Prenyl Transfer

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. 

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To illustrate this, we give as an example investigations on the prenyl transfer catalyzed by FDPSase and by PFTase. These studies show that, with a fluorinated substituent, the reaction is significantly slowed down (Figure 1.21) ... 

Figure 7.27 Relative rates of prenyl transfer catalyzed by FDPSase and PFTase.

Poulter, C.D., and Rilhng, H.C. (1978). The prenyl transfer reaction. Enzymatic and mechanistic studies of the l -4 coupling reaction in the terpene biosynthetic pathway. Acc Chem Res 11 307-313. 

Gibbs, R.A. (1998). Prenyl transfer and the enzymes of terpenoid and steroid biosynthesis. In Comprehensive Biological Catalysis, (M.L. Sinnott, Ed.), Vol. 1, pp. 31-118. Academic Press, London. 

Comprehensive Biological Catalysis—a Mechanistic Reference Volume has recently been published. The fiiU contents list (approximate number of references in parentheses) is as follows S-adenosylmethionine-dependent methyltransferases (110) prenyl transfer and the enzymes of terpenoid and steroid biosynthesis (330) glycosyl transfer (800) mechanism of folate-requiring enzymes in one-carbon metabohsm (260) hydride and alkyl group shifts in the reactions of aldehydes and ketones (150) phosphoenolpyruvate as an electrophile carboxyvinyl transfer reactions (140) physical organic chemistry of acyl transfer reactions (220) catalytic mechanisms of the aspartic proteinases (90) the serine proteinases (135) cysteine proteinases (350) zinc proteinases (200) esterases and lipases (160) reactions of carbon at the carbon dioxide level of oxidation (390) transfer of the POj group (230) phosphate diesterases and triesterases (160) ribozymes (70) catalysis of tRNA aminoacylation by class I and class II aminoacyl-tRNA synthetases (220) thio-disulfide exchange of divalent sulfirr (150) and sulfotransferases (50). 

Nature is generally reluctant to carry out classical, central S- 2 reactions, with the exception of methylations - glycosyl- and prenyl-transfer reactions are very S Nl-like. A much preferred route for displacements of sugar substituents is oxidation by a tightly bound nicotinamide cofactor of a p-hydroxyl, elimination of the leaving group, introduction of the substituent in a Michael addition and reduction, as dealt with in Section 6.8. 

In principle, carbon-carbon bond formation via prenyl transfer might occur via a carbanion generated from addition of an enzymic nucleophile to one of the... 

The prenyl transfer reactions are electrophilic substitution reactions. The reaction mechanism probably includes a carbocation " (Figure 16), evidence for which comes from studies on the related enzyme dimethy-lallyltryptophan synthase. " ... 

R. A. Gibbs, Prenyl Transfer and the Enzymes of Terpenoid and Steroid Biosynthesis. In Comprehensive Biological Catalysis ... 

Poulter has reviewed the evidence supporting an ionization-condensation-elimination mechanism (Vol. 8, p. 24) in the prenyl-transfer reaction. 

Prenylation - transfer of Cl5 or C20 groups from intermediates in cholesterol biosynthesis to cysteine residues four positions from the C-terminus of the protein. 

The first reaction involves a prenyl transfer step in which C(10 of one of the allylic substrates (the donor) is bonded to the C(2)-C(3) double bonds of the other (the receptor) to produce a cyclopropylcarbinyl diphosphate with a CV-2-3 structure (Poulter, 1990). In this manner, famesyl pyrophosphate (8) yields presqualene pyrophosphate (9). Only (/ )-presqualene pyrophosphate has been found in nature. 

This unexpected result was interpreted to mean that the reverse prenyl transferase presents the olefinic n-system of DMAPP in a manner in which both faces of the n-system are susceptible to attack by the 2-position of the indole moiety. The simplest explanation is to invoke binding of the DMAPP in an upside down orientation relative to normal prenyl transferases which permits a facially non-selective S attack on the rr-system as shown in Scheme 20. It was speculated that in this situation the pyrophosphate group is likely anchored in the enzyme active site with the hydrophobic isopropenyl moiety being presented in a conformationally flexible (A B) disposition with respect to the tryptophan-derived substrate (Scheme 20). This is in contrast to the normal mode of prenyl transfer where the nucleophilic displacement at the pyrophosphate-bearing methylene carbon occurs with inversion of stereochemistry at carbon with the hydrophobic tail of DMAPP buried in the enzyme active site. 

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). 

Zang, D.L., Daliels, L., and Poulter, C.D. (1990) Biosynthesis of archaebacterial membranes formation of isoprene ethers by a prenyl transfer reaction./. Am. Chem. Soc., 112,1264—1265. 

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