Antibiotics and inhibition of protein synthesis

Protein synthesis can be divided into a- Steps taking place prior to translation b- Steps in the translation at the ribosomes.

Activation of the amino acids

B- translation of mRNA on ribosomes

Bacterial ribosomes are characterized by their sedimentation rate of 70 S ribosomes. The 70 S dissociate to 50 S and 30 S.

Protein biosynthesis at the ribosomal levels can be classified into: 1.The initiation phase allows the start of protein synthesis 2.Elongation phase of the peptide by one amino acid at each cycle. 3.Termination phase which stops protein synthesis (presence of non-sense codon.

Role of antibiotics in inhibition of bacterial protein synthesis

Inhibition of aminoacyl-tRNA formation: - Both reactions are catalyzed by amino acid specific ligases. - Indolmycin competes with tryptophan in the activation reaction (i.e no enzym- tryptophaneacyl-AMP

- Furanomycin competes with isoleucine and inhibit the transfer reaction

Tetracyclin antibiotic inhibits tRNA binding to ribosomes thus inhibits protein synthesis.

Inhibition of initiation stage: Viomycin antibiotic prevents mRNA binding to the 30 s subunit rather than binding to tRNA

Aminoglycosides: streptomycin inhibits initiation reaction. Neomycin and kanamycin inhibits translocation of the ribosome along mRNA.

Inhibition of elongation - Microlides: erythromycin inhibits peptide chain elongation. - Chloramphincol inhibits peptidyl- transfer reaction.

Inhibition of termination E.g Puromycin is an analogue of the 3´-end of the aminoacyl-tRNA. Peptidyltransferase can not cleave the amide bridge with pauromycin in contrast to the ester bond of peptidyl-tRNA.