Stages and mechanisms of translation, regulation of translation. Antibiotics – ingibitors of transcription and translation
STAGES OF TRANSLATION 1. Recognition 2. Initiation 3. Elongation 4. Termination
Aminoacyladenilate RECOGNITION Aminoacyl-tRNA-synthetase Aminoacyladenilate + tRNA aminoacyl-tRNA + AMP
Activation of amino acids Each amino acid has a specific tRNA There is specific aminoacyl-tRNA-synthetase for each AA
The structure of tRNA
Initiation of Translation The translation complex is assembled at the beginning of the mRNA coding sequence Complex consists of: -Ribosomal subunits -mRNA template to be translated -Initiator tRNA molecule -Protein initiation factors
Initiator tRNA First codon translated is usually AUG The initiator tRNA recognizes initiation codons -Bacteria: N-formylmethionyl-tRNA -Eukaryotes: methionyl-tRNA
Initiation of protein bio- synthesis Methionyl- тRNA binds to P- center
Sites for tRNA binding in ribosomes There are two centers: peptidyl (P) and aminoacyl (А)
Elongation 1) Positioning of the next aminoacyl-tRNA in the A site 2) Formation of the peptide bound (enzyme – peptidyl transferase) between methionine and AA in A- centre. The residue of methionine is transferred on the amino group of another AA 3) Translocation – shift of ribosome by one codon. Methionyl-tRNA is released from P-centre. Dipeptidyl- tRNA moves from A-centre to P-centre.
Termination of Translation Ribosome comes to terminal codon UGA, UAG or UAA No tRNA molecules recognize these codons and protein synthesis stalls Protein termination factors F-1, RF-2, RF-3 split off synthesized polypeptide from the last tRNA Ribosomal complex dissociates
Termina- tion of Trans- lation
POSTTRANSLATIONAL MODIFICATION 1)Preparing of proteins for different functions 2)Direction of proteins to different locations (targeting) 1.Removing of methionine (formylmethionine) 2.Formation of disulfide and other bonds (secondary, tertiary structures) 3. Proteolytic cleavage 4. Modification of amino acid residues: - Hydroxylation - Glycosilation - Phosphorilation 5. Joining of prosthetic groups or cofactors 6. Formation of the quaternary structure
Regulation of the Protein Biosynthesis The operon model (by Jacob and Monod)
Inhibitors of Transcription
Antibiotics inhibiting protein synthesis