LS1a_L17_notes08

LS1a_L17_notes08 - PROF ROBERT LUE 1a L I F E S C I E N C E...

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Unformatted text preview: PROF. ROBERT LUE NOVEMBER 13, 2008 1a L I F E S C I E N C E S 1 Translation: the RNA-directed synthesis of proteins 1. The role of RNA in protein synthesis- Three classes of RNA are required to synthesize proteins- mRNAs are decoded in sets of three nucleotides- The structure and function of transfer RNA- Proofreading by aminoacyl-tRNA synthetase 2. The translation machinery and cycle- The structure of the ribosome- The protein translation cycle- Starting and stopping translation- Elongation factors enhance the accuracy of translation Translation: the RNA-directed synthesis of proteins 1. The role of RNA in protein synthesis- Three classes of RNA are required to synthesize proteins- mRNAs are decoded in sets of three nucleotides- The structure and function of transfer RNA- Proofreading by aminoacyl-tRNA synthetase 2. The translation machinery and cycle- The structure of the ribosome- The protein translation cycle- Starting and stopping translation- Elongation factors enhance the accuracy of translation 2 Kinetic Proofreading in Translation EF-Tu blocks peptide bond formation Ribosome + aa-tRNA EF-Tu GTP Ribosome aa-tRNA EF-Tu GTP Binding of aa-tRNA : Some selectivity because of base-pairing energy for correct match peptide bond synthesis... Proofreading depends on the kinetic competition between k tRNA and k EF-Tu GTP hydrolysis : GTPase activation is slower for mismatched tRNA. Ribosome aa-tRNA EF-Tu GDP + P i k GTPase Ribosome aa-tRNA EF-Tu dissociates first : Peptide bond formation can proceed, tRNA is accepted k EF-Tu Ribosome EF-Tu GDP aa-tRNA dissociates first : amino acid lost! Dissociation rate ( k tRNA )is more rapid for mismatched tRNA k tRNA mismatch k tRNA match The ability of EF-Tu to participate in proofreading is based on the fact that peptide bond formation cannot occur while EF-Tu is bound. Let ʼ s walk through a simpli¡ed picture of how error correction in translation works: An aminoacyl-tRNA bound to EF-Tu * GFP approaches the A site of the ribosome and the exposed triple of nucleotides in the mRNA. As we discussed on the previous slide, there is some initial selectivity at this step due to the lower free energy for a perfect match. Nonetheless, it is possible for a mismatched or partially mismatched tRNA to bind. Because EF-Tu is present, peptide bond formation cannot immediately proceed. In order for Ef-Tu to release, it ¡rst must hydrolyze its bound GTP to GDP. The activation of this GTPase activity is slower when an improperly base-paired tRNA is in the A site, and this provides a ¡rst stage of additional selectivity. When GTP is hydrolyzed to GDP, this creates an essentially irreversible step that allows the ribosomal machinery to make another independent test of whether the correct tRNA is bound. Here ʼ s how it works: the GDP-bound form of EF- Tu will dissociate from the ribosome with a rate denoted k EF-Tu....
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LS1a_L17_notes08 - PROF ROBERT LUE 1a L I F E S C I E N C E...

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