Geranylfarnesyl diphosphate synthase (, FGPP synthase, (all-E) geranylfarnesyl diphosphate synthase, GFPS, Fgs) is an enzyme with systematic name geranylgeranyl-diphosphate:isopentenyl-diphosphate transtransferase (adding 1 isopentenyl unit). This enzyme catalyses the following chemical reaction
The enzyme from Methanosarcina mazei is involved in biosynthesis of the polyprenyl side-chain of methanophenazine.