Isoprenoids are an extensive group of natural products with diverse structures consisting of various numbers of five-carbon isopentenyl pyrophosphate (IPP) units. Isoprenoid compounds serve a variety of essential biological functions - for example, steroids are cyclic isoprenoids, which have distinct biological functions as hormones; retinoids are involved in morphogenesis and are the light-sensitive element in vision; and prenylated proteins such as Ras and other G-proteins are involved in specific signal-transduction pathways. Prenyltransferases catalyze the transfer reactions of prenyl moieties from different prenyl donors (e.g. geranyl (GPP, C10), farnesyl (FPP, C15), or geranylgeranyl (GGPP, C20) diphosphate) to various aliphatic or aromatic acceptors of both low and high molecular substances. They include: (i) isoprenyl pyrophosphate synthases (aka prenyl diphosphate synthases), which catalyze chain elongation to generate linear isoprenyl pyrophosphates; (b) protein prenyltransferases, which catalyze the transfer of the carbon moiety of an isoprenyl pyrophosphate to a protein or a peptide; and (c) aromatic prenyltransferases that catalyze the transfer reactions of prenyl moieties onto aromatic acceptors. (Adapted from PMID:12135472 and PMID:26216239.)