Gram_negative_wall
Gram_negative_wall_key
Gram_positive_wall
Gram_positive_wall_labelled
Gram_positive_wall
The Bacterial Cell Envelope
The Gram Negative Cell Envelope

The diagram above shows a bacterial envelope. It contains an inner membrane, resembling the 'skin' or
membrane of an animal cell, in that it is a typical phospholipid bilayer, but then wrapped around this is a
cell wall layer of tough but flexible peptidoglycan and then wrapped around this is the outer membrane.

The labelled version of the diagram, followed by a key, is shown below. This type of envelope belongs to
bacteria known as
Gram negative bacteria. Gram's stain is a dye used to stain bacteria - if the bacteria
stain purple then they are said to be Gram positive, but if they do not stain then they are said to be Gram
negative and many Gram negative bacteria have the envelope structure shown here. This is important as
it drastically narrows down the choices when one is attempting to identify a strain of bacterium - the first
things we determine are the cell shape and the Gram stain reaction.

Whereas an animal cell, such as the cells that make up your body, have a single membrane skin
consisting of a
double layer of phospholipids, Gram negative bacteria have two such double-layered
membranes. Each membrane can be divided into an outer and an inner
leaflet - a leaflet is a single layer
of lipids (such as phospholipids) and two opposing leaflets are needed to make a stable membrane. The
outer membrane(OM) - the outermost of the two - is often the surface-most layer of a bacterial cell - the
layer that faces the outside world. The inner leaflet is made up of phospholipids (shown in green), like a
typical biological membrane, but the outer leaflet is made up of a type of lipid (fat) unique to Gram
negative bacteria, called
lipopolysaccharide (LPS) - shown in orange. LPS is also called endotoxin, as
the bacteria shed some of it into their environment and if they are inside the human body, then the LPS
has toxic effects on the human body and is a primary inducer of fever when a person is infected by
bacteria such as
Salmonella typhi which causes typhoid fever.
The inner membrane (IM) has two phospholipids (just as in the animal cell membrane). Beneath this
membrane, at the bottom of the digram, we have the internal parts of the cell (the cell cytoplasm), so the
inner membrane (or cytoplasmic membrane) is the innermost layer of the bacteria's 'skin'. To add a
scale, the inner membrane has the usual membrane thickness of about 7.5 nanometres (7.5 millionths
of a millimetre). This is extremely thin, but compared to the size of the bacterium (typically one
thousandth of a millimetre in diameter) this is not so thin and biological membranes are also very strong
and more importantly they automatically repair themselves (unless the damage is too severe). However,
the world is a rough place and so most bacteria have additional structures to protect them. In particular
the layer of material called
peptidoglycan forms a very strong wall around the cell. Peptidoglycan is a
tough, rigid and fibrous material, though in Gram negatives it is quite thin and so quite flexible and
elastic and maintains the shape of the cell. The peptidoglycan sits in the 'space' between the inner and
outer membranes. This space is called the periplasmic space and is actually filled with a gelatinous
substance called
periplasm. Periplasm contains many proteins (not shown in the diagram) including
lipoproteins (proteins that connect to lipids in the membranes) that connect the peptidoglycan to the
inner membrane, anchoring everything in place. The periplasm is typically 13 to 25 nm thick ( 1 nm = 1
nanometre = 1 millionth of a millimetre) depending on growth conditions and contains a 5 to 8 nm thick
layer of peptidoglycan which is closely associated with the OM.

Both membranes contain proteins that float in the lipid sheets. However, the proteins of the inner
membrane are especially diverse and three are shown in the diagram. These proteins may span the
membrane (so-called transmembrane proteins that cross the membrane) or sit in one or other leaflet or
even flip from one side to the other. They have very diverse functions, including the import of nutrients
into the cell and the export of unwanted materials. Others function as sensors that monitor the
environment. The outer membrane contains proteins that form pores called porins that allow nutrients
into the periplasm where they may be processed by periplasmic proteins and then transported into the
cell by inner membrane proteins.

