Biosynthetic pathways Mevalonic acid

BRs belong to the class of molecules known as triterpenoids. Because BRs are a group of modified sterols, the BR biosynthetic pathway can be divided into two major parts the sterol-specific pathway (which converts squalene to campesterol) and the BR-specific pathway (which converts campesterol to the BR). In the sterol-specific pathway, mevalonic acid (which is the starting material in terpenoid biosynthesis) is condensed and cyclized to produce 2,3-oxidosqualene. This precursor is further modified to form the major plant sterols such as sitosterol and campesterol. To become bioactive BRs, sterols must be processed by the BR-specific pathway. 

Precursor selection An important step in designing a microbial labeling process is the identification of one or more suitable labeled precursors. Generally, secondary metabolites are biosynthesized via primary metabolites from five metabolic sources. These are amino acids, shikimic acid (shikimic acid pathway), acetate and its homologues (polyketide pathway), mevalonic acid (isoprene pathway) and carbohydrates. Selection of a suitable precursor is primarily influenced by the biosynthetic pathway(s) involved, but also depends on the desired position of label in the product and the availability of labeled precursors. 

Eicosanoids and terpenoids are still other classes of lipids. Eicosanoids, of which prostaglandins are the most abundant kind, are derived biosynthetically from arachidonic acid, are found in all body tissues, and have a wide range of physiological activity. Terpenoids are often isolated from the essential oils of plants, have an immense diversity of structure, and are produced biosynthetically from the five-carbon precursor isopentenyl diphosphate (IPP). lsopentenyl diphosphate is itself biosynthesized from 3 equivalents of acetate in the mevalonate pathway. 

Terpenoid substances are of broad distribution and diverse function in insects. One set, elaborated by the mandibular glands of Acanthomyops claviger, acts both as a defensive secretion and as an alarm releaser. When fed Cu-labeled acetate or mevalonate, laboratory colonies of these ants produce radioactive citronellal and citral, providing unambiguous evidence for de novo synthesis of these terpenes by the ant. The incorporations of these precursors implicate the mevalonic acid pathway as the likely biosynthetic route. 

Fig. 2 Two possible biosynthetic pathways to pyrethrolone. The [l-13C]D-glucose-derived 13C labels that occur in the mevalonic acid and 2-C-methyl-D-erythritol 4-phosphate (13) pathways are colored in red and green, respectively. The phosphate moiety is indicated as P ...

In the past decade, eight inherited disorders have been linked to specific enzyme defects in the isoprenoid/cholesterol biosynthetic pathway after the finding of abnormally increased levels of intermediate metabolites in tissues and/or body fluids of patients (Table 5.1.1) [7, 9, 10]. Two of these disorders are due to a defect of the enzyme mevalonate kinase, and in principle affect the synthesis of all isoprenoids (Fig. 5.1.1) [5]. The hallmark of these two disorders is the accumulation of mevalonic acid in body fluids and tissues, which can be detected by organic acid analysis, or preferably, by stable-isotope dilution gas chromatography (GC)-mass spectrometry (GC-MS) [2]. Confirmative diagnostic possibilities include direct measurement of mevalonate kinase activities in white blood cells or primary skin fibroblasts [3] from patients, and/or molecular analysis of the MVK gene [8]. 

It had already been stated earlier that clavine alkaloids are formed from L-tryptophan and mevalonic acid, the methyl group in position 6 originating from methionine (53, 54). There was also evidence that 4-dimethylallyltryptophan (3) is an early intermediate in the biosynthetic pathway (58). 

The biosynthesis of monoterpenoids and camphor has been described by several authors (108-llU). Ruzicka (115,116) proposed a unified biogenetic scheme for terpenes. The biosynthetic building blocks for these terpenes are iso-prene units. The biosynthetically active isoprene units are isopentenyl pyrophosphate [l] and dimethyl allyl pyrophosphate [2] the compounds that are derived from acetate via mevalonic acid (Scheme V). Geranyl pyrophosphate [3] is the C-10 precursor for the terpenes (117). Banthorpe and Baxendale (ll8) confirmed the biosynthetic pathway of (iamphor via acetate mevalonate by conducting degradation study of camphor, biosynthesized from l c labelled mevalonic acid. The biosynthesis of camphor is summarised in Scheme VI. 

This brief survey of natural products derived from the 3-deoxy-D-oro6iTk>-heptulosonic acid 7-phosphate pathway illustrates the economy of fimda-mental biosynthetic pathways. The relative economy and simplicity of the biological degradative and energy-yielding reactions is paralleled in the biosynthetic reactions. For example, 3-deoxy-D-oroW o-heptulosonic acid 7-phosphate is a precursor of a host of aromatic products mevalonic acid is the progenitor of the terpenoids and steroids, and 5-aminolevulinic acid of the porphyrins. 

High serum cholesterol levels are an important risk factor for coronary heart disease and an effective way of reducing serum cholesterol is to inhibit sterol biosynthesis. The "statins" work by inhibiting 3-hydroxy-3-methylglutaryl CoA reductase (HMG CoA reductase), a major regulatory enzyme in the cholesterol biosynthetic pathway which catalyses the rate-limiting conversion of HMG CoA to mevalonic acid. 

