Squalene conversion

FIGURE 26 10 The biosyn thetic conversion of squa lene to cholesterol proceeds through lanosterol Lano sterol IS formed by enzyme catalyzed cyclization of the 2 3 epoxide of squalene... 

Like the a2ole derivatives, it inhibits the biosynthesis of ergosterol. However, naftifine [65472-88-0] does not inhibit the cytochrome P-450 dependent C-14-demethylase, but the epoxidation of squalene. Squalene epoxidase cataly2es the first step in the conversion of squalene via lanosterol to ergosterol in yeasts and fungi or to cholesterol in mammalian cells. The squalene epoxidase in C. albicans is 150 times more sensitive to naftifine, C2 H2 N, than the en2yme in rat fiver (15). Naftifine is available as a 1% cream. 

Steroids are heavily modified triterpenoids that are biosynthesized in living organisms from farnesyl diphosphate (Cl5) by a reductive dimerization to the acyclic hydrocarbon squalene (C30), which is converted into lanosterol (Figure 27.12). Further rearrangements and degradations then take place to yield various steroids. The conversion of squalene to lanosterol is among the most... 

Lanosterol biosynthesis begins with the selective conversion of squalene to its epoxide, (35)-2,3-oxidosqualene/ catalyzed by squalene epoxidase. Molecular 02 provides the source of the epoxide oxygen atom, and NADPH is required, along with a flavin coenzyme. The proposed mechanism involves... 

The biomimetic approach to total synthesis draws inspiration from the enzyme-catalyzed conversion of squalene oxide (2) to lanosterol (3) (through polyolefinic cyclization and subsequent rearrangement), a biosynthetic precursor of cholesterol, and the related conversion of squalene oxide (2) to the plant triterpenoid dammaradienol (4) (see Scheme la).3 The dramatic productivity of these enzyme-mediated transformations is obvious in one impressive step, squalene oxide (2), a molecule harboring only a single asymmetric carbon atom, is converted into a stereochemically complex polycyclic framework in a manner that is stereospecific. In both cases, four carbocyclic rings are created at the expense of a single oxirane ring. 

Interest in presqualene pyrophosphate continues and it is claimed that the structure (88) assigned to a squalene precursor is incorrect. Presqualene pyrophosphate has been shown to contain a cyclopropyl ring (89), and both (89) and its parent alcohol have been synthesised. -Mechanisms for the conversion of (89) into squalene have been pub-lished. -... 

Polyene cyclizations are of substantial value in the synthesis of polycyclic terpene natural products. These syntheses resemble the processes by which the polycyclic compounds are assembled in nature. The most dramatic example of biosynthesis of a polycyclic skeleton from a polyene intermediate is the conversion of squalene oxide to the steroid lanosterol. In the biological reaction, an enzyme not only to induces the cationic cyclization but also holds the substrate in a conformation corresponding to stereochemistry of the polycyclic product.17 In this case, the cyclization is terminated by a series of rearrangements. 

The 13C-labelled squalene has been used71 to study the mechanism of its enzymatic conversion to lanosterol (3-/J-hydroxy-8,24-lanostadiene72) by yeast squalene-oxide lanosterol... 

Analysis of the details of the pathway was helped by the discovery by Nancy Bucher (1953) that cholesterol synthesis took place in cell-free post-mitochondrial supernatants. ATP, Mg2+ and NAD+ were required. Tchen and Bloch extended these findings to show that squalene could be formed anaerobically but the conversion of squalene to cholesterol was oxygen dependent, the oxygen of the intermediate lanosterol being derived from 8C>2 not H2180. It therefore became possible to focus either on the conversion of acetate to squalene or on the latter s cyclization to the sterol. 

