FIG. 1.

E. coli lipid A structure. Two glucosamine units are linked by a β 1′-6 glycosidic bond to compose the lipid A backbone. Each glucosamine unit bears a phosphoryl group and two principal R-3-hydroxymyristoyl chains. The acyl chains of the distal glucosamine unit are further acylated with laurate and myristate. The entire structure forms a hexa-acylated 1,4′-bis-phosphate lipid A molecule. In approximately one-third of the lipid A population in E. coli, a pyrophosphoryl group is found in position 1 of the carbon backbone instead of the single phosphoryl group. Additional covalent modifications of lipid A that can be observed under certain conditions are indicated by the dashed lines: 4-amino-4-deoxy-l-arabinose, pEtN, and palmitate.