Figure 2.

Simplified overview of the LPS biosynthesis. Lipid A-Kdo₂ is synthesized on the cytoplasmic surface of the cytoplasmic membrane. The rest of the core is assembled to the lipid A-Kdo₂ and MsbA flips the whole complex to the periplasmic side of the cytoplasmic membrane. The O antigen is synthesized by cytoplasmic membrane-associated enzyme complexes using C55 Und-P as an acceptor for chain assembly and is then flipped to the periplasmic face of the membrane by one of the three pathways: (1) Wzy dependent, (2) ABC transporter dependent or (3) synthase dependent. For simplicity, only the ABC-transporter pathway is represented. Once on the periplasmic side, the O antigen is linked to the lipid A-core by the WaaL ligase and the mature LPS molecule is then transported across the periplasm and inserted into the outer leaflet of the outer membrane by the Lpt (LPS transport) system, a complex that spans the Gram-negative cell envelope to deliver LPS to the outer membrane (E). OM, outer membrane; CM, cytoplasmic membrane.