In contrast to other classes of terpenes that vary greatly in structure and molecular size, the . limiting enzyme of the mevalonate pathway of cholesterol synthesis. Subsequent steps lead to the important C5 building blocks IPP and DMAPP. In contrast to other classes of terpenes that vary greatly in structure and molecular size, the enzymes involved in the pathway have been isolated and studied. Subsequent steps lead to the important C5 building blocks IPP and DMAPP. Terpenes are a large and diverse class of organic compounds, produced by a variety of plants, Terpenes are derived biosynthetically from units of isopentenyl pyrophosphate. One of the intermediates in this pathway is mevalonic acid.
to Different Classes Pathways of the Biosynthetic Terpenes Leading
So it makes perfect sense that the chain elongation reaction should more S N 1-like than S N 2-like. Is this in fact the case? We know how to answer this question experimentally - just run the reaction with fluorinated DMAPP or GPP substrates and observe how much the fluorines slow things down. If the reaction is S N 1-like, the electron-withdrawing fluorines should destabilize the allylic carbocation intermediate and thus slow the reaction down considerably.
If the mechanism is S N 2-like, the fluorine substitutions should not have a noticeable effect, because a carbocation intermediate would not be formed. When this experiment was performed with FPP synthase, the results were dramatic: These results strongly suggest indicate the formation of a carbocation intermediate in an S N 1-like displacement.
In this section, we will briefly examine the reaction catalyzed by an enzyme called squalene synthase, an important enzymatic transformation that involves some very interesting and unusual electrophilic additions, rearrangements, and reactive intermediates. This particular enzyme is also of interest because it represents a potential new target for cholesterol-lowering drugs. Cholesterol, as we discussed earlier in this chapter, is derived from a carbon isoprenoid molecule called squalene.
Squalene, in turn, is derived from the condensation of two molecules of farnesyl diphosphate FPP , a carbon isoprenoid. You may recall that FPP is the product of the C 4 to C 1 , or 'head to tail' electrophilic condensation of isoprenoid chains:.
The condensation of two molecules of FPP to form squalene, however, is something different: The chemistry involved is quite a bit more complicated. The first two steps are familiar: This results in a new carbon-carbon bond between the two FPP molecules, but with incorrect C 1 to C 2 connectivity remember, the overall reaction is a C 1 to C 1 condensation. In step 3, a proton is abstracted and the electrons from the broken C-H bond bridge across a 2-carbon gap to form a cyclopropyl intermediate.
In the second stage of squalene synthesis, the second pyrophosphate group leaves, generating a cyclopropylcarbinyl cation step 4. Because this is a primary carbocation, you probably are wondering about how stable it could be and thus how likely an intermediate. As it turns out, such carbocations are remarkably stable, due to favorable interactions between the empty orbital and orbitals on the three-membered ring the level of bonding theory needed to really understand this idea is beyond the scope of this text, but you may learn about it if you take a class in advanced organic chemistry.
What occurs next is an alkyl shift leading to a tertiary carbocation step 5. Discussion of the final step step 6 will need to be put off - this is a reduction with a hydride nucleophile derived from a coenzyme called NADPH.
Although this may seem like an extremely convoluted and perhaps unlikely! Steven Farmer Sonoma State University. Objectives After completing this section, you should be able to identify a terpene from a given list of organic structures. Key Terms Make certain that you can define, and use in context, the key terms below. Study Notes You are not expected to memorize all the details of the synthetic mechanisms for terpenoids. Isoprene Rule Compounds classified as terpenes constitute what is arguably the largest and most diverse class of natural products.
Monoterpenes and sesquiterpenes The isopentane units in most of these terpenes are easy to discern, and are defined by the shaded areas. Triterpenes Polymeric isoprenoid hydrocarbons have also been identified. Terpenoid Biosynthesis While we can identify isoprene units within a terpenoid structure and use that in its classification, the building block for terpenoid synthesis in nature is isopentenyl diphosphate formerly called isopentenyl pyrophosphate and abbreviated IPP.
Step 5 - Decarboxylation Finally isopentenyl diphosphate IPP , the 'building block' for all isoprenoid compounds, is formed from a decarboxylation-elimination reaction. Nature's Diversity and Ingenuity". Cellular and Molecular Life Sciences. Their Occurrence and Physiological Significance". Frontiers in Plant Science.
Carotenoid-Derived Aroma Compounds; chapter American Journal of Enology and Viticulture. Journal of Chromatography A. Journal of Agricultural and Food Chemistry. Enzyme and Microbial Technology. Annual Review of Plant Biology. Retrieved 19 January Proceedings of the National Academy of Sciences. Retrieved 18 September Retrieved 22 July The Plant of the Thousand and One Molecules".
British Journal of Pharmacology. Types of terpenes and terpenoids of isoprene units. Acyclic linear, cis and trans forms Monocyclic single ring Bicyclic 2 rings Iridoids cyclopentane ring Iridoid glycosides iridoids bound to a sugar Steroids 4 rings. Isoprene C 5 H 8 Prenol Isovaleric acid.