A general type of chemical reaction between two compounds, A and B, such that there is a net reduction in bond multiplicity (e.g., addition of a compound across a carbon-carbon double bond such that the product has lost this 77-bond). An example is the hydration of a double bond, such as that observed in the conversion of fumarate to malate by fumarase. Addition reactions can also occur with strained ring structures that, in some respects, resemble double bonds (e.g., cyclopropyl derivatives or certain epoxides). A special case of a hydro-alkenyl addition is the conversion of 2,3-oxidosqualene to dammara-dienol or in the conversion of squalene to lanosterol. Reactions in which new moieties are linked to adjacent atoms (as is the case in the hydration of fumarate) are often referred to as 1,2-addition reactions. If the atoms that contain newly linked moieties are not adjacent (as is often the case with conjugated reactants), then the reaction is often referred to as a l,n-addition reaction in which n is the numbered atom distant from 1 (e.g., 1,4-addition reaction). In general, addition reactions can take place via electrophilic addition, nucleophilic addition, free-radical addition, or via simultaneous or pericycUc addition. 

A metabolite, molecular entity, or some other event/ process that precedes another component in a longer sequence of events or conversions. For example, the isoprenoid metabolite squalene is a precursor of cholesterol and glucose 6-phosphate is a precursor of glycogen, ribose, and pyruvate. See Series First Order Reaction Pulse-Chase Experiments... 

This enzyme [EC 2.5.1.21], officially known as farnesyl-diphosphate farnesyltransferase (and also referred to as farnesyltransferase and presqualene-diphosphate synthase), catalyzes the conversion of two molecules of farnesyl diphosphate to yield presqualene diphosphate and pyrophosphate (or, diphosphate). In its polymeric form, the enzyme then catalyzes the NADPH-dependent reduction of presqualene diphosphate to form squalene. See also Farnesyl Diphosphate Farnesyltransferase... 

Further detailed study of the substrate specificity of yeast squalene synthetase has been reported (see Vol. 7, p. 130). The enzyme is very sensitive to changes in substrate. For example, 10,11-dihydrofarnesyl pyrophosphate was converted into 2,3,22,23-tetrahydrosqualene with only 60% of the efficiency of farnesyl pyrophosphate whereas 6,7-dihydro- and 6,7,10,11-tetrahydro-farnesyl pyrophosphates were not metabolized. The first of the two binding sites has a greater preference for farnesyl pyrophosphate and this accounts for the formation of the unsymmetrical squalene product when mixtures of farnesyl pyrophosphate and a modified substrate are used. The importance of the methyl groups, especially that at C-3, for binding is emphasized by the low efficiency of conversion of 3-desmethylfarnesyl, , -3-methylundeca-2,6-dien-l-yl (1), and E,E-7-desmethylfarnesyl pyrophosphates. The prenylated cyclobutanones (2) and (3)... 

The all-tra 5 -squalene (C30H50), discovered in shark liver oil in the 1920s, is a triterpene, but one in which the isoprene rule at violated in one point. Rather than a head-to-tail arrangement of six units of isoprene, there appear to be farnesyl units that have been connected tail to tail. Almost aU steroids are biosynthesized from cholesterol. Cholesterol is biosynthesized from squalene, which is first converted to lanosterol. The conversion of squalene to the steroid skeleton is an oxirane, squalene-2,3-oxide, which is transformed by enzymes into lanosterol, a steroid alcohol naturally found in wool fat. The whole process is highly stereoselective. 

Squalene is an important biological precursor of many triterpenoids, one of which is cholesterol. The first step in the conversion of squalene to lanosterol is epoxidation of the 2,3-douhle bond of squalene. Acid-catalysed ring opening of the epoxide initiates a series of cyclizations, resulting in the formation of protesterol cation. Elimination of a C-9 proton leads to the 1,2-hydride and 1,2-methyl shifts, resulting in the formation of lanosterol, which in turn converted to cholesterol by enzymes in a series of 19 steps. 

Reactions described in this chapter that are catalyzed by mixed-function oxidases are those involved in fatty acyl-CoA desaturation (Fig. 21-13), leukotri-ene synthesis (Fig. 21-16), plasmalogen synthesis (Fig. 21-30), conversion of squalene to cholesterol (Fig. 21-37), and steroid hormone synthesis (Fig. 21-47). 

