lecture 19 - Lecture19...

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Lecture 19 Fig 8.12 A special initiator tRNA starts the polypeptide chain Protein synthesis starts with a methionine amino acid usually coded by AUG When you make a peptide bond, there are two sites on ribosome that are important A site - where newly amino acyl tRNAs enter P site - has the growing polypeptide Both the A and P site are attached by their carboxyl group in a amino acyl bond to the OH on a ter- minal ribose of terminal A on the tRNA and are covalently attach The tRNA is making contact, not only with the codon in the mRNA but part of the structure of tRNA makes contacts with the rRNA and ribosomal protein in both the large and small subunit. Initiation vs elongation the first tRNA that enters has to go to the P site not the A site, otherwise there would be nothing to join together, you need two different tRNAs each with an amino acid to make the first peptide bond. need initiation factors to deliver the initiating tRNA into the half P site. The way the ribosome works is that initiation involves just the RNA, the initiation factors, the initiating tRNA with thyam- ine goes into the half P site. half because the ribosome is only the small subunit and doesn’t have the large subunit yet. Once the complex assembled then the unique initiating factors due to GTP hydrolysis end up dissociated and the large subunit joins the small, now the ribosome is ready for elongation. have the first codon from the start of the tRNA in the half P site and an empty A site for the second amino acyl tRNA to enter and the A site will sit over the second codon. Prokaryotic and eukaryotic ribosomes do initiation steps slightly different one assembles on the mRNA first Eukaryotes assemble the factors on the small subunit first then join the message later. Both have a special initiation factor. prokaryotes it is called IF2 and in eukaryotes its called eIF2; they uniquely recognize a unique tRNA that ends up initiating Methionine and only that tRNA goes into the empty P site. In bacteria, part of the recognition of IF2 is having the reaction where to get the Met charged by synthase on this initiating tRNA a formyl group is added to the amino group. Normally, an intermediate amino can have a blocked amino group that terminates protein synthesis. Always adding the second amino acid as a free amino group and adding a carboxyl group of the amino acid before. If you have a blocked amino group you can’t make a peptide bond. The first amino acid can have a blocked amino group because the second amino acid is going to have the free amino group. Eukaryotic protiens don’t have a formylated Met, they use the different features of the tRNA to distinguish it from the elongating tRNA of from Met. Both have special factors that only recognizes the intiating factors of tRNA.
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lecture 19 - Lecture19...

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