Cholesterol isoprenoid units

The biosynthesis of cholesterol may be divided into five steps (l) Synthesis of mevalonate occurs from acetyl-CoA (Figure 26-1). (2) Isoprenoid units are formed... 

Cholesterol is synthesized in the body entirely from acetyl-CoA. Three molecules of acetyl-CoA form mevalonate via the important regulatory reaction for the pathway, catalyzed by HMG-CoA reductase. Next, a five-carbon isoprenoid unit is formed, and six of these condense to form squalene. Squalene undergoes cychzation to form the parent steroid lanos-terol, which, after the loss of three methyl groups, forms cholesterol. 

Cholesterol is synthesized from acetyl-CoA in the liver. Cholesterol and a number of natural products from plants (including rubber) are isoprenoid compounds. The isoprenoid unit is a 5-carbon structure. 

D. In the synthesis of cholesterol, but not of ketone bodies, HMG CoA is reduced by NADPH + H+ to mevalonic add. The enzyme, HMG CoA reductase, is highly regulated (it is inhibited by cholesterol and bile salts and induced by insulin). Mevalonic acid is converted to isopentenyl pyrophosphate, which provides isoprenoid units for the synthesis of cholesterol and its derivatives and for many other compounds. 

Cholesterol biosynthesis proceeds via the isoprenoids in a multistep pathway. The end