Synthesis takes place in four stages, as shown in Figure 21-33 (D condensation of three acetate units to form a six-carbon intermediate, mevalonate (2) conversion of mevalonate to activated isoprene units (3) polymerization of six 5-carbon isoprene units to form the 30-carbon linear squalene and ( ) cyclization of squalene to form the four rings of the steroid nucleus, with a further series of changes (oxidations, removal or migration of methyl groups) to produce cholesterol. 

FIGURE 21-35 Conversion of mevalonate to activated isoprene units. Six of these activated units combine to form squalene (see Fig. 21-36). The leaving groups of 3-phospho-5-pyrophosphomevalonate are shaded pink. The bracketed intermediate is hypothetical. 

Stage (4) Conversion of Squalene to the Four-Ring Steroid Nucleus When the squalene molecule is represented as in Figure 21-37, the relationship of its linear structure to the cyclic structure of the sterols becomes apparent. All... 

Most animal steroids arise from cholesterol, which in turn is derived from squalene. This C30 triterpene, whose biosynthesis is described in Section B, is named after the dogfish Squalus in whose liver it accumulates as a result of blockage in oxidation to cholesterol. Squalene is also a prominent constituent of human skin lipids. Its conversion to cholesterol, which takes place in most animal tissues,117/154-156 is initiated by a microsomal enzyme system that utilized 02 and NAD-PH to form squalene 2,3-oxide (Fig. 22-6, step a). 

Isotopic labeling experiments show that cholesterol is derived from ethanoate by way of squalene and lanosterol. The evidence for this is that homogenized liver tissue is able to convert labeled squalene to labeled lanosterol and thence to labeled cholesterol. The conversion of squalene to lanosterol is particularly interesting because, although squalene is divisible into isoprene units, lanosterol is not—a methyl being required at C8 and not C13 ... 

The biosynthesis of steroids begins with the conversion of three molecules of acetyl-CoA into mevalon-ate, the decarboxylation of mevalonate, and its conversion to isopentenyl pyrophosphate. Six molecules of isopentenyl pyrophosphate are polymerized into squalene, which is cyclized to yield lanosterol. Lanos-terol is converted to cholesterol, which is the precursor of bile acids and steroid hormones. 

AD of 2,6- , -famesyl acetate (3) with 8 as ligand occurred selectively at the terminal double bond to give 5 with 96% ee (position selectivity of about 120 1). In a special solvent system consisting of /ert-butanol-water-methylcyclohexane, at ca. 50% conversion of squalene (4) diol 6 was obtained in 32% yield with 90% ee. Here, the position selectivity was 8 1 [51. 

In the biogenesis of steroids, the enzyme-catalyzed polycyclization of squalene (225) produces the tetracyclic substance lanosterol (225) which is eventually converted into cholesterol (227) Eschenmoser, Stork, and their co-workers (80-82) have proposed that the squalene-1anosterol conversion can be rationalized on the basis of stereoelectronic effects. The stereochemical course of this biological cyclization (83, 84) can be illustrated by considering the transformation of squalene oxide (228) (an intermediate in the biosynthesis of cholesterol (83, 84)) into dammaradienol 229. This transfor-... 

Reactions at Saturated Carbons appear to mimic in principle the biogenetic conversion of squalene into polycyclic triterpenoids. This work has been well reviewed (85-87) and only a few representative examples will be described here. 

Similarly, the calibration graph may be non-linear, particularly if peak heights are used as a quantitative parameter, and during manipulations with low concentrations of the solute when its adsorption on the surface of the support, column walls, etc., occurs to a significant extent. Fig. 1.2 illustrates the improvement that was obtained by the conversion of the sample compound. In the direct determination of morphine by GC, the dependence of the ratio of the peak height of morphine to that of squalene on the amount of compound injected is non-linear and therefore quantitative evaluation is difficult. An analogous calibration graph for the TMS derivative, in contrast, is linear. Hence, if a suitable derivative is used a drawback that could interfere with the GC analysis itself can be overcome [2]. 

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