tRNALadungen

tRNALadungen, or charged tRNA, are tRNA molecules that carry a covalently attached amino acid at their 3' terminus. They are the substrates used by ribosomes to synthesize proteins. Charged tRNAs are produced by a family of enzymes called aminoacyl-tRNA synthetases, one for each amino acid. The charging reaction proceeds in two steps: activation of the amino acid with ATP to form aminoacyl-AMP, followed by transfer of the amino acid from aminoacyl-AMP to the 3' end of the tRNA, yielding aminoacyl-tRNA and releasing AMP and pyrophosphate. The process requires magnesium ions and is energetically costly.

Fidelity is essential. Many synthetases possess an editing site that hydrolyzes mischarged tRNAs, and some organisms rely on separate trans-editing enzymes to correct errors. Correct charging depends on tRNA identity elements and, in many cases, on posttranscriptional tRNA modifications that influence recognition by the synthetase.

In the cell, the pool of charged tRNA reflects amino acid availability and translation demand. During elongation, charged tRNAs are delivered to the ribosome by elongation factors in a GTP-dependent step, enabling codon–anticodon pairing and incorporation of the appropriate amino acid into the growing polypeptide.

Organisms maintain cytosolic and organellar (mitochondrial, chloroplast) sets of aminoacyl-tRNA synthetases, with organellar enzymes often specialized for local genetic code and tRNA pools. Defects in charging can stall translation and trigger cellular stress responses.