The lipopolysaccharide of the outer leaflet of the outer membrane may have chains of sugars
(
polysaccharides) attached to it. Depending on the strain and species these chains may be of variable
length, but they form a thin layer of protective slime over the cell surface. In addition much longer
polysaccharides may also be present, which form a thick
slime capsule around the cell
(polysaccharides may form a slimy gel when mixed with water). This capsule may as thick as the
bacterium and offers superb protection against many threats. Whether or not there is a slime capsule,
there may be a layer of crystalline protein on top of the outer membrane, called an S-layer, which may
offer additional protection. All those structures that occur on the surface may be involved in adhesion -
allowing the bacteria to stick to surfaces.

A fringe of
pili may also cover the cell and flagella may also project through the envelope.

The Gram Positive Cell Wall

The Gram positive envelope lacks the outer membrane altogether, and like an animal cell has a single
membrane which is a double layer of phospholipids. However, on top of this is a very thick wall of
peptidoglycan (10-80 nanometres thick). There may also be an S-layer on top of the peptidoglycan and
there may be a slime capsule also on top of the peptidoglycan. The structure of Gram positive bacterial
envelopes is shown below:
Additionally the peptidoglycan layer may contain long chain molecules called teichoic acids (TAs) and
lipoteichoic acids (LTAs). Lipoteichoic acids (LTAs) are attached to the cytoplasmic membrane and pass
through the peptidoglycan to emerge from its surface as a fringe. The peptidoglycan is a fibrous mesh and
some fibres also stand up on the surface, giving the cell a hairy appearance (though this would be slimy in
real life). The teichoic acids do not attach to the lipids in the membrane but are present on the outer
surface of the peptidoglycan layer and also within it. The thick peptidoglycan layer forms a rigid and very
tough wall that protects the bacterium and maintains its shape.

Gram's stain contains a dye that stains peptidoglycan purple. Peptidoglycan is unique to bacteria, but in
Gram negative bacteria the outer membrane stops the dye molecules from reaching the peptidoglycan,
and so it does not get stained. Some Gram negative bacteria also have no peptidoglycan at all, they may
have a single membrane, perhaps with some slime on top, and no rigid wall, or they may have rigid walls
composed of different materials. There is in fact a very wide range of envelope types in bacteria, but the
two discussed here are the two most commonly encountered.

Importance of the cell envelope

First of all the cell envelope is protective. It protects against both toxic chemicals and physical trauma.
Mechanically, even the envelope of Gram negative bacteria, with a thin peptidoglycan layer, is immensely
strong. For example, the pressure needed to collapse Gram negative bacteria varies from 30 to 100
atmospheres, depending upon species. Compare this  with the pressure needed to collapse the hull of a
military submarine: 20-25 atmospheres for a WWII U-boat and about 73 atmospheres for a modern nuclear
submarine! Usually the inside of the cell will be maintained at a higher pressure than the outside. The
strength of the cell wall also maintains the shape of the cell.

The slime capsule protects the bacteria from noxious chemicals and from predators such as
amoebae (or
in the case of pathogens the white blood cells). The slime may be so extensive as to encapsulate a whole
colony of bacteria. Such slime mats and biofilms and balls of millions of bacteria may form quite large and
easily visible structures.

The lipid membranes act as selective barriers, allowing only certain materials in and out of the cell, rather
like a passport control (but probably more rigorous!). Nutrients are allowed into the cell and unwanted
waste and toxins are removed. In this way the cell can maintain an internal environment different from that
outside the cell - inside is salty, even if the cell is in fresh water and the salts may be different to those in
sea water. Lipid membranes are disrupted by detergents (lipids are fats) and when this happens the cell
contents mix with the surroundings and the cell is destroyed.

The envelope must also sense the environment - the envelope of a cell is its interface with the outside
world. It does this with specific proteins that are designed to detect and respond to specific signals, such
as the presence of nearby food or signals from other bacteria.

In a later update we shall look at how cells, such as bacteria, import and export materials.

