Amino acid activation
- Aminoacyl transferase binds Adenosine triphosphate (ATP) to amino acid, PP is released.
- Aminoacyl TRNA synthetase binds AMP-amino acid to tRNA. The AMP is used in this step.
During amino acid activation the amino acids (aa) are attached to their corresponding tRNA. The coupling reactions are catalysed by a group of enzymes called aminoacyl-tRNA synthetases (named after the reaction product aminoacyl-tRNA or aa-tRNA). The coupling reaction proceeds in two steps:
- aa + ATP ⟶ aa-AMP + PP, (pyrophosphate)
- aa-AMP + tRNA ⟶ aa-tRNA + AMP
The amino acid is coupled to the penultimate nucleotide at the 3’-end of the tRNA (the A in the sequence CCA) via an ester bond. The formation of the ester bond conserves a considerable part of the energy from the activation reaction. This stored energy provides the majority of the energy needed for peptide bond formation during translation.
Each of the 20 amino acids are recognized by its specific aminoacyl-tRNA synthetase. The synthetases are usually composed of one to four protein subunits. The enzymes vary considerably in structure although they all perform the same type of reaction by binding ATP, one specific amino acid and its corresponding tRNA.
The specificity of the amino acid activation is as critical for the translational accuracy as the correct matching of the codon with the anticodon. The reason is that the ribosome only sees the anticodon of the tRNA during translation. Thus, the ribosome will not be able to discriminate between tRNAs with the same anticodon but linked to different amino acids.
Editing mechanisms occur when there is a misactivation of amino acids, where a amino acid is attached to the wrong tRNA molecule. The aminoacyl-tRNA synthetases can hydrolyze the amino acid before it attaches to the wrong tRNA molecule (pre-transfer editing) or deacylate the mischarged tRNA after attachment (post-transfer editing).
The error frequency of the amino acid activation reaction is approximately 1 in 10,000 despite the small structural differences between some of the amino acids.
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