Protein Synthesis Lecture 2 2017s.ppt

Protein Synthesis Lecture 2 2017s.ppt - BMB7010...

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Mechanism of Protein Synthesis BMB7010 Biochemistry Protein Synthesis (Evans) Lecture 2 Berg, Tymoczko and Stryer Chapter 30 7 th Edition pages 900 - 913 Problems 12, 18 Overview:
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1. Protein synthesis starts with N-formylmethionine-tRNA f bound to the P site on the 30S subunit. 2. The anticodon on the initiator tRNA f is based paired to the start codon. 3. The 50S subunit joins the complex forming the complete 70S ribosome. 4. The aminoacyl tRNA encoding the next amino acid then binds to the A site.
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1. The peptide bond is formed by transferring formyl- methionine from the tRNA f in the peptidyl site to the amino group of the amino acid bound to the tRNA in the A site. 2. The reaction takes place at the peptidyl transferase center on the 50 S subunit. 3. The peptide is attached to the tRNA in the A site on the 30S subunit and the other end of the peptide tRNA is now in the P site.
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Peptide Bond Formation 1. Nucleophilic attack by the amino group of the aminoacyl tRNA in the A site on the carbonyl group of the ester linkage of the peptidyl tRNA in the P site to form a tetrahedral intermediate which then collapses to form the peptide bond and releases the deacylated tRNA.    Catalyzed by 23S peptidyl transferase activity.
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Translocation then occurs moving the mRNA so the codon for the next amino acid is in the A site. Translocation is mediated by a protein enzyme, the elongation factor, EF-G . GTP hydrolysis powers the translocation. After translocation the tRNA with the growing polypeptide chain is in the P site on both 30S and 50S subunits. The uncharged initiator tRNA is in the E site and no longer bound to mRNA.
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The initiator tRNA then dissociates and the ribosome is ready for another cycle. The growing polypeptide chain never leaves the P site where it is threaded through the tunnel.
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Formyl group prevents the attack of the free amino group on the linkage between the second amino acid and the tRNA. amino group formylated This reaction would form a stable 6 membered ring and terminate peptide synthesis. Role of Formylation of the Methionine on the Initiator tRNA
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In vitro translation using the mischared tRNA showed that the random UG copolymer that normally incorporates cysteine, incorporates alanine. Thus, the amino acid in aminoacyl-tRNA does not play a role in selecting a codon. Only the codon (in the A site on mRNA)-the anticodon on the incoming tRNA interactions, not the amino acid , determine which amino acid is incorporated into the polypeptide. Cysteine bound to cysteinyl tRNA was converted chemically to alanine.
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Wobble steric freedom in third base in the codon that allows charged tRNA to sometimes recognize more than one codon.
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  • Winter '18
  • podolsky
  • Protein biosynthesis

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