Cholesterol from squalene

Figure 19.18 Biosynthesis of cholesterol from squalene. (Reproduced by permission from Vance DE, Vance JE. Biochemistry of Lipids and Membranes. Menlo Park Benjamin/ Cummings, 1985, p. 412.)...

Hydroxylation reactions play a very important role in the synthesis of cholesterol from squalene and in the conversion of cholesterol into steroid hormones and hile salts. All these hydroxylations require NADPH and O. The oxygen atom of... 

A spectacular example of cationic cyclization is the Johnson polyene cyclization, described in Section 10.8.A. Polyenes such as squalene are expected to assume a steroid-like conformation in the lowest energy form (sec. 1.5.E), based on the biogenetic preparation of cholesterol from squalene. In practice, treatment of polyenes with acid led to a very low yield of tri- or tetracyclic products, giving instead significant amounts of polymeric material. Diligent work over many years prevailed, however, and Johnson solved the many problems (as described in sec. 10.8.A) to make this reaction an excellent and efficient synthetic route to di-, tri-, and tetracyclic molecules. One of the later examples of polyene cyclization uses an allyl silane to quench the cyclization process. A Lewis acid was used to initiate the reaction via reaction with the acetal. Treatment of... 

Within 2 years of the publication of the Whitmore paper, Robinson proposed the formation of the steroids (including cholesterol) from squalene (a C30 polyunsaturated polyisoprene molecule) via an incredible series of intermediates and rearrangements. Later, following the elucidation of the structure of lanosterol, R. B. Woodward and K. Bloch made a brilliant proposal that at once rationalized the biosynthetic origin of both lanosterol and cholesterol and implicated lanosterol as an intermediate in cholesterol bios)mthesis. Their mechanism involved the concerted (bonds made and broken simultaneously) cydization of four rings, as well as four rearrangements... 

The biosynthesis of cholesterol from squalene was examined. Using model structures and intrinsic reaction coordinate calculations, evidence was reported for the asynchronous, concerted reaction of squalene oxide to the protosterol cation. 

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. 

In 1952, Konrad Bloch and Robert Langdon showed conclusively that labeled squalene is synthesized rapidly from labeled acetate and also that cholesterol is derived from squalene. Langdon, a graduate student of Bloch s, performed the critical experiments in Bloch s laboratory at the University of Chicago, while Bloch spent the summer in Bermuda attempting to demonstrate that radioactively labeled squalene would be converted to cholesterol in shark livers. As Bloch himself admitted, All I was able to learn was that sharks of manageable length are very difficult to catch and their oily livers impossible to slice (Bloch, 1987). 

Perhaps the most spectacular of the natural carbocation rearrangements is the concerted sequence of 1,2-methyl and 1,2-hydride Wagner-Meerwein shifts that occurs during the formation oflanosterol from squalene. Lanosterol is then the precursor of the steroid cholesterol in animals. 

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. 

The synthesis of all isoprenoids starts with acetyl-CoA, which in a series of six different enzyme reactions is converted into isopentenyl-diphosphate (-PP), the basic C-5 isoprene unit that is used for the synthesis of all subsequent isoprenoids (Fig. 5.1.1). At the level of farnesyl-PP the pathway divides into several branches that are involved in the production of the various isoprenoid end products. One of the major branches involves the cholesterol biosynthetic part of the pathway, of which squalene is the first committed intermediate in the production of sterols. Following cycliza-tion of squalene, lanosterol is produced. To eventually produce cholesterol from la-... 

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). 

One of the best therapeutic approaches may be to prevent absorption of cholesterol from the intestines by inclusion of a higher fiber content in the diet.66 Supplementation with a cholesterol-binding resin may provide additional protection. Plant sterols also interfere with cholesterol absorption. Incorporation of esters of sitostanol into margarine provides an easy method of administration. Supplemental vitamin E may also be of value.q Another effective approach is to decrease the rate of cholesterol synthesis by administration of drugs that inhibit the synthesis of cholesterol. Inhibitors of HMG-CoA reductase,s hh (e.g., vaLostatin) iso-pentenyl-PP isomerase, squalene synthase (e.g.,... 

Fig. 2. Synthesis of squalene and cholesterol from isopentenyl pyrophosphate, (a) Isomerization of isopentenyl pyrophosphate to dimethylallyl pyrophosphate (b) synthesis of cholesterol.

Cholesterol is synthesized in the body from squalene, a C30 triterpene that is itself prepared from smaller terpenes, as discussed in Section 29.7B. Because the biosynthesis of all terpenes begins with acetyl CoA, every one of the 27 carbon atoms of cholesterol comes from the same two-caibon precursor. The major steps in the conversion of squalene to cholesterol are given in Figure 29.11. 

Animals accumulate cholesterol from their diet, but are also able to biosynthesize it from acetate. The pioneering work that identified the key intermediates in the complicated pathway of cholesterol biosynthesis was carried out by Konrad Bloch (Harvard) and Feodor Lynen (Munich), corecipients of the 1964 Nobel Prize for physiology or medicine. An important discovery was that the triterpene squalene (see Figure 26.6) is an intermediate in the formation of cholesterol from acetate. Thus, the early stages of cholesterol biosynthesis are the same as those of terpene biosynthesis described in Sections... 

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. 

Previous investigations from several laboratories have demonstrated that both microsomal membranes and the cytosolic fraction from rat hver are required for the biological synthesis of cholesterol [1-4]. Specifically, the following conversions have been reported to require both microsomes and cytosol acetate to cholesterol [4] squalene to cholesterol [1] squalene-2,3-oxide to lanosterol [3] lanosterol to cholesterol [1,5] A -cholestenol to cholesterol [6] lanosterol to dihydrolanosterol [7] various 4,4-dimethyl sterols to cholesterol [8] and 7-dehydrocholesterol to cholesterol [9,10]. 

Fig. 8 is a schematic diagram of a cell which shows the known sites in which sterol carrier proteins are involved in cholesterol biosynthesis, utilization and intracellular transfer. SCP, participates in the conversion of squalene to lanosterol and SCP2 participates in the conversion of lanosterol to cholesterol, the conversion of cholesterol to cholesterol ester by ACAT, and probably also in the conversion of cholesterol to 7a-hydroxycholesterol. SCPj transfers cholesterol from cytoplasmic lipid inclusion droplets to mitochondria in the adrenal and SCPj also translocates cholesterol from the outer to the inner mitochondrial membrane. 

The most abundant member of the steroid family in animals is cholesterol, the precursor of all other steroids. Cholesterol is biosynthesized from squalene, a triterpene (Section 26.6). Cholesterol is an important component of cell membranes (Figure 26.1). Its ring structure makes it more rigid than other membrane lipids. Because cholesterol has eight asymmetric carbons, 256 stereoisomers are possible, but only one exists in nature (Chapter 5, Problem 20). 

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