Mechanism of Antibiotics Inhibition In Bacteria
- Peptidoglycan is the major component of the cell wall of almost all bacteria. It has structural roles ie. to maintain the shape of the bacteria and acts as a selective sieve for molecules from the outer environment.
- Peptidoglycan synthesis is most important biogenesis pathways in bacteria and the glycan chains contain alternating units of N-acetylmuramic acid and N-acetylglucosamine
- The pathway initiates in the cytoplasm where the synthesis of nucleotide precursors takes place and continues in the cytoplasmic membrane and finish in the periplasmic space, where the precursor is polymerized into the peptidoglycan layer.
- The peptidoglycan layer has a unique feature of occurrence of the D-isomers
of some amino acids in the stem peptides (particularly D-alanine and D-glutamic acid) and unusual amino acids such as meso-diaminopimelic acid which are not found in proteins.
- A number of proteins involved in this pathway, such as the transglycosylase enzymes, ligase, the transpeptidase or penicillin-binding proteins (PBPs).
- The proteins have been the attack site for antibiotics in most of the cases.
- As the mammalian cells do not possess any cell wall or any macromolecules similar to peptidoglycan, the antibiotics possess the least effect on mammalian cells.
Cytoplasmic Level Inhibition By Antibiotics
- The antibiotic involved in the early stages of the synthesis of peptidoglycan in the cytoplasm is D-Cycloserine and O-carbamyl-D-serine
- D-cycloserine (DCS) is known to inhibit two sequential enzymes in the bacterial cell wall peptidoglycan biosynthetic pathway ie. dipeptide D-alanyl-D-alanine (D-Ala-D-Ala)
- It occurs inside cytoplasm and it involves a racemase enzyme which converts L-alanine to D-alanine and a ligase which couples D-alanine-D-alanyl ligase (Ddl)
- The inhibition by these antibiotics is accompanied by the accumulation of the nucleotide intermediate, UDP-Nacetylmuramyl-tripeptide involving coupling of the dipeptide to three other amino acids and thus blocked
Cytoplasmic- Membrane Level Inhibition By Antibiotics
- The glycopeptide antibiotics which interfere in the assembly of peptidoglycan in the cytoplasmic membrane are vancomycin and teicoplanin
- These are the larger structure which prevents cell wall construction by interfering with transglycosylases and transpeptidases
- The glycopeptide antibiotics are a group cyclic or polycyclic nonribosomal peptides that inhibit Gram-positive bacterial cell-wall synthesis but it is unable to penetrate the outer cytoplasmic membrane of Gram-negative bacteria due to its large size.
- The assembly site is reached by crossing the cell membrane which is done by a lipid, undecaprenyl phosphate, which acts as a carrier molecule, cycling between the inner and outer faces of the membrane
- The linear glycan chain is assembled by the transglycosylase by sequential transfer of the growing chain to each molecule of the lipid intermediate carrier as it crosses the cell membrane. Glycopeptides block this process by binding, not to the enzyme itself, but to the peptidoglycan precursor, specifically to the D-alanyl-Dalanine portion.
- As the glycopeptide binds to the D-alanyl-D-alanine, it prevents the function of the transglycosylases.
- The binding of vancomycin to the d-alanyl-d-alanine terminating pentapeptide is mediated by hydrogen bonding.
- Vancomycin does not penetrate into the cell membrane of bacteria and is to bind to the disaccharide-pentapeptides on the outer face of the cytoplasmic membrane whereas Teicoplanin also binds tightly to the D-alanyl-D-alanine region of the peptidoglycan precursor.
Cell Wall Level of Inhibition By Antibiotics
- The cell wall in the bacteria is the site of assembly of the peptidoglycan layer which is catalysed by transpeptidase enzymes located on the outer face of the cell membrane.
- The b-lactam antibiotics including — penicillins, cephalosporins, carbapenems and monobactams inhibit transpeptidases by acting as alternative substrates.
- These antibiotics mimic the D-alanyl-D-alanine residues and react covalently with the transpeptidases. The b-lactam bond is broken which prevents the formation of assembly of the peptidoglycan which finally leads to cell burst.
[Note: The proper mechanism of beta lactum antibiotics particularly the penicillin has been explained in the article Penicillin | The First Antibiotic]