Camerini 5

Camerini 5 - Lecture 5: Features of protein synthesis;...

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1 Lecture 5: Features of protein synthesis; post- translational processing Today, we will discuss the following: 1. Elongation 2. Termination 3. Fidelity in protein synthesis 4. Energetics of prokaryotic translation 5. Suppressor mutants 6. Eukaryotic translation 1. Antibiotic inhibition of protein synthesis 2. Movie on translation! 3. Regulation of protein synthesis (if time permits)
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2 Three distinct steps to add one amino acid to the growing polypeptide chain. · Occurs many times per polypeptide, the number depends on the number of mRNA codons or amino acids in the protein · The Elongation Cycle is similar in prokaryotes and eukaryotes. · Fast: 15-20 amino acids added per second · Accurate: 1 mistake every ~10,000 amino acids Elongation
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3 Schematic of chain elongation in prokaryotes 1. Committing step: EF- Tu/GTP/AA-tRNA binds to A-site (one phosphate bond broken) 2. Peptide bond formation catalyzed by the 23S rRNA, part of the 50S subunit. The polypeptide chain is transferred from the P-site tRNA to the A-site tRNA. 3. Translocation: movement of peptidyl-tRNA from the A- site to the P-site; and simultaneous transfer of free tRNA from P-site to E- site; movement down one codon driven by EF-G entry into A-site displacing peptidyl-tRNA(one more phosphate bond broken).
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4 Termination Occurs at stop codons: UAG = Amber UAA = Ochre UGA = Opal/Umber Does not depend on “Stop” tRNAs. Stop codons bind “release factors.” Soluble release factors: RF1 responds to UAA or UAG RF2 responds to UAA or UGA RF3, a GTPase (like EF-Tu and binds in a similar A-site location) RF1/RF2 interact with RF3-GTP, have shapes similar to EF-Tu-GTP-AA-tRNA AA and EF-G, bind to the A-site. Like EF-G, GTP hydrolysis drives the movement of the terminal mRNA codon into the P-site, moving the last tRNA into the E-site and off the ribosome. At the same time, the polypeptide chain is released after hydrolysis of the tRNA-peptide bond. Due to similar structure (“molecular mimicry”), EF-G and the release factors compete for the A-site. In eukaryotes, only a single release factor, eRF, is necessary. It recognizes all three STOP codons and interacts with GTP.
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5 Schematic of chain termination Ribosome stalls when it encounters “Stop” Entry of appropriate release factor dimer; RF3 carries GTP to the A-site Release factor promotes hydrolysis of polypeptide from the peptidyl-tRNA in the P-site GTP hydrolysis causes release of RF-1, RF-3 and the last free tRNA present in the ribosome
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6 EF-Tu structure in GDP vs. GTP bound state EF-Tu:GDP (open) EF-Tu:GtP (compact)
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7 EF-Tu and kinetic proofreading What gives rise to specificity in the loading of AA-tRNAs into the A-site? Energy of one GTP bond. How? Kinetic
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This note was uploaded on 03/14/2009 for the course BIOL 97 taught by Professor Edinger during the Fall '08 term at UC Irvine.

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Camerini 5 - Lecture 5: Features of protein synthesis;...

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