TC - Transfer RNAs Flashcards
D614G missense mutation in SARS-CoV-2 (2)
D614 → G614
- Improved infectivity
- Replicates more readily in nasal passages and trachea but not lungs
What are some features of tRNA? (4)
- They all have highly conserved tertiary structures (L-shape) with much internal base pairing
- The 3’-terminus of every tRNA ends in CCA, with each amino acid linked as an ester to the 3’-OH (or in some cases to the 2’-OH) of the last A
- tRNAs contain numerous unusual post-transcriptionally modified bases
- tRNAs possess an ANTICODON that recognises the mRNA CODON by anti-parallel base pairing
What structures are present in tRNA secondary and tertiary structures? (4)
1 = acceptor stem
2 = D loop
3 = anticodon loop
4 = TψC loop (thymidine, pseudouridine, cytidine)
What is the role of pseudouridine (ψ) in tRNAPhe?
**Pseudouridine enhances stability in the tertiary structure **
- Interacts with the ribosome due to increased possibilities for H-bonding
How is the aminoacyl-tRNA ester formed? (2)
The peptide bond -CO-NH- linking amino acids is an amide bond and its synthesis requires the input of energy
- The amino acid is first linked to the 3’-OH (or 2’-OH) of the terminal adenosine of a specific tRNA by an ester bond (amino acyl-tRNA)
- The hydrolysis of the ester bond helps drive peptide bond formation in the ribosome
What ensures specificity in aminoacyl-tRNA linkage?
To ensure an amino acid is only linked to its correct tRNA, highly specific enzymes are used.
- These vital enzymes are called amino acyl-tRNA synthetases
These enzymes ensure that a tRNA is linked to its correct amino acid so must show high specificity and fidelity
Describe the two-step activation process of amino acids
Amino acid + ATP + enzyme → enzyme-AMP-amino acid + PPi
‘ACTIVATED INTERMEDIATE’ **enzyme-AMP-amino acid + tRNA → Aminoacyl-tRNA + AMP + enzyme**
- Each time, the equivalent of two ATPs is used (the enzyme uses a second to drive the reaction)
- TWO-step reaction allows PROOFREADING
mistakes made in Reaction 1 are eliminated in Reaction 2
How is proofreading by amino acyl-tRNA synthetases (e.g. by tRSVal) conducted? (3)
1) Larger amino acid (Phe)
- Cannot enter acylation site
- Not attached to tRNA
2) Smaller amino acid (Ala)
- Enters acylation site
- Forms misacylated tRNA
- Enters hydrolytic active site and is hydrolysed
3) Correct amino acid (Val)
- Enters acylation site
- Forms properly acylated tRNA
- Cannot enter hydrolytic site (correct amino acid added)
Does the anticodon recognise the mRNA codon or the amino acid?
Recognises the mRNA codon
* If an amino acid is linked to the wrong tRNA, then the amino acyl-tRNA still recognises the anticodon and inserts the wrong amino acid
Proved experimentally during an in vitro experiment where cysteine is converted to alanine
What is the role of wobble in tRNA?
Refers to the ability of the first base of an anticodon to base pair with more than one codon
- Allows an aminoacyl-tRNA to dock with more than one codon, so that fewer than 61 tRNAs are needed to decode the 61 codons
Wobble generally involves the nucleotides U or inosine (I) at the first position of the anticodon (matches 3rd position of codon)
How does unusual base-pairing in RNA contribute to tRNA functionality?
U can also pair with G
Therefore:
- U in the first anticodon position can base pair with A or G in the 3rd position of the codon
- G in the first anticodon position can base pair with C or U
What can Inosine bind to and how does this influence wobble binding?
I can base pair with A, U or C using 2 H-bonds
If the first base of the anticodon is inosine (I) then it can base pair with A, U or C
- Therefore a single tRNA can recognise MORE than one codon
