A Class I aminoacyl–tRNA synthetase (aaRS) aminoacylates the 2'-OH group of its cognate tRNA's 5'-terminal A residue, whereas a Class II aaRS aminoacylates the corresponding 3'-OH group. By zooming into the active site region of each tRNA (the region around the ATP's α-phosphorous atom) and comparing the relative dispositions of O2' and O3' on the tRNA's 5'-terminal A residue with respect to the ATP's α-phosphorus atom, determine the class of GlnRS and AspRS.
GlnRS is a Class I aaRS and AspRS is a Class II aaRS.