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The Reaction trajectory of HPPK-catalyzed pyrophosphoryl transfer (Blaszczyk, Li, Yan, and Ji. Structure 2004; 12:467-475. Abstract). Five distinct states are proposed along the reaction coordinate, including apo-HPPK, HPPK•MgATP, HPPK•MgATP•HP, HPPK•MgAMP•HPPP, and HPPK•HPPP. For each catalytic state, a snapshot is provided with a crystal structure, including apo-HPPK (1hka), HPPK•MgADP (1eqm), HPPK•MgAMPCPP•HP (1q0n), HPPK•AMP•HPPP (1rao), and HPPK•HPPP (1rb0). Helices are illustrated as cyan spirals, strands as orange arrows and loops as gray pipes except that loop 3 is highlighted in red. The side chain of W89 is shown as a ball-and-stick model and the ligands as van der Waals spheres (Mg2+ 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.