4 hydroxy 3 methylbut 2 en 1 yl diphosphate synthase

In enzymology, a 4-hydroxy-3-methylbut-2-en-1-yl diphosphate synthase (HMB-PP synthase, IspG, EC 1.17.7.1) is an enzyme that catalyzes the chemical reaction

2-C-methyl-D-erythritol 2,4-cyclodiphosphate + protein-dithiol ⇌ (E)-4-hydroxy-3-methylbut-2-en-1-yl diphosphate + H₂O + protein-disulfide

The substrate of this enzyme is 2-C-methyl-D-erythritol 2,4-cyclodiphosphate (MEcPP) and the product is (E)-4-hydroxy-3-methylbut-2-en-1-yl diphosphate (HMB-PP). Electrons are donated by two reduced ferredoxin proteins per reaction.

This enzyme belongs to the family of oxidoreductases, specifically those acting on CH or CH₂ groups with a disulfide as acceptor. The systematic name of this enzyme class is (E)-4-hydroxy-3-methylbut-2-en-1-yl-diphosphate:protein-disulfide oxidoreductase (hydrating). This enzyme participates in the MEP pathway (non-mevalonate pathway) of Isoprenoid precursor biosynthesis.