Lipid A

Lipid A is a lipid component of an endotoxin held responsible for the toxicity of Gram-negative bacteria. It is the innermost of the three regions of the lipopolysaccharide (LPS), also known as endotoxin molecule, and its composition varies between bacterial species. It is sometimes referred to as the endotoxic principle of bacterial LPS.

Structure

Lipid A is a very conserved component of the LPS. It is a glucosamine disaccharide decorated with phosphate groups and acyl chains. The acyl chains are saturated and vary in number and length. Lipid A with two phosphate groups and six acyl chains is termed 'hexa-acylated'.

Function

Lipid A is responsible for the strong immune response to the LPS. It is recognized by the Toll-like receptor 4 (TLR4) and MD-2 on the surface of certain cells, which leads to an immune response. This includes the production of cytokines, nitric oxide, and eicosanoids.

Clinical significance

Lipid A is an endotoxin and can cause fever, changes in white blood cell counts, and even septic shock, which can be fatal. It is also implicated in the pathogenesis of many Gram-negative bacterial infections and in the phenomenon of endotoxin tolerance.

See also

• Lipopolysaccharide
• Endotoxin
• Gram-negative bacteria
• Toll-like receptor 4