Prenyl diphosphate synthase

Prenyltransferases are a class of enzymes that transfer allylic prenyl groups to acceptor molecules. Prenyl transferases commonly refer to prenyl diphosphate synthases (even though the class of prenyl transferases also includes enzymes that catalyze the transfer of prenyl groups to acceptors that include not only isopentenyl diphosphate (IPP) but also aromatic compounds and proteins etc.). 

This enzyme [EC 2.5.1.1] (also referred to as prenyl-transferase and geranyl-diphosphate synthase) catalyzes the reaction of dimethylallyl diphosphate and isopen-tenyl diphosphate to produce geranyl diphosphate and pyrophosphate (or, diphosphate). The enzyme will not accept larger prenyl diphosphates as substrates. 

Prenylation, the key step in terpene biosynthesis, is catalyzed by prenyltransferases. These enzymes are responsible for the condensation of isopentenyl pyrophosphate (IPP) with an allyl pyrophosphate, thus yielding isoprenoids. Numerous studies have been performed with fluorinated substrates in order to determine the mechanism of the reactions that involve these enzymes prenyltransferases, farnesyl diphosphate synthase (FDPSase), famesyltransferase (PFTase), and IPP isomerase. These studies are based on the potential ability of fluorine atoms to destabilize cationic intermediates, and then slow down S l type processes in these reactions. 

The prenyl diphosphates, GPP, FPP and GGPP, are the central intermediates of terpenoid bios)mthesis. Under the catalysis of monoterpene, sesquiterpene and diterpene synthases, respectively, these substances are transformed into... 

Shibuya et al. (1990) and Gebler and Poulter (1992) provided experimental support that DMAT synthase catalyzes an electrophilic aromatic substitution (Fig. 4) similar to that of other prenyl transferases such as farnesyl diphosphate synthase. A positively charged alkyl intermediate—an allyl carbocation—is... 

Prenyltransferase activities have been studied in C. roseus both at the enzyme level and at the product level. Biosynthetic capabilities were investigated by incubating [1- C]IPP with aliquots of cell-free homogenates prepared from P. aphanidermatum treated and untreated suspension-cultured cells of C. roseus. After elicitation, the total incorporation of IPP into prenyl lipids was decreased, in particular into squalene. But the incorporation of IPP into some (as yet unidentified) compounds was increased (99). The prenyltransferases and subsequent enzyme activities are relatively easily extracted and remain complexed so that the product of one enzyme can be used as a substrate for the next enzyme. With an assay for these enzymes as described in detail in Threlfall and Whitehead (101), about a dozen enzyme activities could be detected in series using cell-free preparations of elicited Tabemaemontana divaricata cells (27). In the elicited C. roseus cells, the activities of IPP isomerase, famesyl diphosphate synthase, squalene synthase, squalene-2,3-epoxidase (and probably also a squalene-2,3-epoxide cyclase) were thus detected. Compared with the control nontreated cells, squalene production seemed to be reduced particularly (99). 

In the case of natural rubber from Hevea brasiliensis, which is almost entirely made of cis-isoprene units, it could be assumed that ci5-prenyltransferase is exclusively involved in phase 2. Unfortunately it is more complex. Authors agree on phase 1, which is traws -condensation catalyzed by a cytosolic soluble trans-prenyl transferase" Famesyl diphosphate synthase has been cloned from rubber latex.The most probable prenyldiphosphate used as a primer for phase 2 is famesyl-PP (15) or geranylgeranyl-PP (16)." " Phase 2 is catalyzed by a still not clearly identified rubber transferase system. Different proteins have been... 

