EXAM 2 - Session 15: Protein Synthesis Flashcards
What is the responsibility of tRNA in mRNA tranlation into protein?
tRNA reads codon of mRNA
* each tRNA can only hold one amino acid
* anti-codon of tRNA base-pairs with codon on mRNA
What is the responsibility of tRNA in mRNA tranlation into protein?
tRNA reads codon of mRNA
* each tRNA can only hold one amino acid
Explain how tRNA attaches amino acids to the growing peptide strand.
- anti-codon of tRNA base-pairs with codon on mRNA
- carries amino acids via ester bond at the 3’ terminus
Describe the process of tRNA activation.
Requires AA, tRNA, enzymes, and energy source
1. correct aa is bound enzyme complex (aminoacyl-tRNA synthase) using ATP –> there is one aminoacyl-tRNA synthase for each tRNA and aa pair
2. AMP is exchanged for last nucleotide of tRNA
3. AMP is released; aa is covalently linked to tRNA via 3’-OH
4. activated tRNA is released and ready for codon recognition
Explain codon redundancy.
Different sequences that end up doing the same thing
* ex. STOP codons: LEU, VAL, ARG
Describe the general features of ribosomes.
1 large and 1 small subunit
* each subunit has characteristic proteins and rRNAs
* rRNAs have 2 degree structure form intermolecular base pairing
* rRNAs have catalytic activity for peptide bond formation
What is the size difference between eukaryotic and prokaryotic/bacterial ribosomes? What does that mean?
Eukaryotic - 80S
Prokaryotic - 70S
* bigger ribosome –> slower
Contrast eukaryotic and prokaryotic ribosomal sequences.
Different protein and rRNA in each.
What is the difference between the surfaces of eukaryotic and prokaryotic ribosomes?
There are 3D differences from rRNA and r-protein
* will affect how different antibiotics will interact with the ribsome
How do antibiotics differ?
Antibiotics can target different steps of translation.
What are the three modes of action of antibiotics?
- blocks tRNA interaction with ribosome
- blocks ribosome moving along mRNA
- blocks interaction of tRNA and mRNA codon
What is the advantage of using combined antibiotics?
One treatment might not be 100% effective in blocking specific function
* targeting different functions can improve bacterial kill-off
Name the three phases of translation.
- Initiation
- Elongation
- Termination
Describe the steps of initiation.
- mRNA initiation sequence binds mRNA to small ribosome subunit
- methionine-tRNA binds to start codon (AUG=met) via anticodon of tRNA
- large subunit binds to small subunit –> MET-tRNA fits into P site of large subunit
Explain why there is always excess mRNA at the 5’ end before the start codon.
The mRNA is always longer than than the codons coding for amino acids at the 5’ end because it is the noncoding intiation sequence.
* allow interactions with small subunit ribosomes
Name and describe the different sites of the large subunit.
E = exit
* site for tRNA done with transfer of amino acid –> ready to leave
P = peptide
* site for tRNA that has growing peptide chain
A = activated aa
* site for tRNA with aa that is ready to be transfered to peptide
Translation: explain the process of elongation.
- ribosome catalyzes formation of peptide bond between amino group of incoming aa and carboxy terminus of growing peptide
- elongation of 1 aa at a time
- continued elongation until stop codon is reached
- requires GTP hydrolysis
Translation: explain the process of termination.
- elongation until stop codon (UAA, UAG, or UGA) at A site
- finished polypeptide is released by hydrolysis from last tRNA
- ribosome splits into subunits (small and large)
- released ribosomes are available for binding to initation sequence of another mRNA
Translation: explain signal peptides.
- signal peptide is held in the membrane of ER
- rest of the growing protein goes into ER lumen
- mRNA encoding a protein targeted to ER remains membrane-bound
- polyribosome is bound to membrane
Explain the difference between translation of proteins with and without signal peptide.
All translation initiates on free ribosomes in the cytoplasm
* WITHOUT signal peptide –> tranlation is completed in cytoplasm
* WITH signal peptide @ N terminal –> ribosome, mRNA, and protein are directed to ER
How does the completed peptide get released from the ER?
Signal peptidase in rER lumen cleaves off protein.
* new protein is released into the ER lumen
* signal peptide is released to cytoplasm and degraded
Explain how the ER becomes rough.
- Signal peptide is responsible for making the ER rough
- signal peptide cleaves protein which is released into ER lumen
- cleavage generates new N-term for protein w/o initiator MET
- signal peptide that is left is released from ER membrane and degraded
- protein in ER gets further processing, delivery to other organelles, or secretion
How are proteins secreted from the rER?
- In the rER lumen –> signal sequence/peptide cleaved
- in the golgi –> 2 additional cuts made = C chain is released
- in the golgi –> A&B chains linked by 2 disulfide bonds
- secretory vesicles bud off golgi
- secretory vesicles fuse to cell membrane after receiving secretion stimulus
What do cytosolic polypeptides lack?
Signal sequence - rather, polypeptides are synthesized ny ribosomes that stay free in cytoplasm.
What is the purpose of protein compaction?
Time: milliseconds to seconds
Purpose: for secondary structure to shield hydrophobic goups from aqueous cytoplasm
What is the purpose of chaperone interactions?
Time: seconds to minutes
Purpose: For many new cytoplasmic proteins –> guides 3D folding of the protein
* once the protein is fully folded –> chaperone will dissociate
Describe the mutation that occurs in Duchenne muscular dystrophy.
mRNA AA codon mutated to premature termination/STOP codon (PTC) –> forced termination
* protein is easily degraded
* result = disorganized cytoskeleton; poor muscle function
In Duchenne muscular dystrophy, how can “read-thru” the premature termination codon (PTC)?
Ataluren (antibiotic)
* facilitates 1/3 or 2/3 match to anti-codon of AA-encoding tRNA
I I I = STOP codon binding release factor
I I X = partial: UAA (STOP) —> actually read as UAC (TYR)
* translation bypasses mutated codon –> restores production of full-length protein
