Protein Synthesis Flashcards
1
Q
Why are proteins important?
A
Crucial for cell growth, proliferation and survival
2
Q
How is protein synthesis regulated?
A
- Expensive process for the cell so is tightly regulated
- regulation can either control overall rates of synthesis throughout the cell or modulate specific reactions
3
Q
What inhibits protein synthesis?
A
- cell stresses
- withdrawal of nutrients
- serum deprivation
- temperature shock
- DNA damage
- viral infection
- hypoxia
4
Q
What does a prokaryotic mRNA contain?
A
- polycistronic = more than one coding region
- messages given are fairly unstable
- absence of nuclear membrane leads to transcription/translation occurring on the same transcript
5
Q
What does a eukaryotic mRNA contain?
A
- usually monocistronic (one coding region)
- capped and polyadenylated so quite stable
- 5’ to 3’ untranslated regions (UTR)
6
Q
What is the CAP structure?
A
- found on eukaryotic mRNAs
- at the 5’ end
- seals end of mRNA protecting it from nuclease digestion
- landing pad for eIF4E (cap binding protein)
7
Q
What is polyadenylation?
A
- poly A tail found at the 3’ end
- protects mRNA from enzymatic degradation
- aids in transcription termination and exportation of mRNA from the nucleus into cytoplasm
8
Q
What are the stages in 3’ polyadenylation of newly synthesised mRNAs?
A
- recognition of AUAAA sequence by specificity components which is cleaved by cleavage factors
- undergoes initial poly(A) polymerisation by poly(A) polymerase
- binds to poly(A) binding protein (PABP)
9
Q
What are general characteristics of tRNA molecules?
A
- single RNA strand of approx 80 nucleotides
- helps decode mRNA sequence into a protein
- functions at a specific site in the ribosome during translation
- aminoacyl tRNA synthase links amino acid to 3’ end of acceptor arm to produce an aminoacyl-tRNA
10
Q
What is the 80S ribosome?
A
- consists of large (60S) and small (40S) subunits
- each subunit contains approx 50% proteins and 50% rRNAs (by mass)
- large and small subunits bind together during initiation
- translation takes place in the cavity between the two subunits
- peptidyl transferase activity is associated with the large 60S subunit
- has three binding sites = E, P, A
11
Q
What are some facts about protein synthesis?
A
- translation must go fast enough to supply protein but slow enough to avoid too many errors
- error rate is 1 in 10^4 incorrect amino acid
- ribosomes add 20 amino acids per second to a polypeptide chain eg the synthesis of actin 375 AAs takes 20 seconds
- it is energetically expensive
12
Q
What are the 3 stages of CAP-dependant protein synthesis?
A
- Initiation:
- small ribosomal unit and initiator tRNA bind
- CAP is recognised and the start AUG codon is scanned
- ternary complex binds large ribosomal unit - Elongation:
- tRNA brings amino acids to the ribosome in the order specified by codons on mRNA
- ribosome catalyses peptide bond formation between the amino group of each amino acid
- more than one ribosome is active on any one mRNA (polysome) - Termination:
- at 3’ end of coding sequence, the ribosome encounters a stop codon
- polypeptide chain is released
13
Q
What is the function of the initiation step of protein synthesis?
A
- initiation of translation is the rate limiting step and is tightly regulated
- more than ten soluble eukaryotic initiation factors (eIFs)
- aim is to bring mRNA to ribosome and initiator tRNA to AUG start codon
- the process is:
1. Assemble of the 43S pre-initiation complex
2. Binding of mRNA to the 43S complex
3. Assembly of the 80S initiation complex
14
Q
What happens during the first step of initiation?
A
- assembly of the 43S pre-initiation complex
- association met-RnA with eIF2 to form ternary complex
- 40S is trapped as a monomer to form the 43S pre-initiation complex
15
Q
What is the structure of eIF2?
A
- polypeptide chain initiation factor eIF2
- made up of three subunits:
1. gamma subunit = GTP binding site
2. alpha subunit = phosphorylation site
3. beta subunit = K boxes (involved in interaction of eIF2B and eIF5)
