The charging of a tRNA with an amino acid can be represented by the following equation:

amino acid + tRNA + ATP → aminoacyl-tRNA + AMP + PPi 

where PPi is pyrophosphate. In the aminoacyl-tRNA, the amino acid and tRNA are linked with a high-energy covalent bond; a large portion of the energy derived from the hydrolysis of ATP is thus stored in this bond and is available to drive peptide bond formation during the later stages of protein synthesis. The free-energy change of the charging reaction shown in the equation is close to zero and therefore would not be expected to favor attachment of the amino acid to tRNA. Can you suggest a further step that could drive the reaction to completion?