Lipid Intermediates in Bacterial Peptidoglycan Biosynthesis

Abstract

Peptidoglycan constitutes one of the major ``Achilles heels'' of bacteria because it is an essential component for cell integrity, and its metabolism is the target for a great number of antibacterials of different natures, e.g., antibiotics such as $β$-lactams and vancomycin, host immune system antimicrobial peptides, and bacteriocins. Peptidoglycan synthesis requires the translocation, across the plasma membrane, of the polymer building block, a disaccharide-pentapeptide. This event is performed via the attachment of the subunit to a lipid carrier, undecaprenyl-phosphate. Lipid intermediates called lipids I and II are generated through the sequential transfer of N-acetylmuramoyl-pentapeptide and N-acetylglucosamine moieties from nucleotide precursors to the lipid carrier by MraY and MurG transglycosylases, respectively. The last membrane intermediate, lipid II (undecaprenyl-pyrophosphate-N-acetylmuramoyl(-pentapeptide)-N-acetylglucosamine), can be further enzymatically modified through the addition of functional groups, amino acids, or peptides, before being flipped towards the outer leaflet of the plasma membrane where the final transfer of the peptidoglycan subunits to the growing polymer is catalyzed by penicillin-binding proteins. The integral membrane proteins FtsW, MurJ, and AmJ are thought to play a major role in the translocation process; however, the exact mechanism and the role of these molecular determinants is yet to be established. The lipid carrier is generated via a pathway involving two steps, first a polymerization reaction of isopentenyl-pyrophosphate catalyzed by the essential cytosoluble UppS enzyme, yielding undecaprenyl-pyrophosphate, followed by a dephosphorylation step ensured by a yet unknown enzyme. At each final transfer of a subunit to the elongating peptidoglycan, the lipid carrier is released in the pyrophosphate form, which is recycled to guarantee the high rate of polymer synthesis. Several integral membrane undecaprenyl-pyrophosphate phosphatases, from two distinct protein families and having their active site facing the extracytoplasmic side, have been identified, BacA and PAP2 enzymes. These enzymes can readily dephosphorylate the released lipid carrier precursor. Thereafter, the lipid is flipped back to the inner side of the membrane, by a yet unknown mechanism, in order to be reused as a glycan acceptor for a new round of peptidoglycan polymerization.