Lipopolysaccharide
Encyclopedia
Lipopolysaccharides also known as lipoglycans, are large molecule
s consisting of a lipid
and a polysaccharide
joined by a covalent bond
; they are found in the outer membrane
of Gram-negative
bacteria, act as endotoxin
s and elicit strong immune responses in animals.
of Gram-negative
bacteria, contributing greatly to the structural integrity of the bacteria, and protecting the membrane from certain kinds of chemical attack. LPS also increases the negative charge of the cell membrane
and helps stabilize the overall membrane structure. It is of crucial importance to gram-negative bacteria, whose death results if it is mutated or removed. LPS is an endotoxin
, and induces a strong response from normal animal immune system
s. It has also been implicated in non-pathogenic aspects of bacterial ecology, including surface adhesion, bacteriophage
sensitivity, and interactions with predators such as amoebae.
LPS is required for the proper conformation of Omptin
activity; however, smooth LPS will sterically hinder omptins.
LPS acts as the prototypical endotoxin because it binds the CD14
/TLR4
/MD2
receptor
complex, which promotes the secretion of pro-inflammatory
cytokine
s in many cell types, but especially in macrophages and B cells. In Immunology, the term "LPS challenge" refers to the process of exposing a subject to an LPS that may act as a toxin.
LPS is also an exogenous pyrogen (external fever-inducing substance).
Being of crucial importance to gram-negative bacteria, these molecules make candidate targets for new antimicrobial
agents.
Some researchers doubt reports of generalized toxic effects attributed to all lipopolysaccharides, in particular, for cyanobacteria.
It comprises three parts:
contained within an LPS is referred to as the O antigen
, O polysaccharide
, or O side-chain of the bacteria.
The O antigen is attached to the core oligosaccharide, and comprises the outermost domain of the LPS molecule. The composition of the O chain varies from strain to strain. For example, there are over 160 different O antigen structures produced by different E. coli strains. The presence or absence of O chains determines whether
the LPS is considered rough or smooth. Full-length O-chains would render the LPS smooth, whereas the absence or reduction of O-chains would make the LPS rough. Bacteria with rough LPS usually have more penetrable cell membranes to hydrophobic antibiotics, since a rough LPS is more hydrophobic. O antigen is exposed on the very outer surface of the bacterial cell, and, as a consequence, is a target for recognition by host antibodies
.
and commonly contains sugar
s such as heptose
and 3-deoxy-D-mannooctulosonic Acid (also known as KDO, keto-deoxyoctulosonate). The LPS Cores of many bacteria also contain non-carbohydrate components, such as phosphate, amino acids, and ethanolamine substitutents.
disaccharide
decorated with multiple fatty acids. These hydrophobic fatty acid chains anchor the LPS into the bacterial membrane, and the rest of the LPS projects from the cell surface. The lipid A domain is responsible for much of the toxicity of Gram-negative bacteria. When bacterial cells are lysed
by the immune system
, fragments of membrane containing lipid A are released into the circulation, causing fever, diarrhea, and possible fatal endotoxic shock (also called septic shock
).
It has recently been shown that a specific enzyme in the intestine (alkaline phosphatase
) can detoxify LPS by removing the two phosphate groups found on LPS carbohydrates. This may function as an adaptive mechanism to help the host manage potentially toxic effects of gram-negative bacteria normally found in the small intestine.
O-antigens (the outer carbohydrates) are the most variable portion of the LPS molecule, imparting the antigenic specificity. In contrast, lipid A is the most conserved part. However, lipid A composition also may vary (e.g., in number and nature of acyl
chains even within or between genera). Some of these variations may impart antagonistic properties to these LPS. For example Rhodobacter sphaeroides
diphosphoryl lipid A (RsDPLA) is a potent antagonist of LPS in human cells, but is an agonist in hamster and equine cells.