Other Variants

The mollicutes are a strange group of bacteria which lack a rigid cell wall and simply have a single typical
bilayered phospholipid membrane. Carbohydrate chains are attached to the outer surface of the
membrane, giving it a protective slimy covering. These cells, like animal cells, are sensitive to
osmotic
lysis
. The contents of cells are quite salty, for example animal cell cytoplasm has a salt content similar to
that of sea water, and when a cell lacking a rigid wall is placed in pure water, then the salts inside it draws
water through the membrane and the cell swells and bursts. This movement of water across the membrane
is called
osmosis and the bursting of the cell is lysis. Mollicutes and animal cells burst when immersed in
fresh (non-salty) water. Cells with a rigid wall, such as plant cells and the typical bacteria described above
do not burst since their rigid walls prevent the cell from swelling too much. Mollicutes are also
pleomorphic - meaning that they are capable of assuming a variety of shapes, since they lack a rigid wall
and are somewhat plastic and elastic. The mollicute
Spiroplasma, however, adopts a helical form (0.2
micrometres in diameter and 3 to 5 micrometres long) as they contain contractile cylindrical protein rods
that both maintain shape and move the cell.

Mollicutes are parasitic on other creatures.
Spiroplasma is a parasite of plants and arthropods.
Mycoplasma is a mollicute that lives in the mucous membranes of animals (such as the mucous lining the
repiratory and genital tracts and the synovial membranes of joints) and may cause pneumonia or arthritis.
Acholeplasma is a mollicute that infects various animal tissues. Anaeroplasma inhabits the rumen of
ruminants and secretes enzymes that destroy the rigid walls of other bacteria and lyses them, allowing the
Acholeplasma to feed on the bacterial cell contents. Ureaplasma need urea as a source of energy and
normally inhabit the mouth, respiratory and genital tracts of humans and other animals. The phytoplasmas
cause more than 600 recognised plant diseases and are spread by insects that feed on the phloem of
plants (such as aphids).

Most mollicutes incorporate sterols from the host into their membranes, which increases the strength and
rigidity of their membranes.
Acholeplasma may synthesise carotenoids to strengthen its membrane in a
similar way. The mollicutes survive without cell walls because of their parasitic nature - their host provides
an osmotically stable environment (the saltiness of animal and plant bodies is well regulated) and so  they
can survive without a rigid cell wall, but would soon be lysed in the external environment.

The
archaebacteria are an ancient group of bacteria that are distant relatives of the bacteria we have
considered so far (the eubacteria or 'true' bacteria). In fact, biochemically all the eukaryotes (animals,
plants, fungi and protoctistans) are more closely related to eubacteria than eubacteria are to
archaebacteria! Archaebacteria probably represent the most ancient life forms on Earth that still exist.
Although quite widespread and turning up all the time in common places, archaebacteria are best known
as inhabitants of extreme environments, such as hot volcanic ponds and extremely salty lakes and highly
acidic waters.

The membranes of eukaryotes and eubacteria all have the same basic structure - they are composed of
two leaflets of phospholipids with proteins embedded and are about 8 nanometres thick - what we call the
unit membrane. These phospholipids all consist of  lipids joined to a phosphate head by ester linkages
(-O-(C=O)-). However, the phospholipids of archaebacteria are unique in that the lipids are joined to the
phosphate head by ether linkages (-O-). This fundamental difference suggest that archaebacteria and
eubacteria diverged early in the Earth's history (maybe 3 billion years ago or so) or had separate origins
and that all other cellular life as we know it ion Earth, evolved from the eubacteria (yes, humans evolved
from bacteria!).

Archaebacteria may possess a rigid cell wall, but whereas the cell wall of eubacteria (usually called simply
bacteria) is usually made of peptidoglycan (murein) the archaebacterial cell wall contains a different
chemical called pseudomurein (pseudopeptidoglycan), but is otherwise similar to the Gram positive
arrangement - with a wall overlying a single unit membrane. Some archaebacteria have a flexible wall of
polysaccharides (long chain sugars) and lipoproteins (proteins with attached lipids) rather than a rigid cell
wall.

