Antibiotics Inhibit Cell Wall Synthesis
The basic structure of bacteria includes cell wall, cell membrane, cytoplasm and nucleus. The cell wall is the outermost layer of the bacterial cell, surrounding the cell membrane. The main component of the cell wall is peptidoglycan. The functions of the cell wall include inhibiting mechanical and osmotic damage, preventing the invasion of macromolecules, and assisting cell movement and growth. Some antibiotics can exert their antibacterial effects by inhibiting the synthesis of bacterial cell walls. This mechanism involves multiple steps and different classes of antibiotics. This class of antibiotics targets the cell wall structure unique to bacteria, and mammalian cells do not have a cell wall, so this class of antibiotics is selective and harmless to mammalian cells.
Chemistry and structure of bacterial cell walls
Peptidoglycan is a major component of bacterial cell walls, also known as mucopeptide. Peptidoglycan consists of alternately arranged n-acetylglucosamine (GlcNAc) and n-acetylmurnac linked by β-1, 4-glucoside bonds into large molecular chains. These sugar chains are cross-linked with each other by short peptide chains (tetrapeptide side chains), forming a grid-like structure that provides the mechanical strength of the cell wall.
The cell wall of Gram-positive bacteria contains a large amount of peptidoglycan, accounting for 50%-90% of the dry weight of the cell wall, while the cell wall of Gram-negative bacteria contains a lower amount of peptidoglycan, accounting for only 1%-10%. The cell wall of Gram-positive bacteria also contains teichoic acid, while the outer wall of Gram-negative bacteria contains lipopolysaccharide (LPS).
Mechanisms of peptidoglycan biosynthesis
Synthesis of nucleotide precursors: In the cytoplasm, nucleotide precursors such as UDP-N-acetylglucosamine (UDP-GlcNAc) and UDP-N-acetyl-L-muramoyl-L-alanine (UDP-MurNAc) are synthesized by a series of enzymatic reactions. These precursors are further assembled into UDP-MurNAc-pentapeptide, a key intermediate for peptidoglycan synthesis.
Formation of lipid linking intermediates: On the medial side of the cell membrane, UDP-MurNAc-pentapeptide binds to undecaprenyl phosphate (C55-P) to form lipid I (lipid I), which is then transferred to the outer side of the cell membrane by the MraY protein. In this process, UDP-GlcNAc is added to lipid I to produce lipid II.
Polymerization and crosslinking: Lipid II is transferred to the outer part of the cell membrane and is catalyzed by high molecular weight penicillin-binding proteins (PBPs). These PBPs, which have transglycosylase and transpeptidase activities, are responsible for inserting new peptidoglycan units into existing peptidoglycan chains and cross-linking adjacent sugar chains via peptide bonds to form a three-dimensional network structure.
In addition, peptidoglycan biosynthesis is also affected by a variety of regulatory mechanisms, including the involvement of cytoskeletal elements and the regulation of enzyme activity at different stages. This process is the target of many antibiotic actions.
Mechanisms of antibiotics inhibit cell wall synthesis
Antibiotics interfering with cell wall synthesis exert their action by inhibiting key steps in peptidoglycan synthesis, an essential constituent for the stability of bacterial cells. Prevention of proper synthesis of cell wall material thus causes eventual disintegration of cell wall integrity, which eventually results in bacterial cell death. The synthesis process of peptidoglycan is rather complex and has multiple steps, comprising some major enzymes responsible for the building, transportation, and insertion of the peptidoglycan units into the cell wall. Different antibiotics selectively interfere with one or more of these steps, and bacterial cell walls are an excellent therapeutic target because of the minimal effects on mammalian cells.
Inhibition of cytoplasmic precursor synthesis
This early stage of peptidoglycan synthesis, occurring in the cytoplasm, involves making necessary precursors, including muramyl pentapeptide. Certain antibiotics inhibit this early stage by inhibiting the enzymes responsible for making D-alanine, an important component of the peptidoglycan precursor. For example, D-cycloserine inhibits the production of useable precursors by inhibiting the action of racemase and D-Ala-D-Ala ligase, thus halting the early stage of the pathway and preventing cell wall formation from proceeding.
