Figure 2. Structure and biosynthesis of Kdo₂-lipid A in E. coli K-12.

The symbols indicate the relevant structural genes encoding each of the enzymes (2, 48). A single enzyme catalyzes each reaction. The lipid A system may have evolved only once, as judged by the available genomes. In almost all cases, as illustrated by E. coli (367), the genes encoding the enzymes of lipid A biosynthesis are present in single copy. At the protein level, orthologs of LpxA and LpxC are the most highly conserved among bacteria. The acyl chain incorporated by LpxA is highlighted in red. The LpxC inhibitor L-161,240 displays antibiotic activity against E. coli that is comparable in potency to ampicillin (64, 65).