Lecture 28 - Protein Synthesis Flashcards
Describe the structure of tRNAs
Have 4 loops –> T-psi-C arm, amino acid arm, D loop, anticodon loop, binds to carboxyl group of the amino acid at the 3’ end of the tRNA
How are amino acids added to a polypeptide (basic)
tRNAs read the complementary sequence of mRNA codons and carries in the correct amino acid
Why are there multiple codons for each amino acid and how do they all bind to the tRNA?
multiple codons for redundancy so that even if one nucleotide is off, the correct amino acid can still be added. Base pairing between tRNA and mRNA doesn’t need to be exact, often only the first two base pairs need to be accurate. The third, at the wobble position, can be different. For example, inosate contains hypoxanthine on a tRNA can bind to UCA, while G can bind to U.
How many reading frames are there?
3, frameshifts can result in a stop codon that creates a truncated protein which is degraded and doesn’t fold properly
How do tRNA synthetases work
Add amino acid to the tRNA. Hydrolyzes ATP bond, attaches AMP to the carboxyl group of the aa, then transfers the aa to the 3’OH or the 2’OH of the 3’ end of the tRNA
Maintains fidelity through two methods: 1) correct aa has highest affinity for active pocket 2) after binding, aa gets pushed to editing site, if it is correct, gets rejected by editing site, if wrong, gets hydrolyzed and released by the editing site
How is protein synthesis initiated in bacteria and eukaryotes?
Bacteria: Met bound by Met-tRNA synthetase to fMet tRNA, Met is then formylated by a transformylase. Shine-Dalgarno sequence on mRNA tells small ribosomal subunit where to bind for translation initiation. IF-3 prevents large ribosomal subunit from binding prematurely. IF-1 binds to A site to prevent tRNAs from binding there. IF-2 -GTP helps fMet-tRNA bind to P-site. GTP hydrolysis removes IF and allows large subunit to bind.
Describe the 3 elongation steps of protein translation
1) Bind aminoacyl-tRNA: tRNA binds to GTP-EF-Tu, which binds to the A-site, GTP hydrolyzed to release EF-Tu, EF-Tu reassociates with GTP with help of EF-Ts.
2) Form peptide bond: high energy ester linkage between aa and tRNA opens up carbonyl carbon for nucleophilic attack to form bond between 1st and 2nd aas, tRNAs then shift such that ends are in E/P sites while codons are in P/A sites
3) Translocation: confromational shift in ribosomes to move one codon downstream so entire tRNAs in E/P site, needs translocase. EF-G binds to A-site and displaces peptidyl-tRNA, old tRNA dissociates from E site
4) repeat until stop codon reached.
Describe the termination of protein translation
3 release factors, proteins mimic tRNA and bind to A-site when stop codon reached
1) bond btwn polypeptide and tRNA hydrolyzed, forms bond with water (carboxylic acid group)
2) Release last tRNA and polypeptide
3) dissociation of ribosome
How are incorrect translation products destroyed?
exposed hydrophobic regions indicate wrong folding and leads to aggregation, so these regions are tagged with ubiquitin (protein) through covalent bonds, proteasome (ATP-dependent protein complex) recognizes ubiquitin and degrades and unfolds protein into small peptides
