Module 4 Section 5

Question 1

Elongation: delivery of charged tRNA to prokaryotic decoding center steps

Answer 1

1. Charged tRNA delivered to A-site by EF-Tu-GTP (elongation factor thermo unstable)
2. A1492 and A1493 (adenosine) ‘flip out’ in response to the correct codon-anticodon BP
3. causes conformational changes to the 30S subunit
4. EF-Tu-GTP hydrolyzed to EF-Tu-GDP
5. tRNA rotates into position (accomodation)
   EF-Tu is eEF1alpha in eukaryotes

Question 2

Elongation: Peptidyl transferase rxn

Answer 2

• peptide bond is formed between 2 AA bound by their tRNAs to the A and P sites on the ribosome
  1. alpha-amino group of amino-acyl tRNA in the A site attacks the carbonyl carbon of the ester bond linking the peptide to the tRNA in the P-site
• alpha-amino is nucleophile
  2. growing chain is transferred to the tRNA in the A site
• as ribosome shifts along mRNA, uncharged tRNA now moves to the E site and the peptidyl-tRNA moves to the P-site
• this frees A site to bind next tRNA
• peptidyl transferase is the enzyme that catalyzes the peptide bond formation

Question 3

Aminoglyceride antibiotics

Answer 3

• cause inappropriate flipping of the A1492/3 for non-cognate tRNAs
• used to kill bacteria through translational fidelity (misfolded proteins = death)
• at high doses, can cause read through of nonsense mutations in eukaryotes

Question 4

Puromycin antibiotic

Answer 4

• inhibitory antibiotic
• structure is very similar to the 3’ end of an aminoacyl-tRNA
• fits into peptidyl transferase center
• not bound to the ribosome by interactions with 16S rRNA
• if bound to chain, the peptide dissociates b’c the puromycin is not bound to the ribosome