Planctomycetes are unusual bacteria with a single unit membrane and a rigid wall made of glycoprotein
(protein with sugars attached). In short, the bacteria exhibit a very wide range of cell envelope structures,
which is a reflection of their diverse evolutionary relationships and diverse biochemistry.

Note on the thickness of biological membranes

It has been pointed out to use that the textbook value of membrane thickness, which is obtained from
standard transmission electron microscopy (TEM) using chemical fixatives, which is some 6-8 nm, is larger
than that measured with techniques like diffraction or predicted on the basis of computed dimensions of
membrane-spanning proteins. The latter give values of around 3 nm (though these measurements may
also be subject to larger errors than usually assumed).

Electron microscopy likely exaggerates the apparent thickness of membranes - chemical fixation may
cause them to swell, and staining with electron-dense reagents will certainly add to their thickness. There
is possibly also observer bias - selecting areas for measurement where the membrane is more readily
visible (although stereology can help address these issues). There is also a difficult issue of defining the
'membrane'. In reality membranes have attached carbohydrate chains, forming the glycocalyx, and some
of these may be incorporated in crude measurements. We conclude that the smaller value, using less
disruptive techniques like neutron diffraction, are likely to be the more accurate measure of the actual
phospholipid bilayer thickness (excluding glycocalyx). To conclude, the average thickness of the bacterial
inner membrane (meaning the lipid bilayers of inner and outer membranes) is probably closer to 3.5 to 4
nm, as obtained by X-ray diffraction. Measurements of the outer membrane give a thickness of 12-14 nm,
based on cryo-fixation TEM, which avoids the problems of chemical fixation and staining.

That said the official value for a mammalian cell membrane is given as 7.5 nm (not sure what method was
used here). Animal cell membranes are different, with different cholesterol components, for example, and
variation between different cell types is to be expected. Textbooks often do not cite their sources and early
biologists often applied 7.5 nm as a ball-park figure to any phospholipid bilayer, possibly without
attempting accurate measurements.

Figures with references can be found in the
bionumbers database.

Outer Membrane Proteins (OMPs)

OMPs. It's easier to understand the bacterial envelope if one has some examples of proteins associated
with it and their functions. Outer membrane contains proteins (OMPs) are either an integral part of the
outer membrane or attached or embedded in its surface. Since the Om interfaces with the external
environment, the functions of these proteins are often readily apparent. In addition there are periplasmic
proteins in the periplasm and inner membrane proteins (IMPs). Often all three classes of proteins
assemble in groups which work together. e.g. to transport materials into or out of the cell. OMPs are also
exposed on the surface of the cell and so are a target for host antibodies during infection by pathogenic
strains of bacteria.

Porins. A major class of OMP is the porin. These associate in triplets (trimers) to form a channel or pore
across the outer membrane. They may be specific or non-specific. For example LamB is a porin in
Escherichia coli which allows maltose and maltodextrin sugars, a nutrient, to pass from the extrenal
medium into the periplasm. The maltose binds to a specific site within the porin channel before diffusing
across (essentially one-dimensional diffusion). OprB is a sugar-specific porin in
Pseudomonas aeruginosa.

Non-specific porins. Some porins, however, are largely non-specific, allowing any substance small
enough to fit inside to cross. OprF is a major OMP in
Pseudomonas and is non-specific. It is also
multifunctional, acting as a structural protein, giving the outer membrane strength, and binds the outer
membrane to the peptidoglycan in the periplasm. It forms channels of two different sizes but is
downregulated in some multidrig resistant strains, reducing the permeability of the OM to antibiotics. The
OM of
Pseudomona aeruginosa is generally only 8% as permeable as that of Escherichia coli, though its
porins allow larger molecules to cross the membrane (up to a molecuylar mass of 3000 in
Pseudomonas
aeruginosa
compared to 500 in Escherichia coli).

