Isopentenylpyrophosphate

If sterol content and conformation are so important for membrane stability, we should study the biosynthesis of sterols (Figure 3). The first enzyme in terpenoid biosynthesis is the 3-Hydroxy-3-Methyl-Glutary1-Coenzyme A-reductase (HMG-CoA-reductase) that catalyzes the synthesis of mevalonate. Two phosphorylations and decarboxylation of mevalonate lead to isopentenylpyrophosphate, the basic C -unit in sterol synthesis. Isopentenylpyrophosphate reacts with its isomer, the dimethylally1-pyrophosphate, in a head/tail-reaction to geranyl-pyrophosphate reaction with another C -unit leads to farnesyl-pyro-phosphate, that dimerizes in a tail/tail-reaction to squalene. After expoxidation of its A -double bond, squalene cyclizes to lano-... 

The addition of mevalonic acid-l-C14 to a pyroclavine- and festuclavine-producing fungus strain yielded inactive alkaloids which, in agreement with the scheme, showed that the carboxyl group of the mevalonic acid is not incorporated. Lowering of the assimilation of mevalonic acid-2-C14 by the addition of dimethylallylpyrophosphate or isopentenylpyrophos-phate supported the assumption that mevalonic acid enters the alkaloid molecule via one of these activated isoprene radicals. This was confirmed by the incorporation of deuterated isopentenylpyrophosphate in alkaloids of the clavine type in saprophytic cultures of a Claviceps strain (128). 

AcetylCoA (C2) can also react with acetoacetylGoA (Cj) to generate hydroxy-methylglutarylCoA (HMGCoA) (C6) and thence the isoprenoid precursor mevalonate (C6). Mevalonate (C(i) ultimately yields the key C-, isoprenoids isopentenylpyrophosphate (CH3G(=GH2)—CH2—CH2—O—P03—P03) (IP-PP) and dimethylallylpyrophosphate (CH3— C(GH3)=GH—CH2—O—P03—P03) (DMA-PP), the immediate precursors of cholesterol and... 

Mevalonate is converted into 3-isopentenylpyrophosphate in three consecutive reactions requiring ATP (Figure 26.8). Decarboxylation yields isopentenyl pyrophosphate, an activated isoprene unit that is a key building block for many important biomolecules throughout the kingdoms of life. We will return to a discussion of this molecule later in the chapter. 

The natural precursors of any flavourings of this group are isopentenylpyrophosphate (IPP) and dimethylallyl pyrophosphate (DMAPP). In the classic pathway these are synthesised from three molecules acetyl/malonyl-CoA via mevalonic acid (MA). Recently a second pathway of the isoprenoid biosynthesis has been detected, in which IPP and DMAPP are synthesised from pyruvate and glyceraldehyde 3-phosphate via deoxyxylulose (DOX) 5-phosphate [322-324],... 

Even though E. coli is a very well-studied bacterium, many interesting mechanistic problems in cofactor biosynthesis in this organism remain unsolved. The mechanisms for the formation of the nicotinamide ring of NAD, the pyridine ring of pyridoxal, the pterin system of molybdopterin, and the thiazole and pyrimidine rings of thiamin are unknown. The sulfur transfer chemistry involved in the biosynthesis of lipoic acid, biotin, thiamin and molybdopterin is not yet understood. The formation of the isopentenylpyrophosphate precursor to the prenyl side chain of ubiquinone and menaquinone does not occur by the mevalonate pathway. None of the enzymes involved in this alternative terpene biosynthetic pathway have been characterized. The aim of this review is to focus attention on these unsolved mechanistic problems. 

Two compounds common in plant metabolism are believed to be precursors of isoprenoid cytokinins in plants adenosine-5 -monophosphate (AMP) and A -isopentenylpyrophos-phate (iPP). As a final product of the mevalonate pathway, the latter substance serves also as a precursor for a wide spectrum of metabolites including some other plant hormones, as abscisic acid, gibberellins and brassinosteroids. The hypothetical scheme of reactions resulting in the formation of iPA, Z and DHZ is given in Fig. 2. The enzyme of entry into isoprenoid cytokinin formation is A -isopentenylpyrophosphate 5 -AMP-A -iso-pentenyltransferase (EC 2.5.1.8, trivially named cytokinin synthetase ). This enzyme activity was first detected in a cell-free preparation from the slime mould Dictyostelium discoideum [7,8]. Later the enzyme from higher plants (cytokinin-independent tobacco callus [9,10] and immature Zea mays kernels [11]) was described and the data were recently summarised in [12], The enzyme is very specific as far as the substrate is concerned [13,14] only the nucleotide AMP can be converted and only iPP (with a double bond in A position) may function as a side chain donor. 

In addition to free cytokinins, cytokinin moieties also occur as constituents of some tRNA species of a wide range of organisms including plants [57]. They are located at the strategic 37 position adjacent to the 3 -end of the anticodon [58]. In contrast to the formation of free cytokinins the biosynthetic pathways of tRNA cytokinins are well understood. The first step in their formation is post-transcriptional isopentenylation of Ade using iPP and unmodified tRNA as substrates. This reaction is catalysed by A -isopentenylpyrophosphate tRNA-A -isopentenyltransferase (EC 2.5.1.8) which was partially purified from yeast [59], E. coli [60] and com [61]. This enzyme is encoded by... 

Gibberellin biosynthesis is initiated by the activation of MVA, followed by its conversion to isopentenylpyrophosphate (IPP). Stepwise condensation leads to the formation of trans-geranylpyrophosphate (GPP), trans-farnesylpyrophosphate (FPP) and trans-geranylgeranylpyrophosphate (GGPP). 

