Lipopolysaccharide forms a major part of the cell envelope of Gramnegative bacteria

In Gram-negative bacteria, the wall is far more complex than for Gram positives and contains glycopeptide, lipopolysaccharide, phospholipid and protein. Also, there is not such a clear distinction between the wall and the membrane as in Gram positives. Up to 20% of the wall contents may be lipids, but only a proportion of these are readily extractable by conventional solvent methods. This is because of the covalent nature of the lipopoly-saccharide linkages.

The cytosol of Gram-negative bacteria is surrounded by a complex cell envelope consisting of at least three layers (Fig. 6.25). The cytoplasmic membrane is composed of the familiar phospholi-pid bilayer with integral and peripheral proteins.

Fig. 6.23 Cord factor.

Sulpholipid

Fig. 6.24 Some mycobacterial lipids. Structures of mycobacterial lipids: (a) trehalose mycolate, where R1 and R2 are mycolic acids; (b) sulpholipid, where R1-R4 are palmitic acid or very long chain branched fatty acids; (c) mycosides A and B, where R1 is a mono- or trisaccharide and R2 is a 12C-18C saturated fatty acid or a mycocerosic acid [e.g. CH3(CH2)2i(CH(CH3)CH2)2CH(CH3)COOH].

Fig. 6.24 Some mycobacterial lipids. Structures of mycobacterial lipids: (a) trehalose mycolate, where R1 and R2 are mycolic acids; (b) sulpholipid, where R1-R4 are palmitic acid or very long chain branched fatty acids; (c) mycosides A and B, where R1 is a mono- or trisaccharide and R2 is a 12C-18C saturated fatty acid or a mycocerosic acid [e.g. CH3(CH2)2i(CH(CH3)CH2)2CH(CH3)COOH].

Then comes a peptidoglycan layer, which is separated from the outer membrane by a periplasmic space. The outer membrane is extremely asymmetric, but contains lipopolysaccharide and enter-obacterial-common antigen in its outer leaflet. Phospholipid molecules are missing from the outer membrane in many Gram-negative bacteria but are present in some species. In those bacteria containing phospholipids in the outer membrane, phos-phatidylethanolamine is, by far, the most common constituent (-85%).

The lipopolysaccharide is involved in several aspects of pathogenicity. It is a complex polymer in four parts (Fig. 6.26). Outermost is a carbohydrate chain of variable length (called the O-antigen), which is attached to a core polysaccharide. The core polysaccharide is divided into the outer core and the backbone. These two structures vary between bacteria. Finally the backbone is attached to a gly-colipid called lipid A. The link between lipid A and the rest of the molecule is usually via a number of 3-deoxy-D-manno-octulosonic acid (KDO) molecules. The presence of KDO is often used as a marker for lipopolysaccharide (or outer membrane) even though it is not present in all bacterial lipopoly-saccharides.

Lipid A is composed of a disaccharide of glyco-samines (Fig. 6.27). The amino groups are substituted with 3-hydroxymyristate while the hydroxyl groups contain saturated (12C-16C) acids and 3-myristoxymyristate. Phosphate and KDO groups are also substituted. Unsaturated and cyclopropane fatty acids, which are common in other lipid types, are absent from lipopolysacchar-ide.

Studies on the synthesis of lipopolysaccharide have made extensive use of bacteria mutants that are deficient in one or more of the necessary reactions. Rothfield, Horecker and their colleagues in the USA initiated such experiments. It was discovered that formation took place in several stages.

Lipid A, which anchors lipopolysaccharide in the membrane, is made first. Hydroxy acids are added first to the disaccharide, followed by KDO and then saturated fatty acids. The hydroxy fatty acids come from acyl-CoA substrates whereas CMP-KDO is the source of the second addition units. After the addition of saturated fatty acids, sugars are added g

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