The biosynthesis of rubber may be divided into three steps (1) initiation, which requires an allylic diphosphate molecule, (2) elongation, in which IPP units are added to a Z-l,4-polyisoprene chain, and (3) termination, in which the polymer is released from the rubber transferase enzyme (Cornish, 1993). In plants, the elongation of Z-l,4-polyiso-prene (natural rubber) requires a small -allylic diphosphate initiator (less than or equal to C20). Famesyl pyrophosphate (FPP) is an effective initiator of polyisoprene biosynthesis (Light et al, 1989) further, because only one molecule of FPP is needed for each molecule of rubber formed, small traces of this substance that are inadvertently present complicate biosynthetic studies. The E-allylic diphosphates are hydrophilic cytosolic compounds, whereas Z-l,4-polyisoprene is hydrophobic and compartmentalized in subcellular rubber particles. A soluble E-prenyl transferase from the latex of Hevea brasiliensis serves as a famesyl diphosphate synthase and plays no direct role in elongation of Z-l,4-polyisoprene (Cornish, 1993). Because the hydro-phobic rubber molecule is produced inside a rubber particle but is formed from hydrophilic precursors from the cytoplasm, the polymerization reaction must take place at the particle surface. 

Despite great diversity in form and function, the terpenoids are unified in their common biosynthetic origin. The biosynthesis of all terpenoids from simple, primary metabolites can be divided into four overall steps (a) synthesis of the fundamental precursor IPP (b) repetitive additions of IPP to form a series of prenyl diphosphate homologs, which serve as the immediate precursors of the different classes of terpenoids (c) elaboration of these allylic prenyl diphosphates by specific terpenoid synthases to yield terpenoid skeletons and (d) secondary enzymatic modifications to the skeletons (largely redox reactions) to give rise to the functional properties and great chemical diversity of this family of natural products. As bacosides are triterpenoid derivatives, they may probably foUow the common biosynthetic pathway of terpenoid production [40]. 

Reaction mechanism of DMAT formation catalysed by DMAT synthase rotation around the C-l/C-2 bond of the allylic carbocation in a fraction of the molecules prior to bond formation with the indole. These findings characterise the prenyl transfer of DMAT synthase as an electrophilic aromatic substitution, mechanistically similar to the electrophilic alkylation catalysed by farnesyl diphosphate synthase (Song and Poulter, 1994). 

The tremendous diversity of volatile monot-erpenes, sesquiterpenes and diterpenes arises from enzymatic modification of the non-volatile prenyl diphosphate intermediates GPP, FPP and GGPP through the action of terpene synthases (TPS) [57, 58], many of which have the distinctive ability to catalyze the formation of multiple products from a single prenyl diphosphate snb-strate [59-61]. A recently isolated monoterpene synthase from Nicotiana suaveolens flowers was found to produce a blend of 5 cyclic and 2 acyclic monoterpenes, all of which are components of N. suaveolens floral scent [62]. In Arabidopsis, two terpene synthases account for the biosynthesis of nearly all sesquiterpenes found in the floral volatile blend [63]. In addition to sesquiterpenes, Arabidopsis flowers also emit monoterpenes dominated by P-myrcene and (5)-linalool. While P-myrcene could likely be synthesized by multi-product monoterpene synthases [64, 65], one single-product monoterpene synthase may be solely responsible for the emission of (5)-linalool [65]. In snapdragon, two specialized single-product monoterpene synthases are responsible for the biosynthesis of the two floral monoterpenes, myrcene and ( )-P-ocimene [66]. The reaction mechanism of TPSs involves the formation of carbocationic intermediates which can then be differentially metabolized to form multiple products [57, 58]. 

The terpenes, carotenoids, steroids, and many other compounds arise in a direct way from the prenyl group of isopentenyl diphosphate (Fig. 22-1).16a Biosynthesis of this five-carbon branched unit from mevalonate has been discussed previously (Chapter 17, Fig. 17-19) and is briefly recapitulated in Fig. 22-1. Distinct isoenzymes of 3-hydroxy-3-methylglutaryl-CoA synthase (HMG-CoA synthase) in the liver produce HMG-CoA destined for formation of ketone bodies (Eq. 17-5) or mevalonate.7 8 A similar cytosolic enzyme is active in plants which, collectively, make more than 30,000 different isoprenoid compounds.910 However, many of these are formed by an alternative pathway that does not utilize mevalonate but starts with a thiamin diphosphate-dependent condensation of glyceraldehyde 3-phosphate with pyruvate (Figs. 22-1,22-2). 

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