In enzymology, a heptaprenyl diphosphate synthase () is an enzyme that catalyzes the chemical reaction (2E,6E)-farnesyl diphosphate + 4 isopentenyl diphosphate 4 diphosphate + all-trans-heptaprenyl diphosphate Thus, the two substrates of this enzyme are (2E,6E)-farnesyl diphosphate and isopentenyl diphosphate, whereas its two products are diphosphate and all-trans-heptaprenyl diphosphate. This enzyme belongs to the family of transferases, specifically those transferring aryl or alkyl groups other than methyl groups. The systematic name of this enzyme class is (2E,6E)-farnesyl-diphosphate:isopentenyl-diphosphate farnesyltranstransferase (adding 4 isopentenyl units). Other names in common use include all-trans-heptaprenyl-diphosphate synthase, heptaprenyl pyrophosphate synthase, and heptaprenyl pyrophosphate synthetase. This enzyme participates in biosynthesis of steroids. Structural studies As of late 2007, 11 structures have been solved for this class of enzymes, with PDB accession codes , , , , , , , , , , and . References EC 2.5.1 Enzymes of known structure