There seems to be separate proofreading active site (s)on such synthetases In a few synthetases that activate amino acids having no close structural relatives. no such proofreading activities have yet identified(selected at the substrate binding level)
• There seems to be separate proofreading active site (s) on such synthetases. • In a few synthetases that activate amino acids having no close structural relatives, no such proofreading activities have yet identified (selected at the substrate binding level)
The codon on mRNA is recognized by the anticodon of tRNA rather than by the activated amino acid Cysteinyl-tRNA H O R H O Cysteine tRNACys synthetase HS-CH-C-C-0-tRNACYs nickel H一CH2-C-C-0-tRNA ATP+H,0 AMP+2 Pi NH Cys-tRNA YS Ala-tRNACY 1. Cys-tRNACyS is chemically converted to Ala-tRNACyS 2. Studies with in vitro protein synthesis systems proved that Ala-tRNA Cys will incorporate Ala at places of Cys using poly (G, t)as templates or hemoglobin mRNA as template (with [4Cl-Ala-tRNACys)
The codon on mRNA is recognized by the anticodon of tRNA rather than by the activated amino acid 1. Cys-tRNACys is chemically converted to Ala-tRNACys ; 2. Studies with in vitro protein synthesis systems proved that Ala-tRNACys will incorporate Ala at places of Cys: using poly(G,U) as templates or hemoglobin mRNA as template (with [14C]-Ala-tRNACys)
Some synthetases are able to Adenylation proofread the incorrectly incorporated amino acids Proofreading: aminoncyl-adernylate is hydrolyzed tRNA binding amino acid C At the aminoacyl- AMP stage hanging with tRNA Proofreading aminoacyl-tRNA is hydrolyzed Wrong amino acid a At the aminoacyl- ERNA Stage COOH Correct amino acid AMP Correctly Aminoacyl-tRNA charged tRNA
At the aminoacylAMP stage At the aminoacyltRNA stage Correctly charged tRNA Some synthetases are able to proofread the incorrectly incorporated amino acids
20. Met-tRNA Met recognizes the initiating AUG codon in almost all cells It was observed that about half of the n-terminal residues of proteins in E coli are met, and the n-terminal of nascent polypeptides is usually modified There are two tRNAs for recognizing AUg and Met in all organisms: one(tRNA Met) brings Met to the initiating AUG and the other(tRNA Met) brings met to internal AUgs The Met charged on tRNAMet(by Met-tRNA synthetase is specifically formylated by a transformylase to form fMet-tRNAMet in bacteria(Met-tRNA Met can not be formylated) fMet-tRNAtMet can only enter and only fMet-tRNAMet can enter the initiation site(site P)on the ribosome in
20. Met-tRNAi Met recognizes the initiating AUG codon in almost all cells • It was observed that about half of the N-terminal residues of proteins in E.coli are Met, and the N-terminal of nascent polypeptides is usually modified. • There are two tRNAs for recognizing AUG and Met in all organisms: one (tRNAi Met) brings Met to the initiating AUG and the other (tRNAMet) brings Met to internal AUGs. • The Met charged on tRNAi Met (by Met-tRNA synthetase) is specifically formylated by a transformylase to form fMet-tRNAfMet in bacteria (Met-tRNAMet can not be formylated). • fMet-tRNAfMet can only enter and only fMet-tRNAfMet can enter the initiation site (site P) on the ribosome in E.coli
Eukaryotes MetHtRNA tRNA Met+ Methionine Archaeans - Initiation fMetH tRNA Met Bacteria +CHO All cells tRNAMet+Methionine → MetHtRNA一→ Elongation Two types of methionine trNA (tRNA Met or tRNAMet in bacteria and tRNAMet are found in all cells Both trNas are recognized by the same Met-tRNA Synthetase
Two types of methionine tRNA (tRNAi Met or tRNAfMet in bacteria, and tRNAMet) are found in all cells. Both tRNAs are recognized by the same Met-tRNA synthetase