Epoxidation of squalene

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. 

This is not discussed in detail since mechanisms of resistance have been carefully reviewed (Ghannoum and Rice 1999). It was pointed out that resistance has not been associated with modification of the structure. For the 1,2,4-triazoles that have been widely used, their effect is due to inhibition of the synthesis of ergosterol that is the dominant component of fungal cell membranes. Resistance is generally associated with modification of the target enzymes, for example, the epoxidation of squalene (Terbinafine) or 14a-demethylase (Fluconazole). Resistance of Candida albicans to the azole antifungal agent fluconazole demonstrated, however, the simultaneous occurrence of several types of mechanism for resistance (Perea et al. 2001) ... 

Terbinafine Inhibits epoxidation of squalene in fungi increased levels are toxic to them Reduces ergosterol prevents synthesis of fungal cell membrane Mucocutaneous fungal infections Oral t duration, days Toxicity Gastrointestinal upset, headache, hepatoxicity t Interactions None reported... 

Butenafine hydrochloride (Mentax) is a benzylamine that is structurally related to the allylamines. As with the allylamines, butenafine inhibits the epoxidation of squalene, thus blocking the synthesis of ergosterol, an essential component of fungal cell membranes. Butenafine is available as a 1% cream to be applied once daily for the treatment of superficial dermatophytosis. 

Stereospecific 2,3-epoxidation of squalene. followed by a non-concerted carbocationic cyclization and a seiies of carbocationic rearrangements, forms lanosterol (26) in the first steps dedicated solely toward steroid synthesis. Cholesterol is the principal starting material for steroid hormone biosynthesis ill animals. The cholesterol biosynthetic pathway is composed of at least 30 enzymatic reactions. Lanosterol and squalene appear to he normal constituents, in trace amounis. in tissues that are actively synthesizing cholesterol,... 

When squalene is written in this form, we see that it is beautifully constructed for cyclization to lanosterol. The key intermediate that initiates the cyclization is the 2,3-epoxide of squalene. Enzymatic cleavage of the epoxide ring is fol-... 

Squalene oxide is formed by epoxidation of squalene catalyzed by the enzyme squalene epoxidase. 

Metalloporphyrins catalyze the autoxidation of olefins, and with cyclohexene at least, the reaction to ketone, alcohol, and epoxide products goes via a hydroperoxide intermediate (129,130). Porphyrins of Fe(II) and Co(II), the known 02 carriers, can be used, but those of Co(III) seem most effective and no induction periods are observed then (130). ESR data suggest an intermediate cation radical of cyclohexene formed via interaction of the olefin with the Co(III) porphyrin this then implies possible catalysis via olefin activation rather than 02 activation. A Mn(II) porphyrin has been shown to complex with tetracyanoethylene with charge transfer to the substrate (131), and we have shown that a Ru(II) porphyrin complexes with ethylene (8). Metalloporphyrins remain as attractive catalysts via such substrate activation, and epoxidation of squalene with no concomitant allylic oxidation has been noted and is thought to proceed via such a mechanism (130). Phthalocyanine complexes also have been used to catalyze autoxidation reactions (69). 

Role of squalene in the biosynthesis of steroids. The biosynthesis of steroids starts with epoxidation of squalene to squalene-2,3-epoxide. The opening of this epoxide promotes cyclization of the carbon skeleton under the control of an enzyme. The cyclized intermediate is converted to lanosterol, then to other steroids. 

Epoxidation of squalene with an enzyme, squalene epoxidase, gives squalene oxide, which contains a single epoxide on one of the six double bonds. 

Steroid biosynthesis occurs by enzyme-catalyzed epoxidation of squalene to 3deld squalene oxide, followed by acid-catalyzed cyclization and an extraordinary cascade of nine sequential carbocation reactions to yield lanosterol (Figure 27.6). Lanosterol is then degraded by other enzymes to... 

The ergosterol (45) biosynthesis in fungi and leishmania utilizes squalene (41) as starting material, which is obtained from long chain precursors like farnesyl pyrophosphate (39) and presqualene pyrophosphate (40). Epoxidation of squalene in the presence of squalene epoxidase furnishes squalene epoxide (42), which is succes-... 

A group of fungicides that inhibit squalene epoxidation has been developed primarily for use against pathogenic fungi in medicine. Epoxidation of squalene is catalyzed by squalene epoxidase (a flavoprotein) that starts the complicated cyclization of squalene. The squalene-2,3-epoxide formed by this enzyme is further metabolized to a protosterol cation intermediate, which is transformed to either cycloartenol in plants (cycloartenol synthase) or lanosterol (lanosterol synthase). Cycloartenol is the precursor to plant sterols, whereas lanosterol is the precursor of cholesterol and the other sterols in animals, and to ergosterol in plants. 

Squalene is very important in nature as it is the precursor of steroids, a family of molecules that serve as hormones in numerous organisms. The formation of the steroid framework from the mono-epoxide of squalene is shown in Figure 2.15. 

FIGURE 12.79 Enzymatic epoxidation of squalene gives squalene oxide, redrawn in a suggestive fashion at the bottom. 

Lanosterol biosynthesis begins with the selective epoxidation of squalene to give (35)-2,3-oxidosqualene, catalyzed by squalene epoxidase. Molecular O2 provides the source of the epoxide oxygen atom, and NADPH is required, along with a flavin coenzyme. The proposed mechanism involves reaction of FADH2 with O2 to produce a flavin hydroperoxide intermediate (ROOH), which transfers an oxygen to squalene in a pathway initiated by nucleophilic... 

Further studies have been reported on the rat liver mierosomal enzyme squalene epoxidase, which catalyses epoxidation of squalene (12) to squalene 2,3-oxide (25). For epoxidase activity there is a requirement for oxygen, NADPH, microsomes, a heat-labile cytoplasmic protein, and a phospholipid. The system is also slightly stimulated by FAD, which might be a component of the electron-transport chain for oxygen activation. The heat-labile supernatant protein has a molecular weight of 44000, but does not bind either squalene, 10,11-dihydrosqualene (which can also act as substrate for the enzyme), or squalene 2,3-oxide, which suggests that it does not have carrier activity analogous to the reported sterol carrier proteins. There is also no evidenee at present that this supernatant acts catalytically. 

Epoxidation of squalene afforded a mixture of squalene 2,3-oxide along with the two trans internal oxides shown above. Controlled acidic hydrolysis allowed selective opening of the terminal epoxide moiety to a diol from which the desired internal oxides were readily separated by thiourea clathrate formation. 

Nature uses epoxides in a multitude of biosyntheses because of their high reactivity. An example is the (5)-2,3-epoxide of squalene 7, which is cyclized to give a pentacyclic triterpene 8 of the gammacerane type [4] (Fig. 2). 

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