A crystallographic glimpse of a nucleotide triphosphate (AMPPNP) bound to a protein surface: external and internal AMPPNP molecules in crystalline N-acetyl-L-glutamate kinase

A large volume of electron density in the crystal of Escherichia coli N-acetyl-L-glutamate kinase is interpreted as an external AMPPNP molecule that makes contacts with the enzyme through its γ-imidophosphate. This external nucleotide differs in a number of respects from the internal AMPPNP bound to the active site, and the possibility that the external AMPPNP is waiting to occupy the active center is discussed.