Protein Synthesis Flashcards
How is the anticodon structured?
The amino acid anticodon is contained in a given amino acyl tRNA for example methionine’s tRNA is methionyl tRNA
The redundancy of the codon production means what type of mutation can occur?
Synonymous variants or silent mutations
What are ribosomes?
Ribonucleoproteins comprising of both RNA and protein
Where are the two populations of ribosomes that carry out protein synthesis?
One bound to the endoplasmic reticulum
The other is free within the cytosol
How fast is protein synthesis?
Very fast
Adds 20 amino acids per second and can synthesise a molecules of actin every 20 seconds
What are the two subunits of ribosomes and what RNAs and proteins do each have?
40S and 60S which forms an 80S ribosome
40S subunits have 18S rRNA and ~33 proteins
60S subunits have three RNAs- 5S, 5.8S and 28S and ~49 proteins
What are the three stages of protein synthesis?
Initiation- assembly of the ribosome on the mRNA together with the first initiator tRNA: aminoacyl tRNA (met-tRNAi)
Elongation- changed tRNAs delivered to the ribosome
Addition of amino acids one at a time building the polypeptide
Termination:
Encounter with a stop codon
End of protein synthesis and release of the polypeptide
What are translation factors and how are they named?
Proteins directly involved in translation are called initiation or elongation factors and have a name that is systematic
Initiation factors are called eIF+ a no.
Elongation factors are called eF+ a no.
Others associated with a ribosomal unit so L or S for large subunit or small subunit e.g. L22
Recall the initiation stage of eukaryotic translation of protein synthesis
eIF 4 and 5 bind to the cap structure on the mRNA
The poly(A) binding proteins bind to the poly(A) tail
The association of these proteins with the RNA results in a circularisation of the RNA
Along with the small ribosomal subunit, we have the coming together of eIF2
Another IF with GTP as a source of energy and a met-tRNA initiator form a complex
The initiator tRNA with eIF2 GTP attaches to the P site of the small ribosomal unit
This results in the formation of the 48S pre-initiation complex
This causes the initiation of scanning where the 48S complex scans to find the start codon- uses ATP
1 base translocated uses 1 ATP molecule
Initiation pauses and the 60S subunit joins to form the 80S ribosome
eIF 2 and 3 dissociate
GTP hydrolyses by eIF 5 this allows the formation of the 80S ribosome
met-tRNAi is associated with P site and a second charged tRNA enters the cleft and associates with the A site in addition to eF1
Consequently, peptidyl transferase catalyses the formation of the first peptide bond between amino acids
Recall the elongation stage of eukaryotic translation of protein synthesis
tRNA charging by aminoacyl tRNA synthesis occurs
eF 1 is involved in the energy-dependent tRNA binding to the ribosome hydrolysing GTP
The formation of the peptide bond by peptidyl transferase then occurs, attaching the elongating chain to the newly introduced amino acid
eF2 facilitates translocation of the ribosome along the message resulting in the tRNAs occupying the P and E sites
The A site is now free and this further consumes energy in the form of GTP
Release of the spent tRNA occurs from the E site allowing recharging of this tRNA requiring more energy from ATP
Recall the termination stage of eukaryotic translation of protein synthesis
Termination happens when the ribosome encounters a stop codon
The entry of the stop codon to the A site results not in the acquisition of a tRNA but of a protein called release factor 1 (erF1)
This triggers hydrolysis of GTP and the terminal peptidyl-tRNA bond, releasing the polypeptide chain
This also results in the dissociation of the ribosome, releasing the mRNA allowing the recycling of the factors and protein machinery
Why is the fact that bacterial and eukaryotic ribosomes are different relevant?
Some antibiotics block protein synthesis and specifically target in bacterial ribosomes
Give four examples of antibiotics which inhibit protein synthesis in bacteria
Streptomycin- small unit, inhibits initiation misreading of genetic code
Tetracyclines- small unit, inhibits aminoacyl tRNA binding to ribosome
Chloramphenicol-large, inhibits peptidyl transferase activity
Erythromycin-large, inhibits of translocation
Neomycin- multiple
What do the A, P and E sites stand for?
These are active sites
A is the amino acid entry site
P is the peptidyl-tRNA binding site
E is the exit site