Efflux porins. Simply reducing membrane permeability is not sufficient to keep drugs and toxins out of the
bacterial cell, however, and some porins are efflux porins which are coupled to efflux systems which pump
unwanted materials out of the bacterium. These include OprJ and OprM in
Pseudomonas. Although more
copies of these efflux porins may be found in bacteria exposed to antibiotics in a clinical or human-made
environmental setting, conferring antibiotic resistance, it is important to understand that they have other
more natural efflux functions, removing plant toxins and naturally occurring antibiotics, for example. These
pumps often recognise and expel a wide variety of toxins, and in at least one case it has been shown that
they tend to recognise a wide range of compounds, but especially those with activated benzene rings, or
similar charge distributions, which are a common feature of many plant secondary metabolites. This wide
specificity also gives them the ability to expel novel toxins, including antibiotics.

OprP is a
Pseudomonas porin specific for phosphate (an essential mineral nutrient). Phosphate starvation
induces the production of these porins which act as electrical wires conducting negatively charged
phosphate ions along a series of binding sites inside the channel before passing them onto a periplasmic
phosphate-binding protein.

Gated porins and iron transport. Some porins are gated, meaning they can be opened and closed. One
example involves iron uptake. Iron is an essential nutrient for aerobic organisms. Since iron is tightly bound
to carrier molecules in the human body it is hard for pathogenic bacteria to acquire optimum levels of iron
and iron often limits their growth. To overcome these problems bacteria may import haemoglobin, and
related molecules, e.g. in
Pseudomonas an OMP called PhuR binds to haemoglobin and haem groups and
initiates their import into the bacterium.
Pseudomonas, and many other bacteria, can also secrete its own
high affinity iron carriers or chelators called
siderophores. In commensals and pathogens living in the
body, these siderophores attempt to steal iron from the body's own carrier proteins. Once they are bound
to iron, specific OMPs recognise the iron-bound siderophore and initiate its transport into the bacterium.
Pseudomonas typically produces up to two siderophores: pyoverdine and pyochelin. Pyoverdin is the
main siderophore used to steal iron from transferrin, the mammalian blood plasma protein which transports
iron around the body. Once bound to iron, pyoverdin binds to its specific receptor, an outer membrane
protein called FpvA. This protein is
TonB-dependant, meaning that it complexes to TonB which is
essential for its function. TonB is a periplasm-spanning inner membrane protein (IMP) involved in passing
signals across the cell envelope. TonB will then open the gate, allowing the iron-pyoverdin to cross the
envelope and enter the bacterial cell.

Pyochelin binds to its own receptor, an OMP called FptA, which is a virulence factor, meaning that its
presence is associated with virulent disease-causing strains, suggesting that it is especially useful in
stealing iron from the host.

Other gated porins include BtuB, an
Escherichia coli receptor for vitamin B12, and is also
TonB-dependant. OprH is a
Pseudomonas OMP thought to act as a gated porin for the import of
magnesium and calcium ions.

Inner Membrane Proteins (IMPs)

Crossing the OM through porins is not the whole story since molecules must also cross the periplasm and
the inner membrane (IM). The inner membrane requires its own transport proteins. Often, however, the
various OM, periplasmic and IM components form a
serial transport system in which three proteins are
arranged in series: an OM transporter/porin connects to a periplasmic transport protein (periplasmic
permease) which connects to an IM transporter, allowing molecules to more easily cross the whole cell
envelope. It is not always clear whether the proteins connect physically or whether they are distinctly
separate (for example materials entering the periplasm may need to diffuse across the periplasm to reach
the IM transporter). Below we consider some inner membrane proteins (IMPs) though our survey will not be
exhaustive. When the three components of a serial transport system are in close proximity or physically
connected then this clearly speeds up the transport process by restricting diffusion of the solute being
transported to one dimension rather than three.

ABC Transporters. ABC (ATP-binding cassette) proteins have an ATP binding domain to utilise energy.
ABC proteins are usually transporters. ABC transporters are ATP-driven transport systems (see energy
systems) that transport molecules across the cell envelope using cell-derived energy in the form of ATP.
They occur in prokaryotes and eukaryotes, but here we shall focus on prokaryotes only. They contain two
membrane-spanning regions that span the inner membrane (which possibly form a channel to transport
materials across the IM) and two regions that extend into the cytoplasm and utilise the ATP. Binding and
hydrolysis of ATP to ADP and inorganic phosphate releases chemical energy which causes a
conformational (shape) change in the transporter.

