosolic tails with those that can recruit activatory motifs

17 Fig Structural basis of Siglec binding to ligands. X-ray crystal structures of the V- set domains of sialoadhesin (Sn) (A) and Siglec-7 (B) are shown complexed with sialic acid. (C,D) Molecular details of interactions of sialic acid with Sn and Siglec-7.

18 Biological functions mediated by sialoadhesin: Interactions of sialoadhesin on macrophages with cells and pathogens. (Right) Red staining shows ring of sialoadhesin expressed by macrophages in marginal zone of spleen and green staining shows Siglec-H on the plasmacytoid dendritic cells

19 Proposed Biological functions mediated by CD22: CD22 glycan-dependent homotypic interactions in equilibrium with CD22–BCR interactions.

20 Biological functions mediated by myelin-associated glycoprotein (MAG)

21 Proposed Biological functions mediated by CD33-related Siglecs

22 Chromosomal organization of CD33-related Siglec clusters in some rodents and primates 9. Why are Siglec homologs found primarily in "higher" animals? 10. Why are some Siglecs evolving rapidly?

23 Proposed Evolutionary Chain of “Red Queen” Effects involving Sialic Acids and CD33-related-Siglecs Host sialic acids are evolving to evade pathogens that exploit them as receptors? Host Sialic acids Pathogens Pathogens are evolving to utilize host sialic acids as receptors. CD33rSiglecs Siglecs are evolving to adjust to the changes of host sialic acids? Host sialic acids are evolving to adjust to the changes of Siglecs? Sialylated Pathogens Pathogens expressing sialic acids are evolving to utilize Siglecs as receptors? Siglecs are evolving to evade sialylated pathogens that exploit them as receptors? Based on Varki &Angata Glycobiology 16:1R-27R, Modified by Takashi Angata: For Glycoforum/Glycowords Primary “Red Queen” Effect Primary “Red Queen” Effect? Secondary “Red Queen” Effect?

24 Proposed Evolutionary Scenario for Multiple Human-Specific Changes in Sialic Acid Biology Varki A. Nature 446: 1023, What changes in sialic acid biology occurred during human evolution?

25 Proposed Evolutionary Scenario for Multiple Human-Specific Changes in Sialic Acid Biology Varki A. Nature 446: 1023, 2007

26 Proposed Evolutionary Scenario for Multiple Human-Specific Changes in Sialic Acid Biology Varki A. Nature 446: 1023, 2007 ST6GAL1 SIGLEC7SIGLEC9 DELETION AMINO ACID CHANGE EXPRESSION CHANGE GENE CONVERSION SIGLEC6SIGLEC11SIGLEC5/14SIGLEC13 CMAH SIGLEC12 SIGLEC1


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