Aminoacyl-tRNA (also aa-tRNA or charged tRNA) is tRNA to which its cognated amino acid is chemically bonded (charged). The aa-tRNA, along with some elongation factors, deliver the amino acid to the ribosome for incorporation into the polypeptide chain that is being produced. A specific cognate amino acid is charged or aminoacylated to each tRNA by aminoacyl tRNA synthetase. This matching is crucial, since it ensures that only the particular amino acid matching the anticodon of the tRNA, and in turn matching the codon of the mRNA, is used in protein synthesis.
Due to the degeneracy of the genetic code, multiple tRNAs will have the same amino acid but different codons. These different tRNAs are called isoacceptors. Under certain circumstances, non-cognate amino acids will be charged, resulting in mischarged or misaminoacylated tRNA. These mischarged tRNAs must be hydrolyzed in order to prevent incorrect protein synthesis.
Aminoacyl-tRNA is produced in two steps. First is the adenylation of the amino acid, which forms aminoacyl-AMP:
- amino acid + ATP ↔ aminoacyl-AMP + PPi
Second, the amino acid residue is transferred to the tRNA:
- aminoacyl-AMP + tRNA ↔ aminoacyl-tRNA + AMP
The net reaction is:
- amino acid + ATP + tRNA ↔ aminoacyl-tRNA + AMP + PPi
The net reaction is energetically favourable only because the pyrophosphate is hydrolysed; that reaction is highly energetically favourable and drives the other reactions. All of these reactions take place inside the aminoacyl-tRNA synthetase specific for that tRNA.
The different aa-tRNAs have varying pseudo-first-order rate constants for the hydrolysis of the ester bond between amino acid and tRNA. In general, proline aa-tRNA is found to be the least stable while the steric hindrance of the methyl group on the β carbon of isoleucine and valine tend to stabilize their aa-tRNAs. Increased ionic strength resulting from sodium, potassium, and magnesium salts has been shown to destabilize the aa-tRNA bond. Increased pH also destabilizes the bond and changes the ionization of the α carbon amino group of the amino acid. The charged amino group can destabilize the aa-tRNA bond via the inductive effect. The elongation factor EF-Tu has been shown to stabilize the bond.
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