Farnesyl synthase

Mevalonate kinase deficiency. Mevalonate kinase and farnesyl-diphosphate synthase are localized in the peroxisome and are involved in the synthesis of isoprenoids. Mevalonate kinase deficiency causes severe developmental delay, dysmorphic features and early death. Mevalonate deficiency has also been observed in the hyperimmuno-globulinemia-and periodic fever syndrome. 

FIGURE 1.4 Proposed biosynthetic route for the biosynthesis of (A) squalene oxide (squalene-2,3-oxide) via the isoprenoid pathway and (B) triterpene saponins of the dammarane-type and oleanane-type from squalene oxide. PP, diphosphate group GPS, geranyl phosphate synthase FPS, farnesyl phosphate synthase NADPH, nicotinamide adenine dinucleotide phosphate. 

This enzyme [EC 2.5.1.21], also known as farnesyltransf-erase, presqualene di-diphosphate synthase, and squa-lene synthase, catalyzes the condensation of two molecules of farnesyl diphosphate to form presqualene diphosphate and diphosphate (or, pyrophosphate). The entire enzyme complex catalyzes the NADPH-depen-dent reduction of presqualene diphosphate to yield squalene. 

This enzyme [EC 2.5.1.10], also known as farnesyl-di-phosphate synthase, catalyzes the reaction of geranyl diphosphate with isopentenyl diphosphate to produce trans,trans-farnesyl diphosphate and pyrophosphate (or, diphosphate). Some forms of this enzyme will also utilize dimethylallyl diphosphate as a substrate. However, the enzyme will not accept larger prenyl diphosphates as an efficient substitute for geranyl diphosphate. 

A descriptor for an enzyme active site that permits binding of a family of related compounds (e.g., mimics of the reaction intermediate) that can be derived from the initial binding and conformational changes in the substrate. This concept arose from the observation that a number of monoterpene cyclases were incapable of discriminating between enantiomers of the reaction intermediate, even though the enzyme catalyzes the synthesis of an enantiomerically pure product from an achiral substrate. An example is trichodiene synthase which catalyzes the cyclization of farnesyl diphosphate to trichodiene. 

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 chiral hydrocarbon germacrene D is a widely spread plant constituent and is considered to be an important intermediate in the biosynthesis of many sesquiterpenes. Schmidt et al. [19, 20] have shown that the plant Solidago canadensis generates both optical antipodes of this compound by enzymatic cyclisation of farnesyl diphosphate using two different enantiospecific synthases. As to be seen in Fig. 17.5, the enantiomeric ratio of germacrene D in Solidago canadensis can vary from individual to individual [21]. 

In the last few years, sesquiterpene synthase from different plants has raised attention. In 2004, Schalk and Clark [88] described a process (patented by Fir-menich, Switzerland) that makes it possible to obtain sesquiterpene synthase and to produce various aliphatic and oxygenated sesquiterpenes from farnesyl diphosphate. For instance, valencene can be obtained in this way. 

Much attention has been paid to the last step of the formation of monoter-penes and sesquiterpenes, which is catalysed by terpenoid synthases. Over 30 complementary DNAs (cDNAs) encoding plant terpenoid synthases involved in the primary and secondary metabolism have been cloned, characterised, and the proteins heterologously expressed [6]. However, because geranyl diphosphate and farnesyl diphosphate are not readily available substrates, their biotransformation by terpenoid synthases is not economically viable. As a result, considerable effort has been put into engineering the total plant terpenoid biosynthetic pathway in recombinant microorganisms. 

The transformation of Arabidopsis thaliana with a cDNA from strawberry fruits encoding a dual (S)-linalool/(S)-nerolidol synthase also led to the production of both (S)-linalool and its glycosylated and hydroxylated derivatives in the leaves [14]. Surprisingly, the formation and emission of (S)-nerolidol was detected as well, suggesting that a small pool of its precursor farnesyl diphosphate is present in the plastids. The newly emitted (S)-linalool and (S)-nerolidol showed the same diurnal emission pattern as the pristine volatiles. 

