tRNA charging
Summary
tRNA charging is the ATP-driven attachment of a specific amino acid to its matching tRNA by an aminoacyl-tRNA synthetase, forming aminoacyl-tRNA. Most testable: the synthetase, not the ribosome, sets which amino acid is used, so a mischarged tRNA inserts the wrong residue.
Detail
Meaning: Charging is the activation of tRNA. An aminoacyl-tRNA synthetase uses ATP to make an aminoacyl-AMP intermediate, releasing pyrophosphate (PPi), then transfers its specific amino acid to the 3′ end (the CCA sequence) of the matching tRNA as an ester bond. Hydrolysis of PPi makes the reaction effectively irreversible. The high-energy bond later drives peptide bond formation. Synthetases recognize both the amino acid and the tRNA, and many proofread their product.
Equation: amino acid + tRNA + ATP → aminoacyl-tRNA + AMP + PPi. AMP is adenosine monophosphate and PPi is pyrophosphate. Energy changes are in J/mol. The net cost is two high-energy phosphate bonds once PPi is hydrolyzed.
On the MCAT: A passage describes a mutant synthetase, or an amino acid chemically changed after attachment to tRNA, then asks which residue enters the protein. The ribosome checks codon-anticodon pairing, not the attached amino acid, so the residue follows the tRNA's anticodon. Energy-accounting questions may also expect you to count the ATP spent on charging.
Common confusion: Students credit the ribosome with checking the amino acid, or say charging uses GTP. Charging uses ATP; GTP powers initiation, elongation and translocation at the ribosome. The choice of amino acid is fixed by the synthetase.
Sources
- Lehninger Principles of Biochemistry
- Biochemistry (Berg, Tymoczko, Stryer)
- Molecular Biology of the Cell
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.