Squalene in steroid biosynthesis

Suggest a biosynthetic route to the monoterpene chrys-anthemic acid that uses a reaction similar to the formation of squalene in steroid biosynthesis,... 

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). 

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. 

The synthesis of tricyclic compounds based on the cyclisation of methyl geranyl-geranoate was described eighteen years ago. However, the discovery of expoxy-squalene as an intermediate in steroid biosynthesis, and the corresponding in vitro cyclisation experiments, have re-awoken interest in this area of diterpene synthesis. The structural and stereochemical course of the in vitro cyclisation of the epoxy-trans-olefin (138) has been studied. Two A/B-trans-fused tricyclic compounds (139) and (140) have been isolated. The levantenolides (142) and (143) have been obtained by two closely related routes. In the first, the butenolide (141), derived from monocyclofamesyl bromide, afforded the a- and P levantenolides (142) and (143) on cyclisation with stannic chloride. In the second, the corresponding acyclic butenolide derived from famesyl bromide was cyclised. 

Non-Arthropod Invertebrates.— The Protozoa are able to synthesise terpenoids [e.g. (99)]. However, there is no evidence for squalene or steroid biosynthesis in Coelenterata, Echinodermata, Nematoda, or Platyhelminthes. In some cases they may once have been able to synthesise their own steroids, and not need a dietary source. Hymenolepis dimunuta is able to synthesise famesol from mevalonic acid. In the phylum Annelida the position is less clear. While some species can synthesise steroids others are only able to synthesise squalene. ... 

As in previous Reports this section is particularly concerned with non-vertebrate species and their ability to synthesize steroids from simple precursors. Further Protozoan studies have confirmed their ability to synthesize steroids. However, in the Porifera one species had this ability but another did not. Coelenter-ate examples again showed the absence of squalene or steroid biosynthesis. 

Recall from the Connections highlight on page 550 that the cationic cyclization of squalene is a key step in steroid biosynthesis. It has been found that the active site of enzymes that catalyze this type of transformation is unusually rich in aromatic amino acids (Phe, Tyr, and Trp). Why might this be so ... 

What is the nature of the substance with the Cs chain Let us recall the S3mthesis of isoprene, the starting point for carotenoids and terpenes, from acetate we see that this synthesis leads to the Cs arrangement we are seeking. Arguments have been advanced previously to implicate the iso-prenoid, squalene, in the biosynthesis of steroids. Squalene is not limited solely to the tissues of the selachians. Traces have been foimd everywhere in animal tissues where it has been sought. When we consider that isoprene units, as we have already indicated, are derived from acetate, we may represent squalene as in Fig. 62 and compare it with Fig. 61. 

FIGURE 8.8 A biological epoxi-dation reaction of the alkene squalene, a step in steroid biosynthesis. The reaction is effected by a flavin hydroperoxide formed by reaction of O2 with the coenzyme reduced flavin adenine dinucleotide, FADH2. 

Squalene is also an intermediate in the synthesis of cholesterol. StmcturaHy, chemically, and biogeneticaHy, many of the triterpenes have much in common with steroids (203). It has been verified experimentally that squalene is the precursor in the biosynthesis of all triterpenes through a series of cyclization and rearrangement reactions (203,204). Squalene is not used much in cosmetics and perfumery formulations because of its light, heat, and oxidative instabiUty however, its hydrogenated derivative, squalane, has a wide use as a fixative, a skin lubricant, and a carrier of Hpid-soluble dmgs. 

Processes of this kind are important in the biosynthesis of steroids and tetra- and pentacyclic terpenes. For example, squalene 2,3-oxide is converted by enzymatic catalysis to dammaradienol. 

Figure 26-3. Biosynthesis of cholesterol. The numbered positions are those of the steroid nucleus and the open and solid circles indicate the fate of each of the carbons in the acetyl moiety of acetyl-CoA. Asterisks Refer to labeling of squalene in Figure 26-2.

