chapter 4 part 3
DESCRIPTION
Chapter 4 Part 3. The Cell Wall of Prokaryotes: Peptidoglycan and Related Molecules. 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 - PowerPoint PPT PresentationTRANSCRIPT
Chapter 4 Part 3
The Cell Wall of Prokaryotes: Peptidoglycan and Related Molecules
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?
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
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
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
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
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
Peptidoglycan• 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• 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
4 amino acids in peptidoglycan linked to NAM
different for Gram + and Gram -
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
How do the different chains of peptidoglycan link together?
• Gram +– Peptide interbridge– Kinds and number of amino acids vary
Peptidoglycan
• Gram + - thick layer of peptidoglycan• Gram - - thin layer of peptidoglycan
• Basis for the gram stain
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
Gram + bacteria very sensitive to the action of penicillin and lysosome
• 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 - vs. Gram +• Gram-negative Bacteria 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 +
• Thick peptidoglycan• (90%)• negatively charged teichoic acid• Cross-linking occurs with a peptide
interbridge (amino acids involved differ)
Gram positive –Teichoic acid
• Polymer of glycerol or ribitol• Joined by phosphate groups• Amino acids are attached
Gram positive –Teichoic acid• 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
Teichoic acid is negatively charged
• Bacteria are stained with + dyes
Gram - bacteria• 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
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
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
LPS = lipopolysacharide• 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
What does LPS do?
• Activate Toll receptors
Different Toll receptors for different pathogens
• Toll receptors part of innate immunity• Some receptors are extracellular while
some are intracellular• Some receptors dimerize
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
Gram stain
• http://www.youtube.com/watch?v=sxa46xKfIOY&list=PLrAEgIY86I6wYIgx3iE-KvyaRFzwuuixr&index=13
Gram Stain
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
Archaea have unusual cell walls
• No peptidoglycan
• Typically no outer
membrane
• Pseudomurein– Polysaccharide similar to
peptidoglycan
– Composed of N-
acetylglucosamine and N-
acetylalosaminuronic acid
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