Flashcards in Protein Translation and Post Translational Modifications Deck (21):
How many subunits in the ribosome?
What is next to the 5' cap and polyA tail?
A UTR (untranslated region)
What are the stop codons?
UAA, UAG and UGA
What is special about methionine?
Where translation starts
What is methionine's codon?
How are ribosome sizes measured?
What enzyme attaches amino acids to tRNA?
Aminoacyl tRNA synthetase
Describe the mechanism of aminoacyl tRNA synthetase
Uses ATP to form an intermediate known as adenylated amino acid, which has the enzyme bound to AMP which is bound to the correct amino acid. This then binds to the tRNA. The AMP then comes off and the enzyme dissociates. The amino acid is transferred to the 3' hydroxyl of the tRNA.
Explain initiation of transcription
eIF-2 binds to Met-tRNA and a molecule of GTP. This forms a complex in the cytoplasm with the 40S subunit. eIFE and eIF4G binds to the 5' cap of mRNA and the preinitiation complex this. Once this occurs the preinitiation complex travels down the mRNA looking for AUG (methionine's codon). When this is the GTP on eIF2 is hydrolysed to GDP and Pi which provides energy to ensure correct base pairing. Once hydrolysed, the conformational change allows 60S subunit to bind. The IF's and bound GDP then dissociates
What is the preinitiation complex?
Met-tRNA, eIF-2 and 40S subunit.
Which site does the first tRNA bind to, P or A?
Which site does the second tRNA bind to, P or A?
A new tRNA binds to the A site, peptidyl transferase catalyses the peptide bond formation between the two amino acids. The second tRNA translocates to the P site and the first tRNA dissociates.
When the ribosome moves over a stop codon, a group of proteins known as release factors bind to the empty A site. Once they have bound, peptidyl transferase hydrolyses the link, (transfers to polypeptide to water) and releases the ribosome. Everything dissociates
What do antibiotics target a lot?
Protein synthesis in prokaryotes
What type of proteins are translated on the rER? How are they singled out?
Those destined to be secreted or transmembrane. Have a special signal sequence 20-24 amino acids long at the start.
What type of amino acids are in the first part of the sequence of transmembrane proteins? Why?
Hydrophobic, so they have an affinity for lipids
Explain how proteins translated in the rER are moved there
Signal sequence on mRNA is bound to by a signal recognition particle (SRP) halting translation. The SRP then binds to a SRP receptor on the rER which then resumes translation. The bound SRP then triggers the assembly of a protein channel within the membrane of the rER which threads the polypeptide through. Once the amino acid sequence is in the rER the signal sequence is chopped off by signal peptidase and is degraded.
What happens in the rER for proteins?
Where are signal peptides in proteins (rER and non-rER)?
Anywhere in non-rER and only at the start for rER