Fig. 1.

Biosynthesis and transport of LPS. (A) The latter steps of Kdo₂-lipid A biosynthesis catalyzed by KdtA (WaaA), LpxL, and LpxM are shown. Two Kdo sugars are added to the tetra-acylated lipid A precursor, termed lipid IV_A. The Kdo sugars are part of the core oligosaccharide and are required for the ordered addition of last two acyl chains by LpxL and LpxM. First, LpxL adds a laurate (C12:0) followed by LpxM that adds a myristate (C14:0) group forming hexa-acylated Kdo₂-lipid A. The remaining core oligosaccharide is extended at the cytoplasmic face of the IM, requiring various glycosyl transferase (not shown). MsbA, an ABC transporter, flips the core–lipid A structure to the periplasmic face of the IM. For simplicity, the O-antigen addition is not shown and is absent in E. coli K-12 strains. The intermembrane translocation of LPS to the OM is conducted by the Lpt system, which forms an envelope-spanning translocation machine. PagP, an OM protein, transfers a palmitate (C16:0) group from mislocalized GPL (red) in the outer leaflet of the OM to LPS. (B, Top) Chemical structure of Kdo₂-lipid A with the acyl chain added by LpxM in blue, and the chain added by PagP is in red. (B, Bottom) Structure of cardiolipin.