Bacterial translation inhibitors
mRNA orders protein synthesis from twenty amino acids. Each codon, constituted of three bases, codes specifically for a specific amino acid to be incorporated into proteins.
tRNA ensures the transfer of amino acid from the cytoplasm to ribosomes. Each tRNA involves:
The assembly of amino-acids into polypeptide is carried out in ribosomes composed of RNA and of proteins. In micro-organisms there are: 70 S ribosomes, 30S subunits with 16 S rRNA and 50S subunits with two molecules, one of 5 S rRNA, and the other of 23S rRNA, and 34 proteins called L for large.
Ribosomes ensure the synthesis of the incipient polypeptide chain, by reading the mRNA with the supply of amino-acyl-tRNA. The reading goes in direction 5 ' ® 3 '. It is complex, one can distinguish three stages:
- Initiation: codon AUG codes for the first amino acid of the polypeptide. Initiation involves the dissociation of ribosomes 70 S into 50S and 30S with interaction of 30S and mRNA in the presence of initiation factors IF1, IF2, IF3 and of GTP.
- Elongation: it consists of the successive addition of amino-acids by peptide bonds. This elongation depends on soluble cytoplasmic proteins called EF (Elongation Factor).
- Termination: after binding of the last amino acid, a stop codon blocks the reaction and releases the protein.
Energy is supplied by ATP and GTP.
These steps lead to the synthesis of proteins: enzymes, carriers, receptors etc. The protein synthesis is done according to the same principle in human cells and microorganisms, but there exist many differences explaining the preferential activity of antibiotics against microorganisms.
One distinguishes drugs which act on the 50S or 30S ribosomal subunits.