Uptake ABC transporters (ABC importers) are involved in importing useful materials into the cell. They
utilise a high-affinity solute binding protein in the periplasm (a periplasmic permease). In Gram positive
bacteria they may be tethered to the cell surface or fused to the transporter protein. These import
nutrients and osmoprotectants (small molecules which ensure that the cell has the correct osmotic
potential, see membranes) into the bacterial cell. They import molecules like sugars, diamino acids, small
peptides, metal ions, anions, siderophores and vitamin B12. There are some 80 types of ABC transporter
in
Escherichia coli alone!

This leads to some complex arrangements which are still being elucidated. For example, in the
Helicobacter pylori, which lives in the mammalian stomach, nickel and cobalt can cross the outer
membrane via the protein transporter NikH, which is associated with TonB, and so is a gated porin of sorts.
TonB provides the energy for transport across the OM (it opens the gate). These ions can then cross the
inner membrane through the NixA transporter, an inner membrane protein. They can also cross the IM,
however, by using an ABC transporter called FecDE, which appears independent of TonB rather than
being bound to it in a complex. Whether the FecDE ABC transporter requires its own OM transporter, or
whether it uses nickel and cobalt ions that have diffused through the periplasm, perhaps after entering via
NikH, is not clear.

ABC exporters transport materials out into the periplasm (where they may subsequently be exported
across the outer membrane). These may export slime capsule polysaccharides,
lipopolysaccharides,teichoic acids, siderophores, antibiotics, toxins and unwanted drugs that have entered
the bacterial cell, etc. They may contribute to antibiotic resistance (although there other transporters than
may do this).

RND (resistance-nodulation-division) Efflux Pumps. RND efflux pumps are another class of exporters
that are also implicated in resistance to antibiotics. Examples include MexAB-OprM and MexXY-OprM in
Pseudomonas aeruginosa. These three-component export pumps consist of an outer membrane efflux
protein (OEP), an efflux porin (such as oprM spanning the outer membrane) an RND protein  (e.g. MexB)
and a membrane fusion protein (MFP, e.g. MexA) which spans the periplasm and apparently joins the OEP
to the RND and possibly forms a channel. Thus we can envisage molecules crossing all three proteins in
series as they are transported across the envelope. In addition to their role in pumping out antibiotics and
other toxins, RND transporters possibly export siderophores and homoserine lactone autoinducer used in
quorum sensing, amongst other things.

Ion Symporters and Antiporters. These transporters are more like those that are likely to be familiar to
college students (see
membranes). Symporters transport two different ion species together in the same
direction across the membrane. They harness a concentyration gradient of one ion species to drag the
other one along with it. As bacteria respire they generate a proton gradient across the inner membrane
(which acts as an electrical capacitor storing electric charge) - they pump protons into the periplasm by
using energy liberated by oxidising fuels such as glucose. These ions are used for a variety of purposes,
from making ATP to driving flagella rotation. They can also be used for transport. For example, the LacY
transporter of
Escherichia coli imports lactose. It is a proton/lactose symporter, allowing a proton flowing
passively down its concentration gradient (diffusion) to cross the inner membrane and enter the cytosol,
along with a lactose molecule which is pulled in with it.

In contrast antiporters also pump two different ion species together, harnessing the concentration gradient
of one of the species, but transport the two ions in opposite directions. For example, the sodium/proton
antiporter of
Escherichia coli expels one sodium ion for every proton taken in.

Periplasmic Proteins

The functions of the periplasm include the detection and processing of nutrients, nutrient transport, the
biogenesis of certain proteins, slime capsules, lipopolysaccharide. In addition to proteins, the periplasm
also contains carbohydrate
glucans, short highly-branched chains of glucose molecules which are
produced under hypo-osmotic conditions; that is when the cell contents are too dilute, which would cause
the loss of water by osmosis, but the presence of glucans in the periplasm rectifies this. These glucans
probably make the periplasm more gelatinous.

