ProteinSynthesisTranslation课件.ppt

Protein Synthesis-Translation Concept of translation Protein synthesis requires: ? mRNA、rRNA、tRNA ? Substrates: 20 amino acids ? Enzymes and protein factors: initiation factor, elongation factor, releasing factor ? ATP、GTP、Mg2+ Prokaryotic mRNA is polycistron. Eukaryotic mRNA is monocistron. The Genetic Code The Genetic Code Is a Triplet Code Three codons for stop signal: UAA, UAG, UGA. 61 codons for 20 amino acids. One codon for start signal: AUG. It also codes for methionine. Nonoverlapping and without Punctuation Open reading frame Frameshift Degenerate, but Unambiguous More fault-tolerant for point mutations. Wobble Base pairs of inosine Fewer than 61 tRNAs are required Enhances the efficiency of protein synthesis The Code Is Almost Universal All organisms use the same genetic code. However, there is intriguing exception of a few minor variations. tRNA and AA Activation tRNA Activation of amino acid Activated amino acid Ala-tRNAAla Ser-tRNASer Met-tRNAMet Summary of AA activation Active form aminoacyl－tRNA Activation site α- carboxyl group Linkage ester bond Activation energy 2 high-energy bonds Protein synthesis fidelity Aminoacyl-tRNA synthetase has the proofreading ability. Prokaryotic Met-tRNAmet Prokaryotic Met-tRNAmet can be formylated to fMet-tRNAimet. Met-tRNAmet + N10-formyl tetrahydrofolate Formyl transferase fMet-tRNAimet + tetrahydrofolate Initiation codons For prokaryotes: fMet-tRNAimet initiation Met-tRNAemet elongation. For eukaryotes: Met-tRNAimet initiation. Met-tRNAemet elongation. Recognized by enzymes or factors in different phases. Ribosome of prokaryotes Three sites on ribosomes A site, P site and E site Translation in Prokaryotes Initiation Three steps in prokaryotic systems – Positioning mRNA on the 30S subunit – Registering fMet-tRNAimet on the P site – Associating the 50S subunit Three initiation