The Mode of Action of Inhibitors

I. Cell wall synthesis Inhibitors

a) Penicillin: Inhibits formation of the bacterial cell wall by blocking cross linking of the cell wall structure. The cell wall is a needed protective casing for the bacterial cell. Inhibits transpeptidase enzymes. Activates lytic enzymes of cell wall. The affected bacterium will eventually lyse because the unsupported cell wall cannot withstand its growth.

b) Cephalosporin: Similar to pencillins in their mode of action but they treat a broader range of bacterial infections. They have structural similarities to pencillins and many people with allergies to pencillins also have allergic reactions to cephalosporins.

c) Bacitracin: Inhibits cell wall production by blocking the step in the process (recycling of the membranes lipid carrier which is needed to add on new cell wall subunits.

d) Carbenicillin:Inhibits transpeptidation enzymes. Activates lytic enzymes of cell wall.

e) Vancomycin:Inhibits transpeptidation in cross-linking peptidoglycans. Interferes with      bacterial cells at many levels, disrupting cell wall synthesis, interfering with RNA, and damaging the plasma membrane.

Mode of action of inhibitors

 

II) Protein Synthesis Inhibitors

   a) Tetracyclines: Inhibits protein synthesis by binding to the subunit of the bacterial ribosome (30S –Small subunit).

   b)Streptomycin:Binds the 30S ribosomal subunit of the tuberculosis bacterium and prevents the ribosome from forming the complex necessary to initiate protein translation. Streptomycin is the first line of chemical defense against Mycobacterium tuberculosis.

   c)Chloromphenicol:Inhibits protein synthesis by binding to a subunit of bacterial ribosome (50S-Large subunit).

   d) Erthromycin (Macrolides):Binds the 50S subunit and blocks translocation of the new protein on the ribosome, thus effectively halting synthesis.

   e) Clindamycin (Lincosamides):Inhibits protein synthesis by binding to a subunit of the bacterial ribosome(50S).

    f) Aminoglycosides:inhibit nucleic acid or protein synthesis in bacteria. They are L-shaped molecules that fit into L-shaped pockets of bacterial ribosomal RNA. When they insert themselves into rRNA, they disrupt ribosomal structure. They don’t have this effect on human cells because the L-shaped pocket is specific to bacteria.

   g) Chloramphenicol:Blocks formation of new peptide bonds during protein synthesis by binding to the 50S subunit of the ribosome.

    h) Fusidic acid:Blocks translocation.

    i) Linezolid:Binds rRNA to prevent translation initiation and thus protein synthesis.

  III)  Nucleic acid Inhibitors

a) Rifamycin:The antibiotic interferes with prokaryotic RNA polymerase and thus, interferes with transcription. Blocks RNA synthesis by binding to and inhibiting RNA polymerase.

b) Fluoroquinolones :inhibit DNA gyrase, a bacterial enzyme that unwinds DNA in preparation for replication and transcription. Both of these disruptions prevent bacteria from dividing to make more bacteria.

Ciprofloxacin: Inhibits DNA gyrase; interferes with DNA replication.

IV) Metabolism Inhibitors: Inhibit synthesis of purine and thymidylate precursor’s folic acid or tetrahydrofolate.

a) Sulfonamides:

• Competitively inhibits dihydropteroate synthase, an enzyme that converts p-aminobenzoic acid (PABA) into folic acid. These drugs can also be incorporated into a compound that resembles dihydrofolate and that in turn can inhibit another enzyme in the pathway, dihydrofate reductase. Sulfonomides inhibit bacteria-specific reaction. Inhibits dihydrofolate reductase, blocking tetrahydrofolate synthesis.

b) Dapsone:

• Interferes with synthesis of folic acid, which is required for the synthesis of purines and thymidine and for the synthesis of the amino acids methionine and gycine.

c) Trimethoprim:

• Inhibits dihydrofolate reductase, blocking tetrahydrofolate synthesis.

v) Photosynthesis Inhibitors 

a) Paraquat (Herbicide):

• Act by accepting electrons from the early acceptors of Photo system- I (PS-I)and then reacting with oxygen to form superoxide.

b) CMU& DCMU:

• CMU-3-4’ chlorophenyl-1-1-dimethyl urea.
• DCMU-3-3-4’ dichlorophenyl-1-1-dimethyl urea.
• Non-cyclic photophosphorylation is inhibited by CMU& DCMU. Competing for the binding site of plastoquinone that is normally occupied by QB.