Antibiotics Inhibiting Protein Synthesis
Protein synthesis in bacteria
Protein synthesis is a highly coordinated, multi-step mechanism essential to cellular functioning. In bacteria, it happens on the 70S ribosome (which has two parts: the 30S subunit and the 50S subunit). It has three stages, beginning, lengthening and ending. Every step involves the cooperation of ribosomal proteins, transfer RNAs (tRNAs) and a series of enzymes.
Starting: The start happens when the mRNA, 30S ribosomal subunit, and initiator tRNA (formyl-methionyl-tRNA in bacteria) join to form the 30S initiation complex. This complex is important to determine which start codon should be chosen.
Conversion: In conversion, amino acids are added to the dwindling polypeptide chain. In this process, the ribosome's A (aminoacyl), P (peptidyl) and E (exit) sites are interacting in coordination. The aminoacyl-tRNA attaches to the A site, and the growing polypeptide is transferred to the coming amino acid. The ribosome then carries down the mRNA to the next codon and adds the next amino acid.
Breaking: Protein production is stopped at a stop codon. release factors help ribosome subunits dissociate and release newly synthesized proteins.
The 70S ribosome is the center of bacterial protein manufacturing. The 30S subunit is primarily responsible for decoding mRNA, while the 50S subunit catalyzes the formation of peptide bonds. The close collaboration between these two subunits ensures the accuracy and efficiency of protein synthesis. Blocking this mechanism is one of the principal objectives of antibacterial reagents.
Mechanisms of antibiotic on protein synthesis
The vast majority of antibiotics that suppress bacterial protein formation interfere with this process in the 30S or 50S units of 70S bacterial ribosomes. This action is a bit different across the classes of reagents, but the goal is to either prevent the correct assembly of the ribosome or to interfere with the elongation process. These antibiotics prevent bacteria from expanding in one or more steps of protein-synthesis.
Antibiotics inhibiting protein synthesis
Antibiotics targeting the 30s ribosomal subunit
Photoaffinity labeling and molecular imprinting showed that the nuclear proteins S3, S7, S8, S14 and S19 in the 30S subunit were tetracycline binding proteins, and S7 protein was a high affinity protein. The 16S rRNA bases G996, U1196, C1054, and C1195 combine with tetracycline to form A reversible combination with hydrogen bonding force, which prevents the aminoacyl tRNA from entering the A site of the ribosome during the extension of the peptide chain, resulting in the extension of the peptide chain being blocked, making the protein required for bacterial survival unable to be synthesized.
Schematic diagram of antimicrobial mechanism of tetracycline.
Long-term use of tetracycline antibiotics will cause bacteria to develop drug resistance, and the drug resistance mechanism is mainly as follows: (1) Bacteria contain more than 30 specific tetracycline efflux pumps, which can discharge tetracycline drugs outside the cell, thereby reducing the concentration of intracellular drugs. At this time, resistance occurs due to the decreased ability of the drug to bind to the ribosome. (2) The synthesis of ribosome protection protein in bacteria causes the change of ribosome configuration. This protein can effectively inhibit the binding of tetracycline and ribosome, but does not change the ribosome function.
Aminoglycoside antibiotics belong to static bacteriostatic drugs, and can be used in combination with reproductive bacteriostatic drugs (β-lactam antibiotics) for synergistic antibacterial. The main site of action of aminoglycoside antibiotics is also the ribosome, which inhibits bacteria by preventing the initiation stage of protein synthesis, the peptide chain extension stage and the termination stage. During the initiation of prokaryotic polypeptide chain synthesis, aminoglycoside antibiotics can bind to 30S subunits and interfere with ribosome 30S initiation complex assembly. During the extension of the polypeptide chain, aminoglycosides bind to the 30S subunit A, resulting in incorrect coding and abnormal protein synthesis. During the termination phase of polypeptide chain synthesis, aminoglycoside antibiotics can prevent the termination codon from binding to the ribosome, further preventing the release of the synthesized peptide chain, and inhibit the dissociation of the 70S ribosome, preventing the protein synthesis cycle.
The resistance of aminoglycoside antibiotics is mainly reflected in: (1) The modification of ribosomes by bacteria (for example, the 16SrRNA methylase produced by actinomycetes protects ribosomes from the inhibition of aminoglycosides by methylating their own 16SrRNA); (2) Aminoglycoside modifiers modify the structure of aminoglycoside antibiotics to prevent them from acting on bacteria, which is the most widespread way for bacteria to develop drug resistance; (3) Modification of cell membrane; (4) Active efflux of drugs.
Antibiotics targeting the 50S ribosomal subunit
The 50S subunit plays an important role in the synthesis and folding of proteins to form tertiary structures. The 50S subunit rRNA is present in the V domain of the nucleoprotein complex peptide transferase center (PTC), which is used to catalyze the peptidyl transfer reaction. The reaction process is the reaction of the amino group of the newly added amino acid (at the A site) with the ester group of the peptidyl tRNA (at the P site), so that the ester bond of the peptidyl group is transformed into a peptide bond, in such a way that the new peptide chain extends from the N terminal to the C terminal to synthesize proteins.
Macrolide antibiotics can be linked to the V domain of the 50S subunit, promoting the dissociation of peptidyl-tRNA from the ribosome, blocking the extension of the peptide chain, and thus inhibiting protein synthesis. Among them, the A2058 base and A2059 base on ribosomal RNA are the main targets of the first and second generation macrolide antibiotics, and the C2' hydroxyl group in the 5-deoxyglycosamine can form hydrogen bonds with A2058 and A2059. In addition, the hydroxyl groups at the C6, C11 and C12 positions of the erythromycin lactone ring can form hydrogen bonds with the 50S large subunit rRNA bases A2062 and U2609 respectively.
It is worth noting that when a carbonyl group is introduced at the C3 position of the lactone ring to form a ketolide derivative, its binding to the ribosome will be stronger. The hydroxyl group of the C2' and the C3' linked dimethylamino group of the telimycin deoxyamino sugar form hydrogen bonds with the 23S rRNA bases A2058 and G2502 of Escherichia coli, respectively. In addition, the sugar can bind to the main chain of G2502 by electrostatic force and to A2059 by hydrophobic force. The ketone group at the C3 position in the lactone ring can also form a hydrogen bonding force with C2610. The pyridoimidazolyl group of C11 and C12 carbamate side chains can also form hydrogen bonds with the 23S rRNA domain II base U790, which increases the binding ability of drug molecules to ribosomes and thus enhances antibacterial activity.
Cephalosporins and chloramphenicol
Cephalosporins, which stop transpeptidisation or transfer by attaching to 23S rRNA on the 50S ribosome subunit in bacteria. This leads to incomplete peptide chains coming apart too early to enable bacteria to build protein. Chloramphenicol stops peptide bond formation by attaching to the L16 protein of the 50S subunit and therefore prevents the movement of amino acids and elongation of peptide chain.
