Aminoacyl trna
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Aminoacyl-tRNA
What is Aminoacyl-tRNA?
An aminoacyl-tRNA (also called "charged tRNA") is a tRNA molecule that has an amino acid covalently attached to its 3'-end. It serves as the adapter molecule in translation - carrying the correct amino acid to the ribosome, guided by its anticodon matching the mRNA codon.
Naming convention: the tRNA is named for both its amino acid and the tRNA it carries. For example, tRNA^Ala carrying alanine becomes alanyl-tRNA^Ala.
Formation of Aminoacyl-tRNA ("Charging")
The process is catalyzed by aminoacyl-tRNA synthetases and occurs in two steps, requiring ATP:
Step 1 - Amino Acid Activation
The amino acid's carboxyl group reacts with ATP, forming a high-energy aminoacyl-AMP-enzyme complex (activated amino acid), releasing pyrophosphate (PPi):
Amino acid + ATP + Enzyme → Enzyme•AMP~Amino acid + PPi
The released PPi is immediately cleaved by pyrophosphatase (PPase) → 2 Pi, which drives the reaction forward thermodynamically.
Step 2 - Transfer to tRNA
The activated amino acid is transferred from the enzyme-AMP complex to the 3'-OH of the terminal adenosine (in the -CCA sequence) of the cognate tRNA, releasing AMP and the free enzyme:
Enzyme•AMP~Amino acid + tRNA → Aminoacyl-tRNA + AMP + Enzyme
The amino acid is linked via a high-energy ester bond to the 3'-OH of the CCA terminus.
Formation of aminoacyl-tRNA (Harper's Illustrated Biochemistry, 32nd Ed.)
Overall charging reaction showing CCA terminus of tRNA (Lippincott's Biochemistry, 8th Ed.)
Aminoacyl-tRNA Synthetases
| Feature | Detail |
|---|---|
| Number | 20 (one per amino acid) |
| Substrate specificity | Each recognizes one specific amino acid AND all its cognate tRNAs ("isoaccepting tRNAs," up to 5 per amino acid) |
| Energy requirement | 1 ATP consumed (→ AMP + PPi, equivalent to 2 high-energy bonds) |
| Error rate | < 1 in 10^4 to 10^6 mischarging events |
| Additional activity | Proofreading (editing) - can remove an incorrectly attached amino acid from the enzyme or the tRNA |
Recognition of tRNA
Synthetases recognize their cognate tRNA through specific nucleotide sequences called recognition sites. The location varies by enzyme:
- Some use the anticodon loop as the primary recognition site
- Others recognize bases elsewhere in the tRNA (e.g., the D arm, acceptor stem)
Regardless of how the synthetase recognizes the tRNA, the anticodon is what determines which mRNA codon the amino acid is incorporated at.
The D arm (dihydrouridine arm) is a key site for recognition by aminoacyl-tRNA synthetases. The TψC arm is involved in binding the aminoacyl-tRNA to the ribosomal surface during protein synthesis. - Harper's Illustrated Biochemistry, 32nd Ed.
Key tRNA Structural Regions Relevant to Charging
| tRNA Region | Role |
|---|---|
| Acceptor arm (3'-CCA-OH) | Site of amino acid attachment (ester bond to 3'-OH of terminal adenosine) |
| Anticodon loop | Codon recognition on mRNA; sometimes a synthetase recognition site |
| D arm (dihydrouridine arm) | Recognition site for aminoacyl-tRNA synthetase |
| TψC arm | Binding to ribosome surface during translation |
The Initiator tRNA - A Special Case
The initiator methionyl-tRNA (written Met-tRNA^Met_i) is unique:
- It recognizes only the AUG start codon that initiates translation
- It does NOT decode internal AUG codons (which specify methionine insertion within a chain - a different tRNA does that)
- In eukaryotes, the methionine on the initiator tRNA is not formylated (unlike in prokaryotes, where it is formyl-methionine)
Role in Translation (Elongation - A Site Binding)
During elongation, the charged aminoacyl-tRNA enters the A site of the ribosome:
- An elongation factor (EF-Tu in prokaryotes; EF1A in eukaryotes) forms a ternary complex with GTP and the incoming aminoacyl-tRNA
- The ternary complex brings the aminoacyl-tRNA to the A site
- Correct codon-anticodon base pairing triggers GTP hydrolysis, which releases EF-Tu/EF1A-GDP
- The aminoacyl-tRNA is then fully accommodated in the A site
- Peptide bond formation follows: the α-amino group of the A-site aminoacyl-tRNA attacks the ester bond of the peptidyl-tRNA at the P site (nucleophilic attack), catalyzed by peptidyl transferase (a ribozyme activity of the large subunit rRNA)
Clinical Relevance - Anti-Aminoacyl-tRNA Synthetase Syndrome
Autoantibodies against aminoacyl-tRNA synthetases are associated with the antisynthetase syndrome, characterized by:
- Inflammatory myopathy (myositis)
- Nonerosive arthritis
- Interstitial lung disease (ILD)
- Raynaud's phenomenon
- "Mechanic's hands"
- Fever
The most common is anti-Jo-1 (anti-histidyl-tRNA synthetase). - Harrison's Principles of Internal Medicine, 22nd Ed.
Summary Box
| Feature | Detail |
|---|---|
| Function | Carries amino acid to ribosome for translation |
| Attachment site | 3'-OH of terminal adenosine (-CCA) |
| Bond type | Ester bond (high-energy) |
| Enzyme | Aminoacyl-tRNA synthetase (20 types) |
| Energy cost | 1 ATP → AMP + PPi (= 2 phosphoanhydride bonds) |
| Fidelity mechanism | Synthetase proofreading/editing activity |
| A site delivery | Via EF-Tu (prokaryotes) or EF1A (eukaryotes) + GTP |
Sources: Lippincott Illustrated Reviews: Biochemistry 8th Ed. | Basic Medical Biochemistry 6th Ed. | Harper's Illustrated Biochemistry 32nd Ed.
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