Limonene Terpinene Phellandrene Umbellulone. Grapefruit mercaptan menthol p-Cymene thymol Perillyl alcohol Carvacrol. Farnesyl pyrophosphate Artemisinin Bisabolol. Terpene synthase enzymes many , having in common a Terpene synthase N terminal domain protein domain. However, sometimes immediate deprotonation is observed corresponding with the more general designation of this enzyme family as synthases rather than cyclases though this later nomenclature would better fit the majority of the enzymes in the family.
Despite that, capture of water by the final carbocation has been detected, with either direct deprotonation to form a hydroxyl group, or even subsequent cyclization before deprotonation, forming a cyclic ether.
Finally, class I terpene synthases display a wide array of catalytic promiscuity. Some are fairly specific while others yield a distinctive range of products from the same substrate [ 28 , 30 , 34 ]. Isoprene synthase ISPS is the only known hemiterpene synthase. ISPS is responsible for the global production of isoprene in nature and biotechnology. ISPS active site contains magnesium ions that interact with the substrate dimethylallyl diphosphate DMAPP catalyzing the elimination of inorganic pyrophosphate to yield isoprene.
The structure of ISPS reveals a shallower active site cavity compared to other class I terpene synthases, even the monoterpene synthases. All monoterpene synthases catalyze the metal-dependent ionization and cyclization of the carbon precursor geranyl pyrophosphate GPP to produce different monoterpenes.
Monoterpene synthases accomplish outstanding structural and chemical diversity in their product assortments, despite their catalysis of the simplest terpene cyclization cascades where they use the shortest linear isoprenoid substrate [ 28 , 30 ]. Limonene synthase from Mentha spicata L. Cineole synthase from Salvia fruticosa Mill. Bornyl diphosphate synthase from Salvia officinalis L. Sesquiterpene synthases are responsible for catalyzing the conversion of farnesyl pyrophosphate FPP into more than known monocyclic, bicyclic, and tricyclic products.
In general, there is low amino acid sequence identity amid sesquiterpene synthases from bacteria, fungi, and plants. Pentalenene synthase from Streptomyces UC, and epi-isozizaene from Streptomyces coelicolor A3 2 , trichodiene synthase from Fusarium sporotrichioides Sherb, and aristolochene synthase from Penicillium roqueforti Thom.
On the other hand, plant sesquiterpene synthases such as epi-aristolochene synthase from Nicotiana tabacum L. Diterpene synthases catalyze the cyclization of the linear C 20 geranylgeranyl pyrophosphate GGPP to produce a range of cyclic and polycyclic diterpenes.
Among the very few characterized diterpene synthases are taxadiene synthase from Taxus brevifolia Nutt. The class II terpene synthases are characterized by being protonation-initiating enzymes. This class is composed of Class II diterpene synthases and triterpene synthases which can be squalene-hopene or oxido-squalene synthases. After the initial carbocation production, these enzymes often catalyze stereochemically complex cyclization reactions producing from one to five rings, followed with subsequent rearrangement.
Similar to the class I terpene synthases, enzymes of this class do not essentially directly deprotonate the final carbocation but sometimes water is captured tailed by deprotonation to form a hydroxylated product.
Also they exhibit a wide range of catalytic promiscuity [ 28 , 30 ]. Amorphadiene synthase ADS is a class I cisoid sesquiterpene synthase. It is a key enzyme in the biosynthesis of the antimalarial drug artemisinin in the plant A. There is no crystal structure reported for ADS, however, a 3D homology model representing the conformation of this enzymes has been recently published Fig.
The model was constructed using another sesquiterpene synthase from A. The created model of ADS showed the characteristic metal-ion binding motifs of class I terpene synthases chelating three magnesium ions in the active site. In addition, the substrate FPP was docked in the active site and its correct orientation was confirmed.
Since ADS belongs to the cisoid family, its multistep mechanism begins with isomerization of the C2-C3 double bond of FPP to produce nerolidyl diphosphate NPP which is ionized into a 2,3-cis-farnesyl cation.
This cation will initially undertake 1,6-cyclization to give bisabolyl cation followed by 1,ring closure to produce the major product amorpha-4,diene. Probing of different amino-acid residues in the active site of ADS helped in providing more insight into its catalytic mechanism. Moreover, efforts of engineering ADS to improve catalytic efficiency and alter product profile have yielded interesting results [ 24 , 40 ]. Schematic representation of the general structure of different terpene synthases.
The DXDD motif is in brown. The three yellow balls represent magnesium ions and the red side chain is the pyrophosphate group of the substrate. Taxadiene synthase TXS , a class I diterpene synthase, catalyzes the first step in biosynthesis of taxol in the bark of T.
The full-length of the enzyme is residue 98 kD but a terminal transit sequence of around 80 amino acid residues is cleaved off after maturation in plastids. The enzyme C-terminal contains conserved metal-binding motifs with three magnesium metal clusters to bind and activate the substrate but the N-terminal and insertion domain lack the characteristic DXDD motif indicating that the enzyme functions as a class I terpene synthase [ 28 , 29 ].
