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