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Trajectory of HPPK-catalyzed pyrophosphoryl transfer. Five distinct states are proposed along the reaction coordinate, including apo-HPPK, HPPK:MgATP, HPPKoMgATPoHP, HPPKoMgAMPoHPPP, and HPPKoHPPP. For each catalytic state, a snapshot is provided with a crystal structure, including apo-HPPK (1.50 Å, PDB: 1HKA), HPPKoMgADP (1.50 Å, PDB: 1EQM), HPPKoMgAMPCPPoHP (1.25 Å, PDB: 1Q0N), HPPKoAMPoHPPP (1.56 Å, PDB: 1RAO), and HPPKoHPPP (1.35 Å, PDB: 1RB0). Helices are illustrated as cyan spirals, strands as orange arrows and loops as gray pipes with loop 3 highlighted in red. The side chain of W89 is shown as a ball-and-stick model and the ligands as van der Waals spheres (Mg ion in black, AMPCPP and AMP in yellow, and HP and HPPP in green). The relative energies of the various states should not be inferred from the diagram (Jaroslaw Blaszczyk et al. Structure 12:467-475, 2004. PubMed: 15016362).