The need for sustainable production of terpenoids, being a very famous class of natural products, is massive. The problem of low natural yield of terpenoids and expensive or difficult chemical synthesis can be overcome by engineering microbial cells to act as biofactories for the sustainable production of terpenoids.
This approach would require transfer of biosynthetic pathways from the native source of terpenoids to these microbes with all its challenges. These microbial factories provide the benefits of the use of cheap carbon sources, ability to increase production yield by genetic manipulation, and environmentally friendly chemistry.
Since all terpenoids originate from the same C 5 precursors IPP and DMAPP produced by MVA or MEP pathway, engineering a platform strain producing large amounts of these precursors is beneficial for manufacturing different types of terpenoids where the terpene synthase responsible for production of a terpenoid of interest can be directly introduced into the platform strain.
In the last few decades, biosynthesis of terpenoids in microorganisms has focused mostly on carotenoids along with precursors for important drugs such as artemisinin and taxol [ 41 , 42 ]. Escherichia coli is one of the most widely used platform organisms.
Numerous reports exploiting its inherent MEP pathway by overexpression for production of terpenoids were successful. Also, efforts were made to introduce the heterologous MVA pathway in E. Numerous terpenoids including amorphadiene and taxadiene were effectively produced in E. One of the drawbacks of E. Another organism that has been widely researched for terpenoid production is the yeast Saccharomyces cerevisiae Meyen ex E. This yeast possess an endogenous MVA pathway, however most of the FPP produced by the pathway is consumed for production of sterols.
This can be achieved by the suppression of competing pathways that drain these precursors along with upregulation of the MVA pathway and expression of the desired terpene synthases.
The major disadvantage of S. In the recent years, interest in using Bacillus subtilis Ehrenberg Corn as a cell factory for terpenoid production has grown. Overexpression of the MEP pathway genes, dxs and idi , increased the production of amorphadiene in B. The production of these carotenoids was further enhanced by overexpression of different MEP pathway genes, in addition to, allowing the systematic analysis of the functionality of the different MEP pathway enzymes [ 8 , 46 , 47 , 48 ].
Furthermore, Photosynthetic microorganisms as cyanobacteria offer an additional advantage in production of terpenoids over both plants and other microbial systems. Similar to plants, they have the ability to directly use CO 2 as their carbon source and light as their source of energy. They can even perform that more efficiently with faster growth rates and improved solar energy conversion than plants. Simultaneously, certain strains of cyanobacteria have the same upsides as other microbial systems where they can grow to high densities in photobioreactors, can be genetically modified, and provide simpler extraction and purification processes for the target terpenoid than plant systems.
Also, they provide better likelihood of functional expression of plant enzymes and metabolic pathways compared to other microbial systems [ 14 , 49 ]. In the medicinal and commercial market, terpenoids will always be valuable compounds of vast interest. The biosynthetic pathways involved in terpenoid production are fully described, however, more insight into the catalytic mechanism of the enzymes involved in these pathways, especially terpene synthase, is of grave importance.
The characterization of different terpene synthases and exploring the structure-function relationships of their amino acid residues with regard to their interaction with the substrate will be the basis of manipulating these enzymes. Protein engineering of terpene synthases will provide the chance to improve the enzymes stability, catalytic efficiency and product specificity aiming at more sustainable production of their respective terpenoids.
In spite of the progress made in understanding microbial metabolic regulation and creating suitable genetic tools, there are several challenges still facing the construction of microbial cell factories for the commercial production of terpenoids. These challenges can be summarized into the precursor supply problem, pathway optimization, microbial tolerance, and efficient product extraction. The future research should focus on further optimization of flux through MEP or MVA pathways to provide high supply of precursors and engineering terpene synthase enzymes to increase the production of desired terpenoids.
Also, efforts should be made to improve microbial tolerance to high levels of terpenoid production and to develop suitable extraction methods of terpenoids, especially volatile ones, during production.
The evolving role of natural products in drug discovery. Nature Reviews Drug Discovery ;4: The success of natural products in drug discovery. A Historical overview of natural products in drug discovery.
The re-emergence of natural products for drug discovery in the genomics era. Nature Reviews Drug Discovery ; Natural products as sources of new drugs over the 30 years from to Journal of Natural Products ; Terpenoids as therapeutic drugs and pharmaceutical agents. Drug discovery and therapeutic medicine. Thoppil RJ, Bishayee A.
Researchers Create ‘Shortcut’ to Terpene Biosynthesis in E. coli
Biosynthesis of Isoprenoids. David Wang's Natural Products Class. Terpene plants for many different purposes — as fragrances . The evidence now indicates that the biosynthetic pathways for the . GPP leads initially to the tertiary. -Terpenes are an enormous class of natural products spanning well over 30, -There are 2 biosynthetic pathways for the production of IPP and DMAPP, the leads to many different carbocyclic skeletons, which are often further oxidized. The condensation of acetyl CoA three units leads to the synthesis of Another part of terpenoid biosynthetic pathway starts in plastid by the Plant genomes appear to encode various farnesyl diphosphate The class of triterpenes includes sterols and triterpenoids, which.