aminoacylation

Aminoacylation is the enzymatic process by which an amino acid is attached to its corresponding transfer RNA (tRNA), forming aminoacyl-tRNA, the substrate used by ribosomes during protein synthesis. In most cells, this reaction is carried out by aminoacyl-tRNA synthetases (aaRS). The process proceeds in two steps: activation of the amino acid with ATP to form an aminoacyl-AMP intermediate, followed by transfer of the activated amino acid to the 3' hydroxyl group of the tRNA’s terminal adenosine, yielding aminoacyl-tRNA and producing AMP and inorganic pyrophosphate (PPi).

Specificity is ensured by a matching between each aaRS and its cognate tRNA. The enzyme recognizes identity elements in the tRNA beyond the anticodon, ensuring correct amino acid charging. Many aaRS enzymes possess editing activities to correct mischarged tRNAs; when a wrong amino acid is attached, the editing site hydrolyzes the misacylated tRNA, preventing misincorporation during translation.

Localization and variations exist among organisms. In most cells, charging occurs in the cytosol; mitochondria and plastids typically encode their own aaRS to charge their resident tRNAs. Some unusual cases involve nonstandard amino acids: selenocysteine is formed on a tRNA charged first with serine and then converted, while pyrrolysine is directly charged by a dedicated pyrrolysyl-tRNA synthetase in certain organisms.

Importance lies in maintaining genetic code fidelity. Accurate aminoacylation is essential for correct protein synthesis, and errors in charging can lead to translational mistakes and protein misfolding. The process is highly conserved and central to the efficiency and accuracy of translation.