Interference with lipid-linked intermediate formation
The next phase occurs at the cellular membrane, where peptidoglycan precursors bind to lipid carriers for transport across the membrane. Antibiotics like bacitracin and tunicamycin target this lipid-linked stage by inhibiting the enzymes responsible for forming and transferring lipid-bound intermediates. Bacitracin, for instance, blocks the dephosphorylation of bactoprenol, a carrier molecule that transports peptidoglycan units to the cell surface. This inhibition prevents the peptidoglycan subunits from reaching the cell wall, disrupting proper cell wall formation.
Transpeptidase and transglycosylase inhibition
PBPs is a necessary enzyme for the synthesis of peptidoglycan, a key component of cell wall. The structure of β-lactam antibiotics such as penicillin and cephalosporin is similar to the terminal structure of peptidoglycan, d-alanyl-d-alanine, which can competitively bind to the active center of the enzyme with covalent bonds, inhibit the cross-linking reaction catalyzed by mucopeptidase, seriously destroy the formation of bacterial cell wall, and cause lytic bacteria, resulting in bacterial lytic death.
Glycopeptide antibiotics such as vancomycin and teicoranin prevent transglycosylase from assembling linear sugar chains to the cell wall by binding to the D-alanyl-D-alanine portion of the dipeptide peptidoglycan precursor. Glycopeptide antibiotics bind to D-alanyl-D-alanine by forming a five-hydrogen bond network, and prevent the transfer reaction of transglycosidase. Teicoranin is a glycopeptide antibiotic that blocks cell wall synthesis by specifically binding to D-alanyl-D-alanine residues.
Antibiotics inhibiting cell wall synthesis at BOC Sciences
Research of antibiotics inhibiting cell wall synthesis
Teixobactin inhibits cell wall synthesis
While using iChip to isolate and culture soil bacteria, Northwestern University's Kim Lewis team discovered a novel antibiotic in Eleftheria terrae, which they named teixobactin. This antibiotic has a good inhibitory effect on a variety of clinically difficult or drug-resistant gram-positive bacteria, and has also shown a good therapeutic effect in a mouse model of methicillin-resistant Staphylococcus aureus (MRSA) infection. Teixobactin is a short peptide consisting of 11 amino acid residues, including four D-type amino acids and a rare L-allo-enduracididine. In the same study, the team also identified the general mechanism of action of teixobactin by inhibiting cell wall synthesis by binding to a precursor component of cell wall synthesis, which is primarily the lipid II.
Chemical structure of teixobactin antibiotic. (Ling, L. L., 2015)
Marinopyrrole A derivative inhibits cell wall synthesis
Marinopyrrole A (MA) was isolated from marine-derived actinomyces in 2008 and has a unique dipyrrole chemical structure that distinguishes it from all currently in clinical use antibiotics. It has a strong killing activity for a variety of clinically harmful pathogens including Methicillin-resistant Staphylococcus aureus (MRSA), but the antibacterial mechanism of this compound has not been resolved. Ray et al. first elucidated the biological function of Marinopyrrole A derivative MA-D1 to kill MRSA by targeting the inhibition of bacterial cell wall synthesis by glucosamine 6-phosphate synthase (GlmS). GlmS is a key rate-limiting enzyme in the hexosamine biosynthesis pathway, the end product of which is UDP-GlcNAc, the core component of bacterial cell wall. MA-D1 and GlmS proteins interact directly and inhibit GlmS enzymatic activity, resulting in the blockage of UDP-GlcNAc synthesis and subsequent cell wall collapse and MRSA death. At the same time, MA-D1 has a low frequency of drug resistance, has good killing activity against linezolid, vancomycin and teicoplanin resistant bacteria, and has good antibacterial activity in GlmS dependent manner in animals with MRSA epidermis and systemic infection.
References
- Ling, L. L., et al. A new antibiotic kills pathogens without detectable resistance. Nature. 2015, 517(7535): 455-459.
- Guo, F., et al. An Optimized Marinopyrrole A Derivative Targets 6-Phosphoglucosamine Synthetase to Inhibit Methicillin-Resistant Staphylococcus aureus. ACS Central Science. 2024.
