Lanosterol

C. A trilerpenoid or irimethylsierol, first found in the non-saponifiable material of wool wax. Lanosterol (4,4,14ot-trimethyl-5a-choiesta-8,24-dien-3 -ol) is the precursor in animals and fungi of other sterols such as... 

Open-chain 1,5-polyenes (e.g. squalene) and some oxygenated derivatives are the biochemical precursors of cyclic terpenoids (e.g. steroids, carotenoids). The enzymic cyclization of squalene 2,3-oxide, which has one chiral carbon atom, to produce lanosterol introduces seven chiral centres in one totally stereoselective reaction. As a result, organic chemists have tried to ascertain, whether squalene or related olefinic systems could be induced to undergo similar stereoselective cyclizations in the absence of enzymes (W.S. Johnson, 1968, 1976). 

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... 

Lanosterol is one component of lanolin a mixture of many substances that coats the wool of sheep... 

The conversion of lanosterol to cholesterol involves 19 steps and is described in the article Cholesterol Biosynthesis Lanosterol to Cholesterol on pp 377-384 in the March 2002 issue of the Journal of Chemical Education... 

The path from squalene (114) to the corresponding oxide and thence to lanosterol [79-63-0] (126), C qH qO, cholesterol [57-88-5] (127), and cycloartenol [469-38-5] (128) (Fig. 6) has been demonstrated in nonphotosynthetic organisms. It has not yet been demonstrated that there is an obligatory path paralleling the one known for generation of plant sterols despite the obvious stmctural relationships of, for example, cycloartenol (128), C qH qO, to cyclobuxine-D (129), C25H42N2O. The latter, obtained from the leaves of Buxus sempervirens E., has apparentiy found use medicinally for many disorders, from skin and venereal diseases to treatment of malaria and tuberculosis. In addition to cyclobuxine-D [2241-90-9] (129) from the Buxaceae, steroidal alkaloids are also found in the Solanaceae, Apocynaceae, and LiUaceae. 

Other, removable cation-stabilizing auxiliaries have been investigated for polyene cyclizations. For example, a sdyl-assisted carbocation cyclization has been used in an efficient total synthesis of lanosterol. The key step, treatment of (257) with methyl aluminum chloride in methylene chloride at —78° C, followed by acylation and chromatographic separation, affords (258) in 55% yield (two steps). When this cyclization was attempted on similar compounds that did not contain the C7P-silicon substituent, no tetracycHc products were observed. Steroid (258) is converted to lanosterol (77) in three additional chemical steps (225). 

Triterpenes. The triterpenes (30 carbon atoms) are widely found in nature, especially plants, both in the free state and as esters or glycosides. A smaller but important group, including lanosterol [79-63-0] (114), occurs in animals. The triterpene hydrocarbon, squalene [111-02-4] (115), occurs in the hver oils of certain fish, especially those of sharks. 

The alcohol fraction is likewise a complex mixture of both aUphatic and cycHc compounds (Table 10). The principal components are cholesterol (34%), and lanosterol and dihydrolanosterol (38%). The aUphatic alcohols account for about 22% of the unsaponifiable products. Sixty-nine components of ahphatic alcohols had been reported up to 1974 (latest reported work as of ca 1997). The hydrocarbons (ca 0.5%) show stmctural similarity to the wool—wax acids or ahphatic alcohols and contain highly branched alkanes as well as cycloalkanes. 

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. 

The most important oxirane, from an anthropocentric viewpoint, is probably squalene oxide (72), a precursor of lanosterol (73) and thus of the maligned but essential cholesterol (74 Scheme 87) 78MI50501). The cyclization of (72) to (73) represents nucleophilic tr-attack on oxirane carbon cf. Section 5.05.3.4.3(t)()), and the process has also been extensively investigated in vitro (68ACR1). Oxiranes are even more ubiquitous in steroid biosynthesis than had been thought, for a cholesterol epoxide has been shown to be a product of mammalian steroid biosynthesis <81JA6974). 

Lanosterol [79-63-0] M 426.7, m 138-140", [a] d +<52.0" (c 1, CHCI3). Recrystd from anhydrous MeOH. Dried in vacuo over P2O5 for 3h at 90°. Purity checked by proton magnetic resonance. 

Squalene monooxygenase, an enzyme bound to the endoplasmic reticulum, converts squalene to squalene-2,3-epoxide (Figure 25.35). This reaction employs FAD and NADPH as coenzymes and requires Og as well as a cytosolic protein called soluble protein activator. A second ER membrane enzyme, 2,3-oxidosqualene lanosterol cyclase, catalyzes the second reaction, which involves a succession of 1,2 shifts of hydride ions and methyl groups. 

Conversion of Lanosterol to Cholesterol Requires 20 Additional Steps... 

Although lanosterol may appear similar to cholesterol in structure, another 20 steps are required to convert lanosterol to cholesterol (Figure 25.35). The enzymes responsible for this are all associated with the endoplasmic reticulum. The primary pathway involves 7-dehydroeholesterol as the penultimate intermediate. An alternative pathway, also composed of many steps, produces the intermediate desmosterol. Reduction of the double bond at C-24 yields cholesterol. Cholesterol esters—a principal form of circulating cholesterol—are synthesized by acyl-CoA cholesterol acyltransferases (ACAT) on the cytoplasmic face of the endoplasmic reticulum. 

Terpenes (and terpenoids) are further classified according to the number of 5-carbon units they contain. Thus, monoterpenes are 10-carbon substances biosynthesized from two isoprene units, sesquiterpenes are 15-carbon molecules from three isoprene units, diterpenes are 20-carbon substances from four isoprene units, and so on. Monoterpenes and sesquiterpenes are found primarily in plants, but the higher terpenoids occur in both plants and animals, and many have important biological roles. The triterpenoid lanosterol, for example, is the precursor from which all steroid hormones are made. 

Isoprene itself is not the true biological precursor of terpenoids. As we ll see in Chapter 27, nature instead uses two "isoprene equivalents"—isopentenvl diphosphate and dimethylallyl diphosphate—which are themselves made by two different routes depending on the organism. Lanosterol, in particular, is biosynthesized from acetic acid by a complex pathway that has been worked out in great detail. 

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 second part of lanosterol biosynthesis is catalyzed by oxidosqualene lanosterol cyclase and occurs as shown in Figure 27.14. Squalene is folded by the enzyme into a conformation that aligns the various double bonds for undergoing a cascade of successive intramolecular electrophilic additions, followed by a series of hydride and methyl migrations. Except for the initial epoxide protonation/cyclization, the process is probably stepwise and appears to involve discrete carbocation intermediates that are stabilized by electrostatic interactions with electron-rich aromatic amino acids in the enzyme. 

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