Squalene takes part in metabolism as precursor for synthesis of steroids and structurally quite similar to (3-carotene, coenzyme qlO, vitamins Ki, E, and D. The squalene in skin and fat tissue comes from endogenous cholesterol synthesis as well as dietary resources in people who consume high amounts of olive and fish oil especially shark liver (Gershbein and Singh, 1969). Squalene is synthesized by squalene synthase which converts two units of farnesyl pyrophosphate, direct precursor for terpenes and steroids, into squalene. As a secosteroid, vitamin D biosynthesis is also regulated by squalene. Moreover, being precursor for each steroid family makes squalene a crucial component of the body. 

In green plants, which contain little or no cholesterol, cydoartenol is the key intermediate in sterol biosynthesis.161-1623 As indicated in Fig. 22-6, step c, cydoartenol can be formed if the proton at C-9 is shifted (as a hydride ion) to displace the methyl group from C-8. A proton is lost from the adjacent methyl group to close the cyclopropane ring. There are still other ways in which squalene is cyclized,162/163/1633 including some that incorporate nitrogen atoms and form alkaloids.1631 One pathway leads to the hop-anoids. These triterpene derivatives function in bacterial membranes, probably much as cholesterol does in our membranes. The three-dimensional structure of a bacterial hopene synthase is known.164 1643 Like glucoamylase (Fig. 2-29) and farnesyl transferase, the enzyme has an (a,a)6-barrel structure in one domain and a somewhat similar barrel in a second domain. 

The formation of the sesquiterpene (+)-5-epi-aristolochene (2) represents, from a biosynthetic point of view, the transformation requiring the fewest steps among those discussed in this review. A single enzyme, tobacco 5-epi-aristolochene synthase (TEAS), converts the biosynthetic precursor, farnesyl diphosphate (6), to... 

The genetic engineering of qinghao (4. annua) has also been paid great attention recently some preliminary results about the early stage of qinghaosu biosynthesis have been reported. For example, amorpha-4,11-diene synthase, an enzyme responsible for the cyclization of farnesyl diphosphate into ring sesquiterpene, has been expressed in Escherichia coli and production of amorpha-4,11-diene (122) was identified. °... 

Zhang, Y.H., et al. (2009). Lipophihc bisphosphonates as dual farnesyl/geranylgeranyl diphosphate synthase inhibitors an X-ray and NMR investigation. J Am Chem Soc 131 5153-5162. 

Overhand, M., Pieterman, E., Cohen, L.H., Valentijn, A.R.P.M., VanDerMarel, G.A., and vanBoom, J.H. (1997). Synthesis of triphosphonate analogues of farnesyl pyrophosphate, inhibitors of squalene synthase and protein farnesyl transferase. Bioorg Med Chem Lett 7 2435-2440. 

The IBP and its products are displayed in Figure 12.1. HMG-CoA, ultimately derived from acetyl-CoA is converted to mevalonate via the enzyme HMG-CoA reductase (HMGR) [8]. This reaction is the rate-limiting step in the pathway. Mevalonate is then phosphorylated via mevalonate kinase (MK) to yield 5-phosphomevalonate [9]. IPP is formed following additional phosphorylation and decarboxylation steps [10]. Isomerization of IPP via the enzyme IPP isomerase yields DMAPP [11]. In mammals, the enzyme farnesyl pyrophosphate synthase (FDPS) catalyzes the synthesis of both GPP and FPP [12]. In plants, a separate GPP synthase has been identified [13]. GPP is a key intermediate in plants as it serves as the precursor for all monoterpenes. In animals, however, GPP appears to serve only as an intermediate in the synthesis of FPP. Very low basal levels of GPP have been measured in cell culture, although cellular GPP levels can become markedly increased in the setting of FDPS inhibition [14]. 

FPP is necessary for the synthesis of both sterols and longer chain nonsterol isoprenoids. The first committed step in sterol synthesis is catalyzed by the enzyme squalene synthesis and involves the head-to-head condensation of two FPP molecules to form squalene [15]. This is followed by cyclization steps, leading to sterol synthesis. The addition of IPP to FPP via the enzyme GGPP synthase yields the 20-carbon GGPP [16]. FPP and GGPP are substrates in the prenylation reactions catalyzed by the enzymes farnesyl transferase (FTase) and geranylgeranyl transferase (GGTase) I and II [17-20]. 

Fnjisaki, S., Hara, H., Nishimura, Y., Horiuchi, K., and Nishino, T. (1990). Cloning and nncleotide seqnence of the ispA gene responsible for farnesyl diphosphate synthase activity in Escherichia coli. J Biochem 108 995-1000. 

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