The structure and mechanism of 6-hydroxymethyl-7,8-dihydropterin pyrophosphokinase

6-Hydroxymethyl-7,8-dihydropterin pyrophosphokinase (HPPK) catalyses the transfer of pyrophosphate from ATP to 6-hydroxymethyl-7,8-dihydropterin (HMDP), and is an essential enzyme in the biosynthesis of folic acid. It is also a potential target for antimicrobial drugs. HPPK from Escherichia coli, which has been the most intensively investigated, is a monomeric protein with a molecular mass of about 18,000. Structures of the enzyme, determined by X-ray crystallography and NMR, have shown that it adopts an alpha/beta fold with a substrate-binding cleft on the surface. Three loop regions surround the enzyme active site and form intimate contacts with the substrates. The enzyme has a fixed order of substrate binding, with ATP binding first, followed by HMDP. Binding of ATP causes a shift in the conformations of the loop regions, which completes formation of the HMDP-binding site. Two magnesium ions bind within the active site, bridging between the phosphate groups in ATP and the enzyme. Both ions appear to play an integral role in ATP recognition and stabilization of the transition state of the reaction. Ligand binding and kinetic studies have shown that the overall rate of the reaction is not limited by the rate of substrate transformation into products on the enzyme, which is relatively fast, but is more likely caused by a slow step associated with product release. These fundamental studies open up the potential for exploitation through the design of specific HPPK inhibitors.