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Chapter 24

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  1. Chapter 24 Protein Synthesis and Controlled Protein Breakdown

  2. Chapter Objectives • Know the process of protein synthesis and the genetic code (don’t memorize the genetic code) • Know how ribosomes work • Initiation, elongation, termination • Know the key steps in protein folding • Understand how protein translation is controlled • Know the process of protein degradation

  3. The Genetic Code • Nonsense mutation – stop codon • Degenerate code – more than one codon per amino acid • Conservation

  4. Overview of Protein Synthesis • Amino acids are activated by aminoacylsynthetases using ATP • Amino acids are added to tRNA • tRNA molecules with their attached amino acid are brought to ribosome • The growing peptide chain is added to each new amino acid • Energy cost = 1 ATP & 2 GTP

  5. tRNA There are not 61 different tRNA molecules


  6. tRNA Wobble • Wobble allows for less tRNA molecules to be needed • Notice orientation!!

  7. Amino Acid Attachment to tRNA • Must attach correct amino acid to tRNA • Wrong amino acid will be incorporated into protein otherwise • Specific aminoacyl-tRNAsynthetase for each tRNA / amino acid pair • Need 20! • No consistent recognition • Sometimes anticodon • Sometimes other regions of tRNA • Always side chain of amino acid • Process driven by ATP hydrolysis

  8. Specificity of Amino Acid Priming

  9. Ribosomes • Prokaryote ribosomes • Subunits • 23S and 5S pieces of RNA and 34 proteins • 16 S piece of RNA and 21 proteins • Total • 50S (large subunit) • 30S (small subunit)

  10. http://www.mrc-lmb.cam.ac.uk/ribo/homepage/mov_and_overview.htmlhttp://www.mrc-lmb.cam.ac.uk/ribo/homepage/mov_and_overview.html

  11. Translation Initiation 30S ribosome subunit binds to Shine-Dalgarno sequence placing AUG in P site Special met-tRNA recognizes IF2 (initiation protein factor 2) IF3 prevents binding of 50S subunit prematurely Shine-Dalgarno 50S subunit then associates IF2&3 are displaced GTP is hydrolyzed

  12. Elongation

  13. Termination This picture is from eukaryotes Think of 60S as 50S and 40S as 30S RF1 and 3 are RF1 and 2 in prokaryotes Still need to bring in IF3 to prevent interaction of 30S with 50S

  14. Overview http://www.mrc-lmb.cam.ac.uk/ribo/homepage/mov_and_overview.html

  15. Better overview http://www.mrc-lmb.cam.ac.uk/ribo/homepage/mov_and_overview.html

  16. Drugs that inhibit Translation • Chloramphenicol – peptidyltransferase • Erythromycin – 50S inhibits translocation • Kirromycin or fusidic acid – prevents EF-Tu release • Sparsomycin –peptidyltransferase inhibitor • Streptomycin – initiation misread • Tetracyclin – inhibits tRNA from binding ribosome

  17. Riboswitches

  18. Selenocysteine • Not a standard amino acid • Made from cysteine and selenium (toxic) • Uses UGA stop codon with special tRNA and EF • Rare

  19. Protein Folding • Chaperones • ATP dependant • Heat Shock proteins • Protein Disulfide Isomerases (PDI) • Peptidylprolineisomerase (PPI)

  20. GroELGroES chaperone

  21. Prion Proteins

  22. Programmed Protein Death • Proteasome • Ubiquitin • Poly • Mono

  23. Ubiquitin Pathway • E1 is ubiquitin activating enzyme (ATP needed • E2 accepts Ub • E3 ligase activity • Multiple E2/E3 combos