Many periplasmic proteins are solute or ion-binding proteins which couple with ABC transporters to import
nutrients (such as amino acids, sugars, vitamins, peptides and ions). These
periplasmic solute-binding
proteins
have a high affinity for their specific solute, and so are able to concentrate a solute that is
otherwise at low concentration outside the bacterial cell. They are typically monomers consisting of two
globular domains joined by a flexible stalk or hinge. The two globules come together to 'pincer' the solute.
The binding protein then interacts with an ABC transporter (permease) to translocate the solute into the
cell.

Periplasmic binding-proteins also function to detect solutes in
chemotaxis. For example, the maltose sugar
receptor (sensor) in
Escherichia coli consists of the maltose-binding protein (MBP) MalE and the Tar signal
transducer. MalE binds the maltose and then interacts with Tar, which spans the inner membrane and
relays the signal to the cytoplasm.

Conclusion on envelope proteins

Hopefully this summary of envelope proteins gives you some feel for the sorts of functions of proteins in
the bacterial envelope and how OMPs, IMPs and periplasmic proteins interact to provide useful functions.
The more we learn about cells the more we move away from the old idea of cells as 'bags of chemicals'
diffusing and mixing at random, to one where processes occur in a much more ordered fashion, often
restricted to small compartments or molecular machines, such as in the three-component serial transport
systems which speed-up the process of diffusion. We also increasingly see not simple enzymes and
chemicals but molecular machines with moving parts, such as the gated porins. This theme will become
increasingly apparent if you read the other cell biology and microbiology articles on ww.cronodon.com.

For more information on some special envelope proteins see the sections on
flagella and pili.

The chemical structure of peptidoglycan

The peptidoglycan in the walls of bacteria is a polysaccharide, that is it is made up of chains of sugar
molecules and so is a
carbohydrate. It is a polymer (made up of molecular chains) of a disaccharide.
Recall that a disaccharide is a sugar made up of two sugar monosaccharide (single sugar) units joined
together. In this instance the sugars are N-acetyl-glucosamine (NAG) and N-acetyl-muramic acid (NAM).
Thus peptidoglycan is made up of chains of alternating -NAG-NAM- units.

Both these sugars are derived from glucose, specifically the isomer beta-D-glucose, see
carbohydrates.
D-glucose or dextrose (grape sugar) is the usual form of glucose found in plants and utilised as fuel by
animal and plant cells. Dextrose contains six carbon atoms and forms an hexagonal ring using five of the
carbon atoms plus an oxygen atom. The carbon atoms (C) are numbered clockwise from the oxygen atom
(O), 1 through 6, in the diagrams below. Dextrose can exist in two isomers that concern us: alpha and
beta, depending whether the hydrogen atom on carbon atom number 1 (rightmost corner of the hexagon
shown below) is above the plane of the ring (alpha-dextrose) or below the plane of the ring (beta-dextrose)
as shown below (where for clarity hydrogen atoms are represented as small balls and a C atom is situated
at each of the five unlabelled corners of the ring):
Gram negative wall labelled
Comment on this article!
The constituents of peptidoglycan
Notice how beta-D-glucose is modified:

In NAG there is a nitrogen containing
amine group (NH2) added to the second carbon atom, hence
glucosamine. This amine group has an acetic acid (ethanoic acid CH3COOH) joined to it: the acetyl
group (CH3CO-), joined to glucosamine with the loss of water (H2O) hence acetyl-glucosamine. Since this
acetyl group is joined to the glucosamine via the nitrogen (N) atom of the amine we call the molecule
N-acetyl-glucosamine.

NAM is NAG with a lactic acid derivative (the lactyl group) joined to the third carbon atom (the bent bond is
for clarity). Since this lactyl group connects to the NAG via an O bridge (-O-) it is an ether and we say that
NAM is a
lactyl ether of glucosamine. To this lactyl group is added four amino acids. Recall an amino
acid is an organic molecule with both a carboxylic acid group, -COOH, and an amine group, NH2, attached
to it.

