In the original publication of the 1QF6 structure (Sankaranarayanan et al., 1999), the authors were able to clarify the secondary and tertiary structure of the bacterial threonyl-tRNA synthetase, and to show that it folds into multiple domains (i.e. independently-folding regions).   The image below from the article shows the 'domain structure' of the enzyme, while also emphasizing its secondary structure (with cylinders representing alpha-helices and thin ribbons representing beta-strands):     (You may notice that the image resolution is somewhat low. This sometimes happens with older publications such as this one, which were not originally released in digital format and were scanned later.)   To create successful conditions for generating the 3D 1QF6 structure, the researchers note that they provided the enzyme with the appropriate tRNA as well as ATP, and that the final crystal structure contained AMP (but not ATP).    Thinking back to the information provided about this enzyme, which one of the following best explains why the nucleotide in the 1QF6 structure has two fewer phosphates than the nucleotide that was originally provided to the enzyme?