4-hydroxy-3-methylbut-2-en-1-yl diphosphate synthase

4-hydroxy-3-methylbut-2-en-1-yl diphosphate synthase (HMB-PP synthase, IspG, ) is an enzyme that catalyzes a redox reaction

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. The enzyme characterised from Thermosynechococcus elongatus contains an iron-sulfur cluster.

The reaction is part of the non-mevalonate pathway (MEP pathway) of isoprenoid precursor biosynthesis.

Nomenclature This enzyme is an oxidoreductase, specifically one 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).

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