It has been speculated that conical Lipid A (e.g., from E. coli) are more agonistic, less conical lipid A like those of Porphyromonas gingivalis may activate a different signal (TLR2 instead of TLR4), and completely cylindrical lipid A like that of Rhodobacter sphaeroides is antagonistic to TLRs.
Lipopolysaccharide gene clusters are highly variable between different strains, subspecies, species of bacterial pathogens of plants and animals.
s. LPS challenge also produces many types of mediators involved in septic shock
. Humans are much more sensitive to LPS than other animals (e.g., mice). A dose of 1 µg/kg induces shock in humans, but mice will tolerate a dose up to a thousand times higher. This may relate to differences in the level of circulating natural antibodies between the two species.
Said et al. showed that LPS causes an IL-10-dependent inhibition of CD4 T-cell expansion and function by up-regulating PD-1
levels on monocytes which leads to IL-10 production by monocytes after binding of PD-1
by PD-L.
Bruce Beutler was awarded a portion of the 2011 Nobel Prize in Physiology or Medicine for his work demonstrating that TLR4 is the LPS receptor.
Molecule
A molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...
s consisting of a lipid
Lipid
Lipids constitute a broad group of naturally occurring molecules that include fats, waxes, sterols, fat-soluble vitamins , monoglycerides, diglycerides, triglycerides, phospholipids, and others...
and a polysaccharide
Polysaccharide
Polysaccharides are long carbohydrate molecules, of repeated monomer units joined together by glycosidic bonds. They range in structure from linear to highly branched. Polysaccharides are often quite heterogeneous, containing slight modifications of the repeating unit. Depending on the structure,...
joined by a covalent bond
Covalent bond
A covalent bond is a form of chemical bonding that is characterized by the sharing of pairs of electrons between atoms. The stable balance of attractive and repulsive forces between atoms when they share electrons is known as covalent bonding....
; they are found in the outer membrane
Outer membrane
The bacterial outer membrane is found in Gram-negative bacteria. Its composition is distinct from that of the cytoplasmic membrane - among other things, the outer leaflet of the membrane includes a complex lipopolysaccharide whose lipid portion acts as an endotoxin - and it is linked to the cell's...
of Gram-negative
Gram-negative
Gram-negative bacteria are bacteria that do not retain crystal violet dye in the Gram staining protocol. In a Gram stain test, a counterstain is added after the crystal violet, coloring all Gram-negative bacteria with a red or pink color...
bacteria, act as endotoxin
Endotoxin
Endotoxins are toxins associated with some Gram-negative bacteria. An "endotoxin" is a toxin that is a structural molecule of the bacteria that is recognized by the immune system.-Gram negative:...
s and elicit strong immune responses in animals.
Functions
LPS is the major component of the outer membraneOuter membrane
The bacterial outer membrane is found in Gram-negative bacteria. Its composition is distinct from that of the cytoplasmic membrane - among other things, the outer leaflet of the membrane includes a complex lipopolysaccharide whose lipid portion acts as an endotoxin - and it is linked to the cell's...
of Gram-negative
Gram-negative
Gram-negative bacteria are bacteria that do not retain crystal violet dye in the Gram staining protocol. In a Gram stain test, a counterstain is added after the crystal violet, coloring all Gram-negative bacteria with a red or pink color...
bacteria, contributing greatly to the structural integrity of the bacteria, and protecting the membrane from certain kinds of chemical attack. LPS also increases the negative charge of the cell membrane
Cell membrane
The cell membrane or plasma membrane is a biological membrane that separates the interior of all cells from the outside environment. The cell membrane is selectively permeable to ions and organic molecules and controls the movement of substances in and out of cells. It basically protects the cell...
and helps stabilize the overall membrane structure. It is of crucial importance to gram-negative bacteria, whose death results if it is mutated or removed. LPS is an endotoxin
Endotoxin
Endotoxins are toxins associated with some Gram-negative bacteria. An "endotoxin" is a toxin that is a structural molecule of the bacteria that is recognized by the immune system.-Gram negative:...
