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 The large molecules

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Date d'inscription : 04/08/2011

MessageSujet: The large molecules   Mer 17 Aoû - 10:33

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The large molecules formed from polysaccharides and lipids are known as lipopolysaccharides. The O-polysaccharide, lipid A and core are the components of a lipopolysaccharide. Lipopolysaccharides are present in the outer membranes of bacteria and function as endotoxins. Eliciting immune responses in animals is also one of their important functions. The negative charge of cell membranes is increased by lipopolysaccharides, which helps in stabilizing the membrane structure. The lipopolysaccharide layer in Gram negative bacteria is useful in maintaining structural integrity.

Lipopolysaccharides

The lipopolysaccharide structure is formed of three components. Information about the components of lipopolysaccharide binding protein are mentioned below.

O-polysaccharide: An O-polysaccharide or O-antigen is formed of a repetitive glycan polymer. The O-polysaccharide is also known as O side chain. The outer membranes of a lipopolysaccharide are formed of O-antigens. These are connected to the core oligosaccharide. Composition of the chain differs in different types of bacteria. The E. coli alone has strains that have 160 different compositions of O-antigens.

Core: The core of lipopolysaccharides is formed of oligosaccharide and sugars such as 3-deoxy-D-manno octulosonic Acid and heptose. The oligosaccharide at the core is attached with lipid A. Apart from carbohydrates, the core also contains amino acids, phosphate and ethanolamine substituents.

Lipid A: The lipid A component of lipopolysaccharides is formed of phosphorylated glucosamine disaccharide, which is surrounded by multiple fatty acids. These fatty acid chains of lipid A help in anchoring lipopolysaccharides to the membrane of bacteria. Toxicity of Gram-negative bacteria is mostly attributed to their lipid A component. When breakdown(lysis) of bacterial cells takes place due to the effect of the immune system, lipid A components circulated in the body results in diarrhea, fever and even endotoxic shocks which could be fatal.

Lipopolysaccharide Endotoxin

The lipid A present in bacteria act as endotoxins. They are released when the lysis of bacteria takes place. The endotoxins are responsible for causing diseases and are found in outer layers of Gram-negative bacteria. The term lipopolysaccharide is used in exchange of endotoxin. The average size of endotoxins is 10 kDa (dalton). However, the endotoxins could be as big as 1000 kDa. Human beings are able to produce antibodies against endotoxins. The number of different types of endotoxins is too large. Therefore, the antibodies may not prove to be effective against all of these endotoxins. Even a small amount of endotoxin is enough to lower blood pressure of the body, induce fever and cause inflammation & coagulation.

Lipopolysaccharide Sigma

The cells of Escherichia coli are enveloped by an asymmetric bilayer formed of phospholipid, lipopolysaccharide and outer membrane proteins (OMPs). A sigma(E)- dependent response is triggered if the integrity of the outer membrane is affected due to chemical and physical stress.

The lipopolysaccharides are an important component of bacteria and form the cell walls. They are not only significant in stabilizing the overall membrane structure, but also in protecting the bacteria against certain chemicals.
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