The biosynthetic pathway to the ergoline nucleus proceeds through 4-dimethylallyl tryptophan (4-DMAT), chanoclavine-I, agroclavine, and lysergic acid. Two cis, trans isomerizations occur one before chanocla-vine-I and the other before agroclavine, as shown by experiments with [2- C]-mevalonic acid and [Z-CH3]-4-DMAT (Fig. 36). The peptide unit is derived from a combination of three amino acids, one of which is always proline. Several genera in the plant family Convolvulaceae Rivea, Ipomoea, etc.) also produce ergot alkaloids. 

The diverse, widespread and exceedingly numerous class of natural products that are derived from a common biosynthetic pathway based on mevalonate as parent, are synonymously named terpenoids, terpenes or isoprenoids. Essentially, they are derived from the basic 5-carbon isoprene unit, biosynthetically as isopentenyl pyrophosphate, which is itself derived from aeetate via mevalonic acid. They may be classified into diverse groups according to the number of isoprene units, e.g.. 

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. 

Lovastatin is a member of a class of drugs (atorvastatin and simvastatin are others in this class) called statins that are used to treat hypercholesterolemia. The statins act as competitive inhibitors of the enzyme HMG-CoA reductase. These molecules mimic the structure of the normal substrate of the enzyme (HMG-CoA) and act as transition state analogues. While the statins are bound to the enzyme, HMG-CoA cannot be converted to mevalonic acid, thus inhibiting the whole cholesterol biosynthetic process. Recent studies indicate that there may be important secondary effects of statin therapy because some of the medical benefits of statins are too rapid to be a result of decreasing atherosclerotic lesions. Statin therapy has been associated with reduced risks of dementia, Alzheimer disease, ischemic cerebral stroke, and other diseases that are not correlated with high cholesterol levels. Although this is still an active area of research, it appears that the pleiotropic effects of statins may be a result of a reduction in the synthesis of isoprenoid intermediates that are formed in the pathway of cholesterol biosynthesis. 

Famesyl groups are produced in the same biosynthetic pathway as cholesterol both are derived from a building block called mevalonic acid (Figure 2). In this pathway, the 15-carbon compound, famesyl pyrophosphate, is produced, and this is dimerized to give the 30-carbon product squalene, which is converted into cholesterol. As will be described below, famesyl pyrophosphate is the source of the famesyl group in farnesylated peptides and proteins. 

Figure 2. The mevalonic acid biosynthetic pathway. The transformation of hy-droxymethyl-coenzyme A (HMG-CoA) to mevalonic acid is the first committed step of the pathway. The enzyme, HMG-CoA reductase, catalyzes this step and is inhibited by the compounds, mevinolin and compactin. Note that farnesyl-pyrophosphate (Farnesyl-PP), the substrate of the protein, farnesyltransferase, can be used to make cholesterol or elongated to make geranylgeranyl-pyrophosphate (Geranylgeranyl-PP). The later compound is the substrate for the protein, geranylgeranyltransferase, or is further elongated to make the long-chain isoprenoids, dolichols, ubiquinones, and isoprenoic acids.

As seen in figure 1, carbohydrates are degraded to pyruvic acid, which is oxidised to acetate in a form which can condense to form fatty acids and polyketides (includes aromatic benzenoid molecules). Fatty acids react with glycerol to give fats or lipids. A different biosynthetic pathway leads from acetate via the condensation product mevalonic acid to mono-, sesqui- and diterpenes which are ingredients... 

Like all steroids, brassinosteroids derive from a single common precursor mevalonic acid. Some phytohormones are synthesized totally or in part via the isoprenoid pathway, such as absdsic acid, gibberellins and cytokinins. The importance of this biosynthetic pathway in processes involved with cell cycle regulation and tumorigenesis in mammals is well documented. Having in mind the similarities between certain regulatory systems in plants and animals the question arose whether brassinosteroids as putative plant steroid hormones would show a specific effect on plant tumor cells. 

The early steps in the ergot alkaloid biosynthetic pathway are outlined in Fig. 1. The first determinant and rate-limiting step is the prenylation of tryptophan to 4-(y,y-dimethylallyl)tryptophan (DMAT), catalyzed by dimethy-lallyl-diphosphate L-tryptophan dimethylallyltransferase (DMAT synthase EC 2.5.1.34) (Heinstein et al., 1971 Gebler and Poulter, 1992). The prenyl group for the DMAT synthase reaction is provided in the form of dimethylallyl diphosphate (DMAPP), which is derived from mevalonic acid. After the formation of DMAT, the free amino group of this intermediate is N-methylated with a methyl group donated by S-adenosylmethionine (AdoMet). The N-methylated DMAT is then converted into chanoclavine I by closure of the... 

The results of an investigation of the biosynthesis of cyclobuxine D from [2- C, (4i )-4- Hi]mevalonic acid in Buxus sempervirens are consistent with the labelling pattern (39) and a biosynthetic pathway from cycloartenol via C-3 and C-20 ketonic intermediates/ Buxozine C (40) is a new alkaloid from B. semper-virens. ... 

Mevalonic acid (MVA) (69) is a very important biosynthetic intermediate used by us to make steroids and by plants to make terpenes. Tracing these biosynthetic pathways required radioactively labelled MVA and J. W. Cornforth has published many such syntheses. MVA has two 1,3-dicarbonyl relationships (70), one of which can be disconnected to (71) and (72). If the OH and CO2H groups are both protected as esters, we can use the Reformatsky method to activate (72) and finally remove both protecting groups in one step. 

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