In GFOR, the NADP is found associated with a classical dinucleotide-binding domain in a conventional fashion. The NADP is effectively buried in the protein-subunit interior as a result of interactions with the N-terminal arm from an adjacent subunit in the tetramer, and with a short helix from the C-terminal domain of the protein. This accounts for NADP's inability to dissociate. The N-terminal arm may also contribute to stabilization of the tetramer. The enzyme has an unexpected structural similarity with the cytoplasmic enzyme glucose-6-phosphate dehydrogenase (G6PD). We hypothesize that both enzymes have diverged from a common ancestor. The mechanism of catalysis is still unclear, but we have identified a conserved structural motif (Glu-Lys-Pro) in the active site of GFOR and G6PD that may be important for catalysis.