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lecture12 - Translation Cricks adapter tRNA...

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1201 Translation Crick’s adapter: tRNA a.k.a. the flashcard aminoacyl-tRNA-synthetases: a.k.a. writing the flashcards The Ribosome: a machine for rapidly retrieving flashcards for translation
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1202 Important Question: How is the correct amino acid attached to each tRNA ? This is as important for correct translation as codon-anti-codon interaction but more challenging because there is no base pairing..
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1203 1. Amino acid is activated 2. This anhydride reacts with tRNA to form aminoacyl tRNA An amino-acid-specific enzyme appends the amino acid to the 3’ribose residue of its cognate tRNA to form amino-acyl-tRNA high energy product: “charged” hydrolysis of PPi - completion
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1204 Aminoacyl-tRNA Synthetases 2 classes: structural differences anticodon recognition: many class I do, many class II won’t 2’-OH vs. 3’-OH amino acid specificity
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1205 How do Aminoacyl-tRNA Synthetases distinguish between tRNAs? Much effort has been made to determine what the major identity elements are in tRNAs interacting with Aminoacyl-tRNA Synthetases.
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1206 - not clear how this can be so accurate given that the molecular interactions between tRNAs and aminoacyl tRNA synthetases only provide a limited amount of Δ G differences. - Proofreading at a different active site requiring conformational changes (kinetic parameters) may explain the observed fidelity. 1 valine for every 50,000 iso-leucines Charging tRNAs is very accurate
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1207 monomer some deep pockets Glutaminyl-tRNA synthetase (GlnRS) with tRNA Gln + ATP class I
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1208 class II enzyme dimer with two tRNA Asp
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1209 binds tRNA from the minor groove of acceptor stem bends it hairpin at 3’ end binds tRNA from the major groove of acceptor stem 3’ continues in helical path class I class II
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1210 Different stereo chemistries for class I and II result in different OH groups being amino-acylated
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1211 Biotechnology challenge: Expanding the genetic code
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1212 Translation Crick’s adapter: tRNA a.k.a. the flashcard aminoacyl-tRNA-synthetases: a.k.a. writing the flashcards The Ribosome: a machine for rapidly retrieving flashcards for translation
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1213 Ribosomes RNA-protein complex 2520kd in E.coli, 4220kd in eukaryotes • binds mRNA • brings in tRNA •A aminoacyl site - incoming charged tRNA •P peptidyl site - peptide bond formation •E exit site - exiting discharged tRNA • supervises and proofreads base-pairing • catalyzes peptide bond formation • undergoes movement • allows translation regulation factors to regulate initiation, elongation, and termination
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1214
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1215 2° structure prediction of rRNAs (prokaryotic)
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1216 Cryoelectron microscopy
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1217 Model building: Prokaryotic ribosome tRNA sites aminoacyl (A) site peptidyl (P) site exit (E) site narrow channel in 30S for mRNA 15Å tunnel in 50S for peptide lined with hydrophilics conformational change upon correct basepairing in 30S, then 50S catalyzes bond formation
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1218 interface between subunits more highly conserved than solvent exposed surface Eukaryotic ribosome
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1219
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1220
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