NE102 Lecture Notes 2

Ribosome binds at the 5’ utr& initiates

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Unformatted text preview: Ribosome binds at the 5’ UTR & initiates polypeptide synthesis at the start codon 2. Elongation Polypeptide chain elongates successively adding amino acids 3. Termination When a stop codon is encountered, polypeptide is released and ribosome dissociates INITIATION Requires several eukaryotic initiation factors (eIFs) eIF2 – binds to initiator Met tRNA & is bound to GTP – active (allosteric regulation) Other eIFs grab the small subunits Other eIFs grab hold of the mRNA eIF4E specifically binds the 5’ cap Poly-A binding protein binds poly-A tail Protein-protein interactions brings the complex together Ribosome complex moves down mRNA until it reaches a start codon Requires ATP Release of eIFs causes GTP to become GDP When eIF2 is attached to GDP, it is inactive. To become active again, a new GTP molecule must be attached to eIF2 Large subunit comes in. Translation pt. 2 19:00 Chaperons facilitate folding of polypeptides Translation pt. 2 19:00 Chaperonins have a “barrel structure” within which protein folding is accelerated. Hsp70 chaperones bind “nascent” (still attached to ribosome and translating) polypeptides and prevent premature folding Ribosomes require “help” form proteins to carry out translation 3 stages: 1. Initiation 2. Elongation 3. Termination INITIATION (CONT.) Question: How do cells regenerate eIF2-GTP from eIF2-GDP? eIF2B : a GTP exchange factor (aka, GEF) ELONGATION Elongation : The building of the polypeptide chain by the ribosome Translation elongation requires several “eukaryotic Elongation Factors” (eEFs) [Sidenote] Three distinct sites on the ribosome: P (pedtidyl) site A (aminoacyl) site Translation pt. 2 19:00 Exit site First step, tRNA brings amino acid and binds to P site and A site Peptide bond forms Translocation and release of tRNA Repeats over and over again, until it reaches a stop codon. TERMINATION Translation termination requires “Release Factors” Release Factors insert into A site and release the protein strand and complex disassembles Cells have mechanisms in place to regulate translations Two general modes: a.1. Cells can regulate the rate at which of SPECIFIC mRNAs are translated • Regulated polyadenylation: Length of the poly-A tail can be lengthened or shortened to influence translation a.2. Cells can regulate “GLOBAL” translation (i.e., all or most mRNAs) Regulation of eIF2 Cells inhibit eIF2 & eIF2B by phosphorylation when they encounter “stress” – stops global translation (b/c most mRNAs need active eIF2) Sources of stress: Loss of nutrients Accumulation of unfolded or improperly folded proteins Viral infection Others… Sidenote: Neurons in the cortex & hippocampus of Alzheimer patients contain very high levels of eIF2 phosphorylation. Integrating Concepts through p53/ Introduction to Recombinant DNA 19:00 What is p53? Integrating Concepts through p53/...
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Ribosome binds at the 5’ UTR& initiates polypeptide...

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