In enzymology, a 2-amino-4-hydroxy-6-hydroxymethyldihydropteridine diphosphokinase (EC 2.7.6.3) is an enzyme that catalyzes the chemical reaction

ATP + 2-amino-4-hydroxy-6-hydroxymethyl-7,8-dihydropteridine ${\displaystyle \rightleftharpoons }$ AMP + (2-amino-4-hydroxy-7,8-dihydropteridin-6-yl)methyl diphosphate

Thus, the two substrates of this enzyme are ATP and 2-amino-4-hydroxy-6-hydroxymethyl-7,8-dihydropteridine, whereas its two products are AMP and (2-amino-4-hydroxy-7,8-dihydropteridin-6-yl)methyl diphosphate.

This enzyme belongs to the family of transferases, specifically those transferring two phosphorus-containing groups (diphosphotransferases). The systematic name of this enzyme class is ATP:2-amino-4-hydroxy-6-hydroxymethyl-7,8-dihydropteridine 6'-diphosphotransferase. Other names in common use include 2-amino-4-hydroxy-6-hydroxymethyldihydropteridine pyrophosphokinase, H2-pteridine-CH2OH pyrophosphokinase, 7,8-dihydroxymethylpterin-pyrophosphokinase, HPPK, 7,8-dihydro-6-hydroxymethylpterin pyrophosphokinase, and hydroxymethyldihydropteridine pyrophosphokinase. This enzyme participates in folate biosynthesis.

This enzyme catalyses the first step in a three-step pathway leading to 7,8 dihydrofolate. Bacterial HPPK (gene folK or sulD) is a protein of 160 to 270 amino acids.^[1] In the lower eukaryote Pneumocystis carinii, HPPK is the central domain of a multifunctional folate synthesis enzyme (gene fas).^[2]

As of late 2007, 23 structures have been solved for this class of enzymes, with PDB accession codes 1DY3, 1EQ0, 1EQM, 1EX8, 1F9H, 1F9Y, 1G4C, 1HKA, 1HQ2, 1IM6, 1KBR, 1Q0N, 1RAO, 1RB0, 1RTZ, 1RU1, 1RU2, 1TMJ, 1TMM, 2BMB, 2CG8, 2F63, and 2F65.