N-Acetyl-L-glutamate kinase (NAGK), a member of the amino acid kinase family, catalyzes the second and frequently controlling step of arginine synthesis. The Escherichia coli NAGK crystal structure to 1.5 A resolution reveals a 258-residue subunit homodimer nucleated by a central 16-stranded molecular open beta sheet sandwiched between alpha helices. In each subunit, AMPPNP, as an alphabetagamma-phosphate-Mg2+ complex, binds along the sheet C edge, and N-acetyl-L-glutamate binds near the dyadic axis with its gamma-COO- aligned at short distance from the gamma-phosphoryl, indicating associative phosphoryl transfer assisted by: (1) Mg2+ complexation; (2) the positive charges on Lys8, Lys217, and on two helix dipoles; and (3) by hydrogen bonding with the y-phosphate. The structural resemblance with carbamate kinase and the alignment of the sequences suggest that NAGK is a structural and functional prototype for the amino acid kinase family, which differs from other acylphosphate-making devices represented by phosphoglycerate kinase, acetate kinase, and biotin carboxylase.