48. Kevin Ahern's Biochemistry - Translation II
1. Contact - [email protected] 2. Kevin's lectures with The Great Courses - https://www.thegreatcoursesplus.com/b... 3. Kevin's Lecturio videos for medical students - https://www.lecturio.com/medical-cour... 4. Course materials at https://kevingahern.com/biochemistry-... 5. Course video channel at • 27. Kevin Ahern's Biochemistry - Citric A... 6. Metabolic Melodies at https://teeheetime.com/category/lyric... 7. Kevin's Free Biochemistry books - https://kevingahern.com/biochemistry-... 8. Kevin's Pre-med Audio course on Listenable - https://listenable.io/web/courses/143... Highlights Translation II 1. Ribosomes have three sites for binding/holding/releasing tRNAs. They are the A,P, and E sites, corresonding to the order in which tRNAs move through them. 2. Initiation of protein syntheis starts with binding of IF1 and IF3 to the 30S ribosomal unit. The mRNA is aligned with the Shine-Dalgarno site. 3. IF2 (when bound to GTP) acts to carry the Met-tRNAf to the P site of the 30S subunit and base pairs it with the AUG start codon. 4. Hydrolysis of the GTP in IF2 results in release of the IF2 and IF1 from the initiation complex. That, coupled with binding of the 50S subunit yields the 70S initiation complex with Met-tRNAf in the P site and the A and E sites open. 5. In elongation, EF-Tu (a G protein coupled to GTP) carries a charged tRNA to the A site of the complex. If the tRNA anti-codon base pairs properly with the codon in the mRNA, it stays matched with the codon and GTP is hydrolyzed on EF-Tu and EF-TuGDP is released. If the tRNA anti-codon does not form a stable base pairing with the complex, the complex dissociates. 6. Next, the peptide group on the tRNA in the P site is transferred and covalently linked via peptide bond to the amino acid on the tRNA in the A site. This reaction is catalyzed by the 23S rRNA in the 50S subunit. 7. The tRNA in the A site along with the peptide it is covalently attached to is transferred to the P stie as the "empty" tRNA in the P site is moved to the E site. EF-G-GTP is involved in the process and GTP is hydrolyzed in the process. EF-G-GTP has a similarity to the tRNAaminoacid-EF-Tu-GTP complex and may act to displace it. 8. As the old tRNA is released from the E site, the empty A site accepts the aminoacyl tRNA corresponding to the next codon. The process is repeated until a termination signal is reached. 9. The process of translation termination begins when a stop codon appears in the A site of the ribosome. Termination of translation requires action of release factors (RF1, RF2 - I mistakenly said there were 3. There are 2). 10. RF-1 and RF-2 carry water to the A site, thus releasing the completed polypeptide from the ribosome. 11. Eukaryotic translation is mechanistically similar to prokaryotic translation. Differences include ribosomes (40S vs 30S and 60S vs 50S), rRNAs (28S, 18S, 5.8S, and 5S), mRNAs (cap at 5' end and polyA at 3' end, both involved in translation), and lack of formyl group on inititator tRNA's methionine. 12. Other translational differences in eukaryotes relate to the structure of eukaryotic cells. Targeting of proteins to organelles, the membrane, or outside of the cell, starts during protein synthesis. Proteins destined to leave the cytoplasm have a signal sequence, consisting of a stretch of hydrophobic amino acids near their amino terminus. 13. When the signal sequence emerges from the ribosome during translation, it is recognized by the signal recognition particle (SRP), which takes the entire ribosome/mRNA/polypeptide complex to the endoplasmic reticulum. There it interacts with the SRP receptor and in the process it links the ribosome with the emerging polypeptide sequence to the translocon. 14. The polypeptide passes through the translocon channel as it is being made and when the signal sequence completely exits the translocon, a signal peptidase clips it free of the rest of the polypeptide. The translation process continues until the stop codon is reached and then everything releases from the translocon. 15. The polypeptide remains in the endoplasmic reticulum where it is further process to travel to the Golgi for additional processing and targeting. Other sequences in the polypeptide may help to direct modifications and/or the final destination of the mature protein.
