Prenyltransferases

There are three main classes of enzymes involved in the processing of isoprenoid building blocks to form the vast range of structures present in the terpene family (Liang et al. 2002). 

The isoprenyl pyrophosphate synthases (IPPS) catalyse the formation of long-chain prenyl pyrophosphates by the addition of an aUylic cation generated from DMAPP, GPP or FPP to one or more molecules of IPP. Terpene synthases are responsible for the conversion of prenyl pyrophosphates into the classic terpene structures a large number of these are cyclic structures so these synthases are also known as terpene cyclases. Protein prenyltransferases catalyse the addition of isoprenoid chains to a protein or peptide. Here we consider mainly the first two of these. 

---

In the next section, we will demonstrate how the QM/MM refinement strategy was combined with advanced sampling techniques to resolve the ambiguous electron densities of a substrate complexed with Orf2, an aromatic prenyltransferase, and successfully clarify the relaxed substrate specificity and product regioselectivity. 

Koehl P Relaxed specificity in aromatic prenyltransferases. Nat Chem Biol 1(2) 71—72... 

BURKE C.C., WILDUNG, M.R., CROTEAU, R., Geranyl diphosphate synthase cloning, expression, and characterizatin of this prenyltransferase as a heterodimer, Proc. Natl. Acad. Sci. USA, 1999, 96,13062-13067. 

Kahrel Y, Takahashi S, Yamashita S et al (2006) Manipulation of prenyl chain length determination mechanism of ci s-prenyltransferases. FEES J 273 647-657... 

PRION PLAQUE EORMATION PROTEIN OXIDATION (In Aging Disease) PROTEIN PRENYLTRANSFERASES... 

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. 

Prenyltransferases are responsible for the synthesis of c A-polyisoprene in natural rubber particles but can also be used to synthesize polyisoprenes in vitro. 

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.). 

The reactions catalyzed by prenyltransferases are unique and interesting from a mechanistic viewpoint. The reaction starts with elimination of the diphosphate ion from an allylic diphosphate (APP) to form an allylic cation, which is attacked by the IPP molecule, with stereospecific removal of a proton to form a new C-C bond and a new double bond in the product. By repeating this type of condensation between IPP and the allylic prenyl diphosphate product, prenyltransferase can synthesize a prenyl diphosphate with a certain length and stereochemistry fixed by ifs specificify (seeFig. 10) [252]. 

The prenyl chain elongation catalyzed by prenyltransferases is quite unique because the reaction proceeds consecutively and terminates precisely at discrete chain lengths according to the specificities of the enzymes. The chain length of products varies so widely that it ranges from geraniol (CIO) to natural rubber (C > 5000). 

Fig. 11 Natural rubber is produced from a side branch of the ubiquitous isoprenoid pathway, with 3-hydroxy-methyl-glutaryl-CoA (HMG-CoA) as the key intermediate derived from acetyl-CoA by the general mevalonic-acid pathway. Mevalonate diphosphate decarboxylase (MPP-D) produces IPP, which is isomeiized to DMAPP by IPP isomerase (IPI). IPP is then condensed in several steps with DMAPP to produce GPP, FPP and GGPP by the action of a trani-prenyltransferase (TPT). The cA-l,4-polymeiization that yields natural rubber is catalyzed by cA-prenyltransferase (CPT), which uses the non-allylic IPP as substrate. Reprinted from [248], with permission from Elsevier...

The cDNAs of the cA-prenyltransferase of H. brasiliensis was successfully identified and expressed in E. coli. The in vitro polymerization of IPP after initiation with FPP using the expressed c/x-prenyltransferase resulted in low degrees of polymerization [267, 268]. After addition of rubber particles to this polymerization, the molecular weight increased tremendously [269], It can be concluded that the rubber particles are essential for rubber biosynthesis. Katarina Cornish established a detailed structural model of the in vivo synthesis of natural rubber in the rubber particle monolayer membrane and partially explained this behavior (see Fig. 12) [251],... 

FIGURE 3.5 Biosynthetic route to isoflavonoids (and some derivatives) from the 5-deoxyflavanone liquiritigenin. A possible route to the retrochalcone echinatin is also shown. Unlabelled arrows indicate biosynthetic steps for which the enzyme(s) have not been characterized. Enzyme abbreviations are defined in the text and in Table 3.1, except for P2CP, pterocarpan 2-C-prenyltransferase P4CP, pterocarpan 4-C-prenyltransferase. 

The formation of phytoalexins such as glyceollins and phaseollins requires C-prenylation by a range of pterocarpan prenyltransferase (PTP) activities, with dimethylallyl pyrophosphate (DMAPP) as the prenyl donor. For glyceollins and phaseollins, prenylation occurs at position C-2 or C-4 of glycinol or C-10 of 3,9-dihydroxypterocarpan. ° ° However, there are differing activities in other species. For example, in Lupinus albus (white lupin) a prenyltransferase acting at the C-6, -8, and -3 positions of isoflavones has been identified.PTPs have also been characterized in detail for the formation of prenylated flavanones in Sophora flavescens (see, e.g., Ref. 207). However, no cDNA clones for flavonoid-related prenyltransferases have been published to date. 

Welle, R. and Grisebach, H., Properties and solubilization of the prenyltransferase of isoflavonoid phytoalexin biosynthesis in soybean. Phytochemistry, 30, 479, 1991. 

A partially purified prenyltransferase from Ehrlich tumor cells yielded 2,3-dehydrodolichol phosphate containing 17-19 isoprene units." It is very likely that this compound is formed by the action, on the corresponding diphosphate," of a contaminating phosphatase and, therefore, the probable reaction sequence is that phosphatase action precedes saturation of the a-isoprene unit, as shown in Scheme 1. 

Rubber also contains almost entirely Z double bonds. Consistent with this fact is the finding that the prenyltransferases catalyzing formation of rubber promote loss of the pro-R proton rather than the pro-S proton of mevalonic acid (see Eq. 22-2). There appear to be two types of prenyltransferase in animal mitochondria giving rise to E and Z double bonds, respectively.64 In contrast, the rubber tree contains a 137-residue protein, the rubber elongation factor. This small protein binds to E prenyltransferases causing them to form Z double bonds.65 The bacterium Micrococcus hit cits synthesizes all E polyprenyl alcohol diphosphates up to the 0 5 nonaprenyl compound solanesyl diphosphate.66... 

Inhibition of these prenyltransferases blocks growth of tumor cells. Many prenyltransferase inhibitors are apparently nontoxic to normal cells and are undergoing human clinical trials as anticancer drugs.763 79a b Among other important polyprenyl... 

However, there are also examples of prenyltransferase in which substrates are only poorly accepted with low rate constants (Kc.lt/Knl). especially when their structures differ significantly from the natural substrates of the enzymes. There are two ways to avoid such drawbacks in future (1) enzyme optimisation by site-directed mutation and (2) significantly increased catalyst overexpression, so that the amount of enzyme is not the limiting factor. 

Fig. 5 Regio-selective prenylation of substituted indoles by novel, heterologously expressed His6-tagged prenyltransferases [71]...

---