Translation and post-translational modification Flashcards
Describe the structure of typical mRNA.
mRNA has a 7-methylguanylate cap at the 5’ end followed by a 5’ untranslated region (UTR) You then get the coding region in the middle At the end you get a 3’ UTR and the poly-A tail
What enzyme is involved in the transfer of an amino acid to tRNA?
Aminoacyl tRNA synthetase
What is the Methionine codon?
AUG
What is the significance of Methionine?
It is the first amino acid in virtually all polypeptides
What are the stop codons?
UAA, UGA, UAG
How do tRNAs work?
- there is one tRNA per amino acid
- the amino acid gets stuck onto the correct correct tRNA using aminoacyl tRNA synthetases
- There is one aminoacyl tRNA synthetase per amino acid
- They are important in the fidelity of translation
How does Aminoacyl tRNA synthetase work?
Aminoacyl tRNA synthetase gets activated (by ATP hydrolysis) to form adenylated amino acid which binds with a molecule of AMP and binds to the relevant amino acid This adenylated amino acid complex then binds to the tRNA and transfers the amino acid to the 3’ end of the tRNA The adenylated amino acid and AMP dissociate
What are the three stages of translation?
Initiation – Elongation - Termination
What components are make up the preinitiation complex?
40S ribosomal subunit - Methionine tRNA - eIF2 (initiation factor) - GTP
GTP powers the reaction
Describe initiation.
The preinitiation complex recognises initiation factors (eIF4E and G) on the 7MeG cap and bind to the mRNA It moves along the mRNA in a 5’ to 3’ direction until it reaches the first in-frame AUG (methionine) Here the GTP is hydrolysed providing energy to ensure correct base pair matching The GDP and eIF2 then dissociate causing a conformational change, which allows the binding of the 60S ribosomal subunit to form a full ribosome
Describe elongation.
The next charged tRNA comes and binds to the A site of ribosome Peptidyl transferase catalyses the formation of a peptide bond between the two amino acids The tRNA from the P site then dissociates and the ribosome moves along
What proteins facilitate elongation?
Elongation factors
Describe termination.
When the A site of the ribosome moves over the first in-frame stop codon, a release factor (a type of protein) binds instead of a tRNA The release factor transfers the growing polypeptide chain to water thus terminating the polypeptide and allowing it to detach from the ribosome
How do lots of antibiotics work?
lots of antibiotics work by inhibiting protein synthesis
they exploit the differences between eukaryotes and prokaryotes
where does protein synthesis actually take place?
it takes place in the cytoplasm as from here it is passed through membranes to enter the compartments
the only exception is some protein synthesis happens in the mitochondria
how do you get proteins into the compartments?
- the secretary and transmembrane proteins are synthesised in the rough endoplasmic reticulum
- Proteins that are destined to be secretory or transmembrane have a special sequence (first 20-24 amino acids) called a SIGNAL SEQUENCE.
- The signal sequence is rich in HYDROPHOBIC AMINO ACIDS
Where is the signal sequence found and what does it consist of?
It is found at the N terminus It mainly consists of hydrophobic amino acids
Describe how proteins enter the ER.
The signal sequence binds to a signal recognition particle (SRP) and this binding halts translation
The SRP then binds to an SRP receptor on the ER membrane and translation resumes
The binding of SRP to the SRP receptor triggers the assembly of a protein channel through which the polypeptide is threaded into the RER
What extra feature do proteins that are destined to be transmembrane have?
They have an extra hydrophobic sequence to hold them in the membrane
What happens once the protein has entered the compartment?
The signal sequence is cleaved by signal peptidase
- we end up with the main protein which is folded
Describe, in full, the post translational modification involved in the production of insulin.
You begin with preproinsulin, which has an N-terminus signal sequence which allows it to move across the membrane into the ER lumen
Once it enters the ER the signal sequence is cleaved and it 3 disulphide bonds form between cysteine residues. The protein folds… this makes it proinsulin.
before it is packaged into the secretory vesicles, it is proteolytically cleaved into two positions to release C chains
this leaves us with fully functional insulin which consists of the A and B chains held together by disulfide bonds
why modify proteins?
give examples of modifications
- modifying proteins increases the diversity of the proteins
examples include
- Disulphide bond formation
- Proteolytic cleavage
- Glycosylation (addition of carbohydrate)
- Phosphorylation (addition of phosphate)
- Prenylation, Acylation (addition of lipid groups
- Hydroxylation
