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Chapter 4 Part 3

Chapter 4 Part 3

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Chapter 4 Part 3

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  1. Chapter 4 Part 3 The Cell Wall of Prokaryotes: Peptidoglycan and Related Molecules

  2. Things to look up • General structure of sugars • How do sugars bind together? • What is the difference between a and b-glycosidic linkages? • General structure of amino acids • Peptide bonds • What 2 groups link together 2 amino acids?

  3. Cell Walls • Eukaryotes – plants; differ chemically from prokaryotes; simplier in structure and less rigid • Bacteria • Peptidoglycan • Destroyed by lysozyme – cell lysis • Archae • lack peptidoglycan but contain walls made of other polysaccharides or protein

  4. Functions of the Cell Wall • Support/cell shape • Surrounds the plasma membrane and protects it and the interior of the cell from adverse changes in the outside environment • Prevents cells from rupturing • Point of anchorage for flagella

  5. Functions of the Cell Wall • Contributes to the ability of bacteria to cause disease • Important in the attachment to host cells • Barrier to some molecules • Site of action of some antibiotics • Chemical composition of the cell wall differentiates gram + from gram - bacteria

  6. Site of action of some antibiotics • Why is it important that antibiotics work on the cell wall? • Eukaroytes (besides plants) do not have cell walls • If kill bacteria, human cells will still live

  7. Peptidoglycan • Found in both gram + and gram – bacteria • Consists of a sugar backbone • Different chains of these sugars are linked together by peptide bonds between amino acids

  8. Consists of a sugar backbone of alternating repeats of N-acetylglucosamine (NAG, G) and N-acetylmuramic acid (NAM, M) NAM and NAG in b1-4 glycosidic linkage NAM is cross-linked between strands by short peptides attached to NAM Peptidoglycan

  9. Consists of four amino acids (peptides) L-alanine, D-glutamic acid, either lysine or diaminopimelic acid (DAP) and D-alanine Alternating pattern of L and D amino acids Unique because it is always L-amino acids found in other proteins Peptidoglycan

  10. 4 amino acids in peptidoglycan linked to NAMdifferent for Gram + and Gram -

  11. How do the different chains of peptidoglycan link together? • Different for gram + and gram – bacteria • Gram - • Linkage of amino group of DAP and carboxyl group of terminal D-alanine group

  12. How do the different chains of peptidoglycan link together? • Gram + • Peptide interbridge • Kinds and number of amino acids vary

  13. Peptidoglycan • Gram + - thick layer of peptidoglycan • Gram - - thin layer of peptidoglycan • Basis for the gram stain

  14. Bacterial Cell Wall Types • Gram type describes the structure of cell wall which influences the way it stains • Thicker peptidoglycan holds crystal violet • Gram + • Counterstain is pink • Gram -; thinner peptidoglycan

  15. Penicilin interferes with the final linking of the peptidoglycan rows by a peptide cross-bridge Lysozyme is an enzyme found in tears and saliva that breaks the b-1,4-glycosidic bonds between NAM and NAG Gram + cell wall is mostly peptidoglycan so it is more sensitive than gram – cell walls Gram + bacteria very sensitive to the action of penicillin and lysosome

  16. Gram-negativeBacteria have only a few layers of peptidoglycan Thin peptidoglycan Only about 10% of the cell wall is peptidoglycan 2 layered membrane Periplasm between the two layers Gram - vs. Gram +

  17. Thick peptidoglycan (90%) negatively charged teichoic acid Cross-linking occurs with a peptide interbridge (amino acids involved differ) Gram +

  18. Polymer of glycerol or ribitol Joined by phosphate groups Amino acids are attached Gram positive –Teichoic acid

  19. Lipoteichoic acid (lipid + teichoic acid) Spans the peptidoglycan layer and is linked to the plasma membrane Teichoic acid Linked to the peptidoglycan layer Purpose: stability, passage of ions, gives negative charge to the cell Gram positive –Teichoic acid

  20. Teichoic acid is negatively charged • Bacteria are stained with + dyes

  21. Contain an inner and outer membrane Peptidoglycan in between thin layer bound to lipoproteins in the outer membrane Outside the cytoplasmic membrane is the periplasmic space, a fluid filled space Contains degradative enzymes and transport proteins Proteins transported here by the SecYEG system Gram - bacteria

  22. Functions of the outer membrane of gram - bacteria • Gives a negative charge to the cell • Important in evading phagocytosis and host defense • Pathogenic properties • Selective barrier • Pore for entrance of hydrophilic molecules • Barrier to certain antibiotics and digestive enzymes

  23. Outer membrane of gram - bacteria • LPS • Lipoproteins • Anchor to the peptidoglycan • Porins • Proteins that form pores (channels) in the outer membrane • Wide enough to allow passage of small hydrophilic molecules • Large hydrophobic molecules cannot penetrate

  24. lipid A endotoxin properties, which may cause violent symptoms in humans Anchor to the membrane a core polysaccharide 6 or 7 C- sugars (Gal, Glu, NAG, Ketodeoxyoctonate or KDO, etc.) O-specific polysaccharide 6 C- sugars (Gal, Glu, Man, Rhm, etc.), repeating units of 4-5 sugars, often branched Reaches out into the environment Function as antigens – differentiate different bacteria LPS = lipopolysacharide

  25. What does LPS do? • Activate Toll receptors

  26. Toll receptors part of innate immunity Some receptors are extracellular while some are intracellular Some receptors dimerize Different Toll receptors for different pathogens

  27. Gram Stain • The structural differences between the cell walls of gram-positive and gram-negative Bacteria are thought to be responsible for differences in the Gram stain reaction • Alcohol can readily penetrate the lipid-rich outer membrane of gram-negative Bacteria and extract the insoluble crystal violet-iodine complex from the cell

  28. Gram stain •

  29. Gram Stain

  30. Some organisms have no cell walls • Mycoplasma • Intracellular parasite • Can only survive inside of their host • no need for cell wall but have tough membranes • More resistant to rupture than other bacteria • Another difference from other bacteria is that mycoplasma contain sterols that help protect from lysis

  31. Archaea have unusual cell walls • No peptidoglycan • Typically no outer membrane • Pseudomurein • Polysaccharide similar to peptidoglycan • Composed of N-acetylglucosamine and N-acetylalosaminuronic acid

  32. Archaea have unusual cell walls • Thermoplasma has no cell wall (extremely stable lipid membrane) • S-Layers • Most common cell wall type among Archaea • Consist of protein or glycoprotein