Thioanalogues (259-271) of isopentenylpyrophosphate and 4-hydroxy-3-methylbut-2-enyl pyrophosphate (HMBPP, the most potent natural antigen) has been synthesized and tested as a prototypic potent Ny9N52 (type of lymphocytes) activators. ... 

Biosynthesis. In Claviceps purpurea, and presumably also in higher plants, E.a. are biosynthesized from tryptophan and isopentenylpyrophosphate (see Terpenes). Synthesis proceeds via 4-dimethylallyl-tiyptophan, which is converted into the alkaloid chanoclavine (by hydroxylation, methylation, decarboxylation and formation of a new C-C bond). All the other clavine alkaloids and the lysergic acid derivatives are derived from chanoclavine. The peptide moieties of the ergotamine and ergotoxin alkaloids are formed by a multienzyme complex (Fig. 2). E.a. are prepared from the sclerotia of rye, previously inoculated with Claviceps and they are also produced by culture of the fungus on artificial growth media. 

Hemiteipenes terpenes composed of a single iso-prene unit (CsHg). The group is represented by only a few members, the mc important being isoprene, which is formed by removal of pyrophosphate from isopentenylpyrophosphate ( active isoprene) (Fig.). 

Fig. 94. Formation of all-trans-prenyl pyrophosphates 1 Isopentenylpyrophosphate A-isomerase 2 prenyltransferase...

The five-carbon isoprene skeleton constitutes a ubiquitous biosynthetic building block in all cells. Nature constructs a daunting array of primary and secondary metabolites from the two basic precursors, dimethylallyl pyrophosphate (DMAPP) and isopentenylpyrophosphate (IPP) (Fig. 16) [95]. 

Lynen F, Eggerer H, Henning U, Kessel I (1958) Famesyl-pyrophosphat und 3-Methyl-A3-butenyl-l-pyrophosphat, die biologischen vorstufen des squalens. Angew Chem 70 738-742 Bloch K, Chaykin S, Phillips AH, de Waard A (1959) Mevalonic acid pyrophosphate and isopentenylpyrophosphate. J Biol Chem 234 2595-2604... 

Fig. 95.1 Terpene biosynthesis pathways and their subcellular localization in the plants. Different classes of terpenes are respectively formed in the cytosol or the plastid by two independent pathways in the plants, that is, acetate-mevalonate pathway (MEV) (cytosol) and methylerythritol 4-phosphate (MEP) or deoxyxylulose 5-phosphate pathway (DXP) (plastid). Mraioterpcmes, diterpenes, and tetraterpenes are derived from IPP and DMAPP Irran the plastidial MEP ot DXP pathway. Sesquiterpenes and triterpenes are biosynthesized from IPP and DMAPP from the cytosol pathway. Black square with a white question mark suggests a possible transport of IPP (isopentenylpyrophosphate) from the plastid to the cytosol. Other metabolites involved in the different steps are DMAPP dimethylallylpyrophosphate, FPP famesylpyrophosphate, GASP D- glyceraldehyde- 3-phosphate, GPP geranylpyrophosphate, GGPP geranylgeranylpyro-phosphate. TPSs in the circle correspond to terpene synthases. Broken arrows show several enzymatic steps (Adapted from Aharoni et al. [8] and Sallaud et al. [154])...

Usually, carotenoid is used to indicate C4o-compounds consisting of eight isopentenylpyrophosphate (IPP) units. Some variations in carbon numbers of carotenoids are also observed (Table 106.1). Cao-carotenoids are synthesized by the combination of two famesol instead of the usual geranylgeraniol unit. C45- and Cso-carotenoids are synthesized by the addition of one and two IPP units to C4o-carotenoids, respectively. Unique carotenoid derivatives, such as carotenoid glycoside fatty acid esters and carotenoid sulfates, are also distributed in specific organisms (Table 106.1). 

The steps required to convert mevalonic acid to the active-isoprenoid intermediate have been worked out with some assurance. The initial step involves the phosphorylation of mevalonic acid to mevalonic acid-5-phosphate by an enzyme called mevalonic kinase. This enzyme was found in yeast by Tchen (1958). The properties of the mevalonic kinase of liver have been described in detail by Levy and PopjAK (1960). The kinase is inhibited by p-chloromercuribenzoate but not by iodoacetamide. The enzyme requires Mg++, Mn++, or Ca++ and ATP or inosine triphosphate. The kinase is specific for the (+) form of mevalonic acid. Mevalonic acid-5-phosphate is phosphorylated further to give mevalonic acid-5-pyrophos-phate (de Waard and Popjak, 1959 Henning et al. 1959). The purified enzyme (Bloch et al., 1959) requires a divalent metal ion for activity (Mg++ is preferable) and has no pronounced pH optimum. Mevalonic acid pyrophosphate then undergoes simultaneous dehydration and decarboxylation to yield isopentenylpyro-phosphate (Lynen et al., 1958 Chaykin et al., 1958). The enzyme concerned with the dehydration and decarboxylation has been purified (Bloch et al., 1959) and shown to have a pH optimum between 5.5 and 7.4 and to require a divalent metal ion (Mg++, Mn++, Fe++ or Co++). The series of reactions in which mevalonate is converted to isopentenylpyrophosphate is outlined in Figure 6. Brodie et al. (1963) have established a new pathway for the biosynthesis of mevalonic acid from malonyl CoA. The importance of this particular pathway in the synthesis of sterols is still unknown. 

Natural mbber is a polymer of 2-methyl-l, 3-butadiene, also called isoprene (Section 25.26). On average, a molecule of mbber contains 5000 isoprene units. As in the case of other naturally occurring terpenes, the five-carbon compound actually used in the biosynthesis of mbber is isopentenylpyrophosphate (Section 25.17). 

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