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  and  - 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  1-4 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  -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 fIOY&list=PLrAEgIY86I6wYIgx3iE- KvyaRFzwuuixr&index=13http:// fIOY&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