A crystallographic glimpse of a nucleotide triphosphate (AMPPNP) bound to a protein surface: external and internal AMPPNP molecules in crystallineN-acetyl-<scp>L</scp>-glutamate kinase

A large volume of unexplained electron density in a crystal of N-acetyl-L-glutamate kinase (NAGK) is now interpreted as an external, very extended, metal-free AMPPNP molecule that occupies two alternative positions and that makes contacts with the protein exclusively through its gamma-imidophosphate. This external nucleotide is compared with the active-site nucleotide and the reasons for its extended shape, lack of complexed metal and peripheral binding are analyzed. Further, the possibility that this bystander AMPPNP is waiting to occupy the active centre is discussed.