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 bglycosidic 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 Nacetylglucosamine (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 45 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=sxa46xK
fIOY&list=PLrAEgIY86I6wYIgx3iEKvyaRFzwuuixr&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 Nacetylglucosamine and Nacetylalosaminuronic 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