Squalene takes part in metabolism as precursor for synthesis of steroids and structurally quite similar to (3-carotene, coenzyme qlO, vitamins Ki, E, and D. The squalene in skin and fat tissue comes from endogenous cholesterol synthesis as well as dietary resources in people who consume high amounts of olive and fish oil especially shark liver (Gershbein and Singh, 1969). Squalene is synthesized by squalene synthase which converts two units of farnesyl pyrophosphate, direct precursor for terpenes and steroids, into squalene. As a secosteroid, vitamin D biosynthesis is also regulated by squalene. Moreover, being precursor for each steroid family makes squalene a crucial component of the body. 

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-... 

Squalene and phytosterols are components present in the unsaponifiable lipid fraction of foods (as tocopherols). Squalene is an intermediary in cholesterol biosynthesis, and 33.9-58.4 mg/100 g of it was found in the lipid fraction of quinoa (Jahaniaval et al., 2000 Ryan et ah, 2007) squalene is the biochemical precursor of the whole family of steroids, and besides their effective antioxidant activity, tocotrienols have other important functions, in particular in maintaining a healthy cardiovascular system and a possible role in protection against cancer (Nesaretnam, 2008). Squalene is used as a bactericide and as an intermediate in many pharmaceuticals, organic coloring materials, rubber chemicals, and surface-active agents (Ahamed et ah, 1998). 

Cationic cyclizations of polyolefins to oligocyclic terpenes such as that catalyzed by squalene-cyclases in the biosynthesis of steroids have long fascinated chem-... 

Biosynthetically-patterned microbiological transformations exploit the substrate flexibility of enzymes involved in the biosynthesis of secondary metabohtes. These biotransformations are sometimes known as analogue biosynthesis or precursor-directed biosynthesis. This approach to biotransformation can be useful in preparing analogues of biologically-active microbial metabolites for structure-activity studies, a feature that has been exploited with penicillins using a cloned isopenicillin N synthase. The structures of the substances that are transformed and of their products can also shed light on the stereo-electronic constraints of enzymatic steps and on the nature of biosynthetic intermediates, a feature that has been exploited in studies on the cyclization of squalene to the triterpenes and steroids. 

The Mollusca are an interesting phylum taxonomically. From the limited data available the classes Bivalva and Cephalopoda are not able to synthesise steroids or squalene. However, in the class Gastropoda only the order Stenoglossa cannot synthesise steroids. In the order Stylommatophora there is a limited ability, and steroid biosynthesis is present in Archaeogastropoda, Mesogastropoda, and Basommatophora. ... 

Cholesterol and many of its biosynthetic precursors are highly insoluble in aqueous media. Yet, cholesterol biosynthesis, utilization and intracellular transfers occur in environments which involve both aqueous and nonaqueous components. For example, the enzymes involved in the conversion of squalene to cholesterol, the conversion of cholesterol to cholesterol esters, and the conversion of cholesterol to 7a-hydroxycholesterol are associated with the endoplasmic reticulum (microsomes). The conversion of cholesterol to pregnenolone, an essential first step in steroid hormone biosynthesis, occurs in mitochondria. In addition, transfers of cholesterol from cytoplasmic lipid inclusion droplets through the cytosol to the mitochondria are essential for steroid hormone production. 

The next step in the biosynthesis of steroids features an unusual head-to-head coupling reaction of two famesol pyrophosphates (OPP, not shown in diagram) to afford the alicyclic triterpene squalene 3-1, a compound found in shark liver oil (Scheme 2.3). Note that this product is in fact symmetrical about the newly formed bond. The next reaction in the sequence, which has only recently been uncovered, comprises oxidation of the terminal double bond to an epoxide. Opening of the oxirane leads to a domino-like series of ring-closing reactions and also concomitant migration of methyl groups. This chain reaction can be, and in fact has been, duplicated in the laboratory in the absence of enzymes. This series of reactions leads to the hypothetical steroidal carbocation 3-2. 

Squalene is an intermediate in cholesterol and steroid biosynthesis. It is formed from presqualene pyrophosphate (Figure 19.21) in the walls of the endoplasmic reticulum using electrons from NADPH. In the reaction, the pyrophosphate is removed from the molecule. 

The most abundant member of the steroid family in animals is cholesterol, the precursor of all other steroids. Cholesterol is an important component of cell membranes its ring structure causes it to be more rigid than other membrane lipids. In the biosynthesis of cholesterol, squalene is converted to lanosterol, which is converted to cholesterol. 

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