tRNA is a type of molecule used by cells to transport amino acids from the cytosol to ribosomes for use in protein synthesis. Cells re-use individual tRNA molecules rather than synthesizing new tRNA molecules each time one is used. A class of enzymes called aminoacyl-tRNA synthetases bind specific free cytosolic amino acids to specific tRNA molecules. This endergonic reaction is powered by ATP.
To begin, a specific synthetase's active site is shaped in such a way that an ATP molecule and a specific free amino acid can fit. The active site, in its initial conformation, does not have a region for a tRNA molecule to fit.
Which of the following must occur before a tRNA molecule can become involved in a reaction?