Isoprenoids are natural compounds synthesized from isopentenyl diphosphate (IPP), which is a five-carbon compound, via allylic diphosphates with various carbon numbers (5 x n). Organisms have an appropriate set of prenyl diphosphate synthases that catalyze the condensation of IPP with allylic diphosphates to produce other allylic diphosphates with higher carbon numbers.
In this pathway multiple units of isopentenyl diphosphate (IPP) undergo a series of polymerizations to form a polyisoprenoid chain.
Additional isoprenoid units are added to a maximal length that is determined by the specific enzyme. Most organisms generate polyprenyl chains of predominantly one length. Once completed, the polyprenyl chain is incorporated into other molecules, such as quinones. The enzyme that attaches the polyprenyl chain to the quinone precursor molecule does not have a preference towards a particular length. Thus, the length of the polyprenyl chain in the mature quinone molecule is determined by the predominant polyprenyl diphosphate synthase enzyme of the organism.

Three genes for prenyl diphosphate synthases (prenyltransferases) exist in the chromosome of Escherichia coli: farnesyl pyrophosphate synthetase (Fpp synthetase, ispA; EC 2.5.1.10), octaprenyl pyrophosphate synthetase (Opp synthetase, ispB), and undecaprenyl pyrophosphate synthetase (Upp synthetase; EC 2.5.1.31). Fpp synthetase catalyzes the condensation of dimethylallyl pyrophosphate (DMAPP) and IPP to yield geranyl pyrophosphate (GPP), which the enzyme uses in a second condensation reaction with IPP to yield the ultimate product, farnesyl pyrophosphate (FPP). The other two prenyltransferases use FPP as the starting molecule in further rounds of sequential condensation with IPP. Opp synthetase (ispB) generates the long-chain polyprenyl pyrophosphate-like isoprenoid quinones (ubiquinone-8, menaquinone-8, and dimethylmenaquinone-8), which have an all-trans-octaprenyl side chain.
Upp synthetase generates undecaprenyl pyrophosphate (UPP, C55-PP), which contains a trans,cis-mixed isoprenoid chain. UPP is required as a lipid carrier of glycosyl transfer in the biosynthesis of a variety of cell wall polysaccharide components in bacteria.


================== Geranyl pyrophosphate synthases: =======================
Geranyl-diphosphate (GPP) is the universal precursor of several classes of compounds, including the monoterpenes, a diverse group of compounds crucial for numerous biological roles. In plants monoterpenes are involved in defence against herbivores and pathogens, allelopathic interactions and pollination. GPP is produced by Geranylgeranyl pyrophosphate synthetase (EC 2.5.1.29) which catalyze the condensation of isopentenyl diphosphate (isopentenyl pyrophosphate, IPP) with dimethylallyl-diphosphate (dimethylallyl pyrophosphate, DMAPP) following a head-to-tail mechanism.


========REFERENCES:=====================

1a. Apfel CM, Takács B, Fountoulakis M, Stieger M, Keck W. Use of genomics to identify bacterial undecaprenyl pyrophosphate synthetase: cloning, expression, and characterization of the essential uppS gene.J Bacteriol. 1999 Jan;181(2):483-92.PMID: 9882662

1. Okada K, Minehira M, Zhu X, Suzuki K, Nakagawa T, Matsuda H, Kawamukai M. The ispB gene encoding octaprenyl diphosphate synthase is essential for growth of Escherichia coli. J Bacteriol. 1997 May;179(9):3058-60. PMID: 9139929

2. Asai K, Fujisaki S, Nishimura Y, Nishino T, Okada K, Nakagawa T, Kawamukai M, Matsuda. The identification of Escherichia coli ispB (cel) gene encoding the octaprenyl diphosphate synthase. H.Biochem Biophys Res Commun. 1994 Jul 15;202(1):340-5.
PMID: 8037730
3. C. M. Apfel, B. Takács, M. Fountoulakis, M. Stieger, and W. Keck. Use of Genomics To Identify Bacterial Undecaprenyl Pyrophosphate Synthetase: Cloning, Expression, and Characterization of the Essential uppS Gene J. Bacteriol., January 15, 1999; 181(2): 483 - 492. PMID: 9882662