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Lecture 21

Lecture 21. Protein Translation. Genetic Code. Amino Acid Activation 2. Polypeptide initiation 3. Polypeptide elongation 4. Polypeptide termination 5. Polypeptide Folding and Processing:. The five stages of protein synthesis. Ribosome Structure. Ribosomes. Ribosomes and mRNA. tRNA.

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Lecture 21

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  1. Lecture 21 Protein Translation

  2. Genetic Code

  3. Amino Acid Activation 2. Polypeptide initiation 3. Polypeptide elongation 4. Polypeptide termination 5. Polypeptide Folding and Processing: The five stages of protein synthesis

  4. Ribosome Structure

  5. Ribosomes

  6. Ribosomes and mRNA tRNA mRNA

  7. Ribosome structure

  8. Initiation • Requirement of initiation: • 30S ribosomal subunit • mRNA • Initiation fMet-tRNAfMet • Initiation factors (proteins) • GTP • Mg2+ Initiation complex

  9. Shine-Dalgarno sequence is important for aligning initiation codon

  10. Elongation Step 1: Binding of Aminoacyl tRNA Involves a G protein EF-Tu Ribosome is the GAP (promotes hydrolysis of GTP). EF-Ts is GEF (Promotes exchange of GDP for GTP)

  11. Kinetic Proofreading The duration of the codon-anticodon binding interaction is compared to the rate of GTP hydrolysis and the dissociation of EF-Tu-GDP Ribosome(n) + aa-tRNA•EF-Tu •GTP k-1 k1 Ribosome(n) • aa-tRNA•EF-Tu •GTP k2 Pi Ribosome(n) • aa-tRNA•EF-Tu •GDP aa-tRNA EF-Tu •GDP k3 k4 If k4 > k3 then reject amino acyl-tRNA Ribosome(n) •EF-Tu •GDP

  12. Elongation Step 2: Formation of the peptide bond Catalyzed by peptidyl transferase A protein? A rRNA?

  13. An antibiotic that interferes with proofreading Streptomycin Causes misreading by: Pyrimidines interchanged in 1st and 2nd codon position. Pyrimidines may be mistaken for adenine in 1st position Resistance conferred by a base change in rRNA of small ribosome subunit.

  14. Peptide bond formation

  15. Elongation Step 3: Translocation Another GTP binding protein EF-G

  16. Chain Termination Release factors recognize specific codons.

  17. Cost of protein synthesis To achieve specificity and speed, four high-energy phosphate compounds are used for each peptide bond (2) used for amino acid activation (1) used in amino acyl-tRNA binding (1) used in translocation -90% of biosynthetic energy in E. coli isspent on this process -Rate: about 30-40 amino acids per sec, 10-20sec per average length polypeptide (400 amino acids)

  18. Antibiotics/ Other Translation Inhibitors

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