Alternative structures are reported for the inner core where the heptose may be substituted by a phosphate, pyrophosphate, or phosphorylethanolamine group. Enteric bacterial LPS cores typically consist of 8–12 sugar units. The oligosaccharide moiety is the core unit of LPS. The position 6 ’ is attached to the oligosaccharide region. Furthermore, there are fatty acids ester-linked at positions 3 and 3′ and amide linked at positions 2 and 2′. The two glucosamine monomers are linked between position 1 and 6, and both of them are phosphorylated to produce bisphosphorylated β-(1-6)-linked glucosamine disaccharide. The common architecture of Lipid A is a disaccharide, with glucosamine being the monomer. The Lipid A structures were first studied based on Enterobacteria. Lipid A is the most conserved part which is responsible for the toxicity of endotoxins, while, the effect of polysaccharides is negligible. Structure of bacterial lipopolysaccharides (Source:, accessed on ). The general structure of all endotoxins is a polar heteropolysaccharide chain, with three distinct domains: the O-antigen region, a core oligosaccharide part and a Lipid A part (Figure 1). They are the integral part of the outer cell membrane and are responsible for the organization and stability of the bacteria. Structure of EndotoxinsĮndotoxins, also known as lipopolysaccharides (LPS), are mostly found in the outer membrane of Gram-negative bacteria. Detergents can be used to separate endotoxin from a protein surface, however an additional step is required to remove the surfactant from the product. In the presence of proteins, affinity chromatography and two-phase extraction methods can take advantage of the physical-chemical interaction between endotoxin and protein to completely remove endotoxin. Due to the difference in sizes of endotoxins and water as well as salt and other small molecules in protein-free solutions, ultrafiltration can be employed. Endotoxin molecules tend to form micelles or vesicles in aqueous solution. Hydrophobic interactions between the lipid A portion and sorbent are also considered to be important attributes that removal techniques can take advantage of. Anion exchange and affinity chromatography are based on cationic functional ligands such as diethylaminoethanol, histidine, polymyxin B, poly ( ε-lysine), and poly (ethyleneimine). The interaction between the anionic phosphate in LPS and the cationic ligands on the sorbents are mostly utilised as the mechanism of endotoxin removal. The selection of a suitable endotoxin removal system is based on the properties of the bioproducts being purified. The use of tailor-made endotoxin-selective adsorbent matrices for endotoxin removal is reported elsewhere. Many purification methods have been developed for endotoxin removal, including LPS affinity interactions, two-phase extractions, ultrafiltration, affinity chromatography and anion exchange chromatography. Endotoxins must be removed from proteins prepared from Gram-negative bacteria prior to its administration into the human and animal bodies to avoid any adverse side effect. Gram-negative bacteria are widely used in the biotechnology industry for recombinant DNA production, where endotoxin contamination can occur at any point within the processes. Various removal methods particularly chromatography-based techniques are covered in this article according to the relevant applications. For most cases, endotoxin removal is favoured at a highly ionic or acidic condition. The endotoxin removal strategies are also discussed in the light of the different interaction mechanisms involved between endotoxins and bioproducts particularly plasmid DNA and proteins. In this article, we review the structures of endotoxin and the effects that it causes in vivo. Gram-negative bacteria are widely used for the production of gene-based products such as DNA vaccines and bio-drugs, where endotoxin contamination can occur at any point within the process and its removal is of great concern.
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