<P> 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: </P> <P> 1 . aa + ATP--> aa - AMP + PP, (pyrophosphate) 2 . aa - AMP + tRNA--> aa - tRNA + AMP </P> <P> 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 (roll over in illustration). 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 . </P> <P> 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 . </P>

Where does the amino acid attach to trna