, and induces a strong response from normal animal immune system
Immune system
An immune system is a system of biological structures and processes within an organism that protects against disease by identifying and killing pathogens and tumor cells. It detects a wide variety of agents, from viruses to parasitic worms, and needs to distinguish them from the organism's own...
s. It has also been implicated in non-pathogenic aspects of bacterial ecology, including surface adhesion, bacteriophage
Bacteriophage
A bacteriophage is any one of a number of viruses that infect bacteria. They do this by injecting genetic material, which they carry enclosed in an outer protein capsid...
sensitivity, and interactions with predators such as amoebae.
LPS is required for the proper conformation of Omptin
Omptin
Omptins are a family of bacterial proteases. They are aspartate proteases, which cleave peptides with the use of a water molecule. Found in the outer membrane of gram-negative enterobacteria such as Shigella flexneri, Yersinia pestis, Escherichia coli, and Salmonella enterica...
activity; however, smooth LPS will sterically hinder omptins.
LPS acts as the prototypical endotoxin because it binds the CD14
Cluster of differentiation
The cluster of differentiation is a protocol used for the identification and investigation of cell surface molecules present on white blood cells, providing targets for immunophenotyping of cells...
/TLR4
TLR 4
Toll-like receptor 4 is a protein that in humans is encoded by the TLR4 gene. TLR 4 is a toll-like receptor. It detects lipopolysaccharide from Gram-negative bacteria and is thus important in the activation of the innate immune system...
/MD2
Lymphocyte antigen 96
Lymphocyte antigen 96 is a protein that in humans is encoded by the LY96 gene.The protein encoded by this gene is involved in binding lipopolysaccharide with TLR 4. It is also known as "MD2".-Interactions:...
receptor
Receptor (biochemistry)
In biochemistry, a receptor is a molecule found on the surface of a cell, which receives specific chemical signals from neighbouring cells or the wider environment within an organism...
complex, which promotes the secretion of pro-inflammatory
Inflammation
Inflammation is part of the complex biological response of vascular tissues to harmful stimuli, such as pathogens, damaged cells, or irritants. Inflammation is a protective attempt by the organism to remove the injurious stimuli and to initiate the healing process...
cytokine
Cytokine
Cytokines are small cell-signaling protein molecules that are secreted by the glial cells of the nervous system and by numerous cells of the immune system and are a category of signaling molecules used extensively in intercellular communication...
s in many cell types, but especially in macrophages and B cells. In Immunology, the term "LPS challenge" refers to the process of exposing a subject to an LPS that may act as a toxin.
LPS is also an exogenous pyrogen (external fever-inducing substance).
Being of crucial importance to gram-negative bacteria, these molecules make candidate targets for new antimicrobial
Antimicrobial
An anti-microbial is a substance that kills or inhibits the growth of microorganisms such as bacteria, fungi, or protozoans. Antimicrobial drugs either kill microbes or prevent the growth of microbes...
agents.
Some researchers doubt reports of generalized toxic effects attributed to all lipopolysaccharides, in particular, for cyanobacteria.
Composition
- O antigen (or O polysaccharide)
- Core oligosaccharideCore oligosaccharide75px|thumb|Lipopolysaccharide. The core includes both the inner and outer core.Core oligosaccharide is a short chain of sugar residues within Gram-negative lipopolysaccharide...
- Lipid ALipid ALipid A is a lipid component of an endotoxin held responsible for toxicity of Gram-negative bacteria. It is the innermost of the three regions of the lipopolysaccharide molecule, and its hydrophobic nature allows it to anchor the LPS to the outer membrane...
O-antigen
A repetitive glycan polymerPolymer
A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...
contained within an LPS is referred to as the O antigen
Antigen
An antigen is a foreign molecule that, when introduced into the body, triggers the production of an antibody by the immune system. The immune system will then kill or neutralize the antigen that is recognized as a foreign and potentially harmful invader. These invaders can be molecules such as...
