MolBio8 - 45 Flashcards
What is the total number of codons?
61
What is the start codon?
AUG - methionine
What are the three possible stop codons?
UAA, UAG, UGA
What is the ORF?
Open reading frame
What is the region of tRNA that binds with the codon called?
The anti-codon
What gives tRNA its structure?
Intramolecular base pairing
Name two modified tRNA nucleotides
Pseudouridine, dihydrouridine
How many possible nucleotide modifications are there?
> 50
What do modified nucleotides facilitate?
Interaction with other proteins, enzymes etc
Why is there not a 1:1 ratio of tRNA to codon?
Wobble pairing allow same anticodon to bind more than one codon
Name one instance of wobble pairing
Deamination of G to create inosine, which can apri U C or A
How is coupling of amino acid to tRNA achieved?
By aminoacyl-tRNA synthetases
How to synthetases work?
First primes amino acid by adding AMP to c-terminus, then uses adenylated amino acid to form aminoacyl-tRNA
Why is aminoactyl-tRNA known as charged?
Energy of the ATP hydrolysis is still contained within the ester linkage
What does translation of mRNA into precise amino acid sequence require?
Two adapters: the synthetase that binds a/a to tRNA, and tRNA that pairs correct a/a to the correct codon
Which terminus is the protein built on?
C-terminus
What performs protein synthesis?
50 ribosomal proteins and several ribosomal RNAs
What are the functions of the two rRNA subunits?
Large - catalyzes polymerisation; small - facilitates tRNA/mRNA interaction
What are the three sites of rRNA?
A-P-E
Outline protein construction by rRNA
tRNA enters A-site, peptidyl-transferase catalyses amino acid addition, tRNA moves to P-site, rRNA moves small subunit 3 nucleotides, tRNA leaves E-site
What are elongation factors?
Factors that help translation and improve accuracy
Describe the function of EF-1
Thought only to bind correctly to tRNA that have correct amino acid coupling, it adds a checkpoint to translation, as before peptidyl transferase works, it must hydrolyse GTP to GDP and dissociate
What is the purpose of the EF lag?
Allow time for incorrectly bound tRNAs to fall off. Some correct ones also fall off, but at a slower rate
What is a ribosome also known as, and why?
Ribozyme - RNA that catalyses a reaction
In which rRNA segment is the bulk of enzymatic activity?
Larger subunit
Outline translation initiation
mRNA with cap and tail bound by eIF-4G/E to form loop, which binds to small rRNA, methionine tRNA scans for AUG, EF hydrolyse and dissociate, so does eIF-2, and large rRNA binds, translation ensues
What does eIF stand for?
eukaryotic Initiation Factor
What is the Kozak sequence?
ACCAUGG - helps initiate translation
What is a polysome?
Multiple ribosomes binding one mRNA
How far apart are ribosomes on polysomes?
80 nucleotides or so
What are stop codons recognised by?
Release factors
Describe release factors
Look like charged tRNAs - molecular mimicry - and enter the A-site
When does protein folding commence?
Immediately after leaving ribosome
Describe initial protein folding
Puts hyrophobic side chains in the middle to achieve a lower energy state, creating a molten globule - roughly the correct folding
Describe the correct protein folding pathway
Multistep process and it is important that the steps occur in the correct order - incorrect folds may reduce the energy state, but blocks further folding
What rescue proteins by reversing incorrect folding steps?
Chaperones
What are the two major classes of protein chaperones?
hsp60 and hsp70
What does hsp stand for, and why?
Heat shock proteins, because their expression is elevated when the temperature is above normal so they can mop up temperature-denatured proteins
Where do hsp70s work?
Directly on proteins as they exit the ribosome, binding to exposed hydrophobic amino acids
What do hsp60s do?
Put misfolded proteins into isolation
How do hsp60s work?
Hyrdophobic entrance binds to the protein partially unfolding it, with a GroES cap that seals the protein inside for 15s to allow refolding
What happens to incorrectly folded, irretrievable proteins?
Polyubiquinated to mark these for destruction in the proteosome - almost 1 in 3 immediately!
How can incorrect folding cause disease?
If they aggregate - they become large and protease resistant, sometimes causing a chain reaction by causing well folded proteins to change (prions)
What diseases implicate aggregate placques?
CJD, Huntington’s and Alzheimer’s - amyloid placques made up of cross-beta filaments
What are prions?
Misfolded proteins that can convert normal proteins to the same cross-beta filament
