AminoacyltRNAs

Aminoacyl-tRNAs are tRNA molecules that have been charged with a specific amino acid by aminoacyl-tRNA synthetases (aaRS). They serve as the direct substrates that deliver amino acids to the ribosome during protein synthesis, translating the genetic code into polypeptide chains.

Charging a tRNA involves two steps. First, an amino acid is activated by adenosine triphosphate (ATP) to form aminoacyl-adenylate (aminoacyl-AMP). In the second step, the activated amino acid is transferred to the 3' end of the tRNA, forming an aminoacyl-tRNA with an ester bond between the amino acid and the terminal adenosine's 3' hydroxyl group. Each aminoacyl-tRNA synthetase is typically specific for one amino acid and its corresponding tRNAs, ensuring that the correct amino acid is attached to the appropriate tRNA.

Accuracy is maintained by proofreading activities within aaRS, which can correct misactivated amino acids (pre-transfer editing) or mischarged tRNAs (post-transfer editing). Misacylation can otherwise lead to incorporation of the wrong amino acid into a growing polypeptide, so editing mechanisms are essential for translational fidelity. In some cases, editing may involve separate trans-editing enzymes rather than the synthetase itself.

Once charged, aminoacyl-tRNAs are delivered to the ribosome by elongation factors (EF-Tu in bacteria, EF1A in eukaryotes) in a GTP-dependent process. The ribosome then assesses codon–anticodon pairing, catalyzing the formation of peptide bonds and continuing elongation.

There are about twenty standard aminoacyl-tRNA synthetases, one for each amino acid, with variations across organisms and organelles. Aminoacyl-tRNAs are fundamental to translating the genetic code with high accuracy.