Reaction trajectory of pyrophosphoryl transfer catalyzed by 6-hydroxymethyl-7,8-dihydropterin pyrophosphokinase

6-hydroxymethyl-7,8-dihydropterin pyrophospholkinase (HPPK) catalyzes the Mg2+-dependent pyrophosphoryl transfer from ATP to 6-hydroxymethyl-7,8-dihydropterin (HP). The reaction follows a bi-bi mechanism with ATP as the first substrate and AMP and HIP pyrophosphate (HPPP) as the two products. HPPK is a key enzyme in the folate biosynthetic pathway and is essential for microorganisms but absent from mammals. For the HPPK-catalyzed pyrophosphoryl transfer, a reaction coordinate is constructed on the basis of the thermodynamic and transient kinetic data we reported previously, and the reaction trajectory is mapped out with five three-dimensional structures of the enzyme at various liganded states. The five structures are apo-HPPK (ligand-free enzyme), HPPK(.)MgATP(analog) (binary complex of HPPK with its first substrate) and HPPK-MgATP(analog) HP (ternary complex of HPPK with both substrates), which we reported previously, and HPPK-AMP-HPPP (ternary complex of HPPK with both product molecules) and HPPK-HPPP (binary complex of HPPK with one product), which we present in this study.