The sequence of amino acids joined to NAM varies, but the sequence shown above is the most common
one and is found in most Gram negative bacteria and some Gram positives. These amino acids are:
L-alanine, D-glutamic acid,
meso-diaminopimelic acid (DAP) and D-alanine. The usual amino acids found
in proteins are 20 or so L-amino acids. L and D amino acids are mirror images of one another, they are
isomers or different forms of the same chemical. Bacterial envelopes are unusual in containing two
D-amino acid isomers. Most biological systems use L-amino acids only. Also unusual is
meso-diaminopimelic acid, which is a diamino acid, meaning it contains a carboxylic acid group and two
amine groups.

These amino acids are joined to one-another in a short side-chain by peptide bonds (-NH-CO-) between
the amino group of one amino acid and the carboxylic acid group of another (with the loss of one molecule
of water per peptide bond formed). See
proteins for more information on peptide bonds. Thus, NAM has a
short peptide side-chain attached to it.

The sugars in each peptidoglycan are joined by
beta-(1,4)-glycosidic links or bonds. This means they
are joined by an O atom (one water molecule, H2O, being removed from the bonding sugars for each
glycosidic link formed) bridging carbon atom 1 on one sugar (left-hand sugar in our diagram) to carbon 4
of the adjacent sugar (right-hand sugar in our diagram) as shown below:
peptidoglycan chain
In our example 4 sugars, or two disaccharide units are shown linked together. In bacteria each chain typically
contains between 10 and 65 disaccharide units joined together in this fashion. The many chains making up
the bacterial cell wall do not just flop about! Rather, they are bonded to one-another to form a fairly rigid 3D
structure or giant molecule. This is where the spare amino group on DAP comes in handy, since it can bind
to the unused carboxylic acid group of D-alanine, forming a peptide bond between two adjacent chains, a
cross-link. Not all DAPs form these cross-links, but many do and they can cross-link with chains, above,
below, or to the sides, so that all the chains are cross-linked together. The more cross-links that form teh
more rigid and stronger the cell wall, but some flexibility is needed is the cell wall is to stretch as the cell
grows and bacteria can regulate the degree of cross-linking.

This type of peptidoglycan, the most common type, is called the
m-Dpm-direct type, since cross-links occur
directly to meso-diaminopimelic acid (m-Dpm or DAP). This is a group A (or type 1) peptidoglycan.

Group A peptidoglycans are those in which the crosslinks utilise the third amino acid in the side-chain
(DAP in this case). In non-direct Group A peptidoglycans, D-alanine crosslinks to the third amino acid (e.g.
lysine in place of DAP) indirectly through a variable number of additional amino acids, forming a
peptide
bridge
.

In
group B (or type 2) peptidoglycans the cross-links involve the second amino acid of the side-chain,
D-glutamic acid which, like the terminal D-alanine, also has a spare carboxylic acid group for peptide bond
formation.

Variations in the side-chain generally occur in the third amino acid which must be a diamino acid to form the
cross-link in the direct crosslink types. In the indirect cross-links a diamino acid (e.g. lysine) must occur in the
peptide bridge to link the carboxylic acid ends of the two peptide side-chains.

Peptidoglycan growth

Bacteria need to synthesise more peptidoglycan when they grow and divide. Divisomes are molecular
machines involved in cell division and these contribute to cell growth by synthesising new cell wall material at
the plane where cell division is destined to occur, for example at the middle of a rod-shaped bacterium or the
equator of a spherical bacterium. This requires elongating peptidoglycan chains. To do this enzymes called
autolysins break the beta 1-4 glycosdic bonds and peptide cross-links to allow new sugar units to be
inserted. Then enzymes called
transglycosidases insert and link new sugar units into the breaks. A further
set of enzymes, the
transpeptidases reform the peptide cross-links. Penicillin type antibiotics inactivate
the transpeptidases, preventing the formation of new cross-links in growing cells resulting in failed growth
and a weakened cell wall which is prone to rupture, killing the cell.


This page was last updated on 12/9/2013.