, O polysaccharide
Polysaccharide
Polysaccharides are long carbohydrate molecules, of repeated monomer units joined together by glycosidic bonds. They range in structure from linear to highly branched. Polysaccharides are often quite heterogeneous, containing slight modifications of the repeating unit. Depending on the structure,...
, or O side-chain of the bacteria.
The O antigen is attached to the core oligosaccharide, and comprises the outermost domain of the LPS molecule. The composition of the O chain varies from strain to strain. For example, there are over 160 different O antigen structures produced by different E. coli strains. The presence or absence of O chains determines whether
the LPS is considered rough or smooth. Full-length O-chains would render the LPS smooth, whereas the absence or reduction of O-chains would make the LPS rough. Bacteria with rough LPS usually have more penetrable cell membranes to hydrophobic antibiotics, since a rough LPS is more hydrophobic. O antigen is exposed on the very outer surface of the bacterial cell, and, as a consequence, is a target for recognition by host antibodies
Antibody
An antibody, also known as an immunoglobulin, is a large Y-shaped protein used by the immune system to identify and neutralize foreign objects such as bacteria and viruses. The antibody recognizes a unique part of the foreign target, termed an antigen...
.
Core
The Core domain always contains an oligosaccharide component that attaches directly to lipid ALipid A
Lipid A is a lipid component of an endotoxin held responsible for toxicity of Gram-negative bacteria. It is the innermost of the three regions of the lipopolysaccharide molecule, and its hydrophobic nature allows it to anchor the LPS to the outer membrane...
and commonly contains sugar
Sugar
Sugar is a class of edible crystalline carbohydrates, mainly sucrose, lactose, and fructose, characterized by a sweet flavor.Sucrose in its refined form primarily comes from sugar cane and sugar beet...
s such as heptose
Heptose
A heptose is a monosaccharide with seven carbon atoms.They have either an aldehyde functional group in position 1 or a ketone functional group in position 2 ....
and 3-deoxy-D-mannooctulosonic Acid (also known as KDO, keto-deoxyoctulosonate). The LPS Cores of many bacteria also contain non-carbohydrate components, such as phosphate, amino acids, and ethanolamine substitutents.
Lipid A
Lipid A is, in normal circumstances, a phosphorylated glucosamineGlucosamine
Glucosamine is an amino sugar and a prominent precursor in the biochemical synthesis of glycosylated proteins and lipids. Glucosamine is part of the structure of the polysaccharides chitosan and chitin, which compose the exoskeletons of crustaceans and other arthropods, cell walls in fungi and...
disaccharide
Disaccharide
A disaccharide or biose is the carbohydrate formed when two monosaccharides undergo a condensation reaction which involves the elimination of a small molecule, such as water, from the functional groups only. Like monosaccharides, disaccharides form an aqueous solution when dissolved in water...
decorated with multiple fatty acids. These hydrophobic fatty acid chains anchor the LPS into the bacterial membrane, and the rest of the LPS projects from the cell surface. The lipid A domain is responsible for much of the toxicity of Gram-negative bacteria. When bacterial cells are lysed
Lysis
Lysis refers to the breaking down of a cell, often by viral, enzymic, or osmotic mechanisms that compromise its integrity. A fluid containing the contents of lysed cells is called a "lysate"....
by the immune system
Immune system
An immune system is a system of biological structures and processes within an organism that protects against disease by identifying and killing pathogens and tumor cells. It detects a wide variety of agents, from viruses to parasitic worms, and needs to distinguish them from the organism's own...
, fragments of membrane containing lipid A are released into the circulation, causing fever, diarrhea, and possible fatal endotoxic shock (also called septic shock
Septic shock
Septic shock is a medical emergency caused by decreased tissue perfusion and oxygen delivery as a result of severe infection and sepsis, though the microbe may be systemic or localized to a particular site. It can cause multiple organ dysfunction syndrome and death...
).
Biosynthesis and Transport
LPS modifications
The making of LPS can be modified in order to present a specific sugar structure. Those can be recognised by either other LPS (which enables to inhibit LPS toxins) or glycosyltransferases that use those sugar structure to add more specific sugars.It has recently been shown that a specific enzyme in the intestine (alkaline phosphatase
Alkaline phosphatase
Alkaline phosphatase is a hydrolase enzyme responsible for removing phosphate groups from many types of molecules, including nucleotides, proteins, and alkaloids. The process of removing the phosphate group is called dephosphorylation...
) can detoxify LPS by removing the two phosphate groups found on LPS carbohydrates. This may function as an adaptive mechanism to help the host manage potentially toxic effects of gram-negative bacteria normally found in the small intestine.
Variability and effect upon specificity
Acyl
An acyl group is a functional group derived by the removal of one or more hydroxyl groups from an oxoacid, including inorganic acids.In organic chemistry, the acyl group is usually derived from a carboxylic acid . Therefore, it has the formula RCO-, where R represents an alkyl group that is...
chains even within or between genera). Some of these variations may impart antagonistic properties to these LPS. For example Rhodobacter sphaeroides
Rhodobacter sphaeroides
Rhodobacter sphaeroides is a kind of purple bacteria; a group of bacteria that can obtain energy through photosynthesis. Its best growth conditions are anaerobic phototrophy and aerobic chemoheterotrophy in the absence of light. R. sphaeroides is also able to fix nitrogen...
diphosphoryl lipid A (RsDPLA) is a potent antagonist of LPS in human cells, but is an agonist in hamster and equine cells.
It has been speculated that conical Lipid A (e.g., from E. coli) are more agonistic, less conical lipid A like those of Porphyromonas gingivalis may activate a different signal (TLR2 instead of TLR4), and completely cylindrical lipid A like that of Rhodobacter sphaeroides is antagonistic to TLRs.
Lipopolysaccharide gene clusters are highly variable between different strains, subspecies, species of bacterial pathogens of plants and animals.
Immune response
LPS function has been under experimental research for several years due to its role in activating many transcription factorTranscription factor
In molecular biology and genetics, a transcription factor is a protein that binds to specific DNA sequences, thereby controlling the flow of genetic information from DNA to mRNA...
s. LPS challenge also produces many types of mediators involved in septic shock
Septic shock
Septic shock is a medical emergency caused by decreased tissue perfusion and oxygen delivery as a result of severe infection and sepsis, though the microbe may be systemic or localized to a particular site. It can cause multiple organ dysfunction syndrome and death...
. Humans are much more sensitive to LPS than other animals (e.g., mice). A dose of 1 µg/kg induces shock in humans, but mice will tolerate a dose up to a thousand times higher. This may relate to differences in the level of circulating natural antibodies between the two species.
Said et al. showed that LPS causes an IL-10-dependent inhibition of CD4 T-cell expansion and function by up-regulating PD-1
PD-1
Programmed Death 1, or PD-1, is a Type I membrane protein of 268 amino acids. PD-1 is a member of the extended CD28/CTLA-4 family of T cell regulators. The protein's structure includes an extracellular IgV domain followed by a transmembrane region and an intracellular tail...
levels on monocytes which leads to IL-10 production by monocytes after binding of PD-1
PD-1
Programmed Death 1, or PD-1, is a Type I membrane protein of 268 amino acids. PD-1 is a member of the extended CD28/CTLA-4 family of T cell regulators. The protein's structure includes an extracellular IgV domain followed by a transmembrane region and an intracellular tail...
by PD-L.
Bruce Beutler was awarded a portion of the 2011 Nobel Prize in Physiology or Medicine for his work demonstrating that TLR4 is the LPS receptor.