Prenyl transferase reactions

It is also possible that the pyrophosphate moiety of the substrate functions as the base in the terminating deprotonation step of some cyclizations. The assistance of the pyrophosphate in deprotonation has been implicated in the prenyl transferase reaction (70), and the process has been recently modeled (87). Should such assistance apply in monoterpene cyclization, a spatial correlation must exist between the position of the pyrophosphate and the proton removed from the proximal face of the transient cation. Such an experimental observation would provide indirect evidence for this additional function of the pyrophosphate group in olefin synthesis. 

Dimethylallyl pyrophosphate is the substrate for the first prenyl transferase reaction, and it is condensed with a molecule of isopentenyl pyrophosphate to give the Cio compound geranyl pyrophosphate. The addition of a second isopentenyl pyrophosphate unit produces the compound farae-syl pyrophosphate. The condensation of two molecules of famesyl pyrophosphate results in formation of the C30 compound squalene, the precursor of cholesterol and other sterols. The C20 compound, GGPP, is formed from... 

Fig. 7. The proposed mechanism (ionization-condensation-elimination) of the prenyl transferase reaction.

The prenyl transferase reactions have continued to attract attention and 9... 

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. 

Reaction type 6B of Table 10-1 is allylic rearrangement with simultaneous condensation with another molecule. The reaction, which is catalyzed by prenyl-transferases,3073 occurs during the polymerization of polyprenyl compounds (Fig. 22-1,Eqs. 22-2,22-3). Experimental evidence favors a carbocation mechanism for all of these reactions.308 309 See Chapter 22. 

With the preceding reviews of the enzymology of monoterpene cyclization and of model studies relevant to the cyclization process, it is possible to formulate a unified stereochemical scheme for the enzymatic cyclization of geranyl pyrophosphate (Figure 4). The proposal which follows is consistent with the implications of parallel advances in related fields, most notably the contributions of Cane (8,16,24,25,52), Arigoni (67) and Coates (68,69) on the stereochemistry of sesquiterpene and diterpene cyclizations, and of Poulter and Rilling (29,70) on the stepwise, ionic mechanism of prenyl transferase, a reaction type of which several monoterpene, sesquiterpene and diterpene cyclizations are, in a sense, the intramolecular equivalents. 

Elongation consists of the sequential condensation of IPP and allylic diphosphate through the action of the prenyl chain elongating enzyme, commonly called prenyl transferase. The reactions catalyzed by prenyltransferases start with the formation of allylic cations after the elimination of pyrophosphate ions, to form allylic prenyl diphosphate. This is followed by addition of an IPP with stereo-specific removal of the proton at the 2-position. This is the key reaction which determines cis- or rrans-configuration of the double bonds contained in the linear isoprenoid chain. Cis-trans isomerism is dependent on the nature of the prenyltransferase involved in elongation catalysis. A comprehensive review of cis- and traAW-prenyltransferases was recently undertaken by Takahashi and Koyama. CA-prenyltransferases are much less well known than their trawi-homologues. 

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. 

As mentioned earlier, terpenoids are built from IPP, which is synthesized by all living things from carbohydrates. The initiator, DMAPP, is synthesized from IPP by the isomerase enzyme. Then one unit of IPP adds to the DMAPP, catalyzed by the trans-prenyl transferase enzyme. It is important to note that this reaction requires metaP cofactors to allow the activation and breakage of the pyrophosphate end group and the insertion of an additional IPP unit. This step constitutes the elementary step of initiation, and yields geranyl pyrophosphate (GPP), with the release of a pyrophosphoric acid, HOPP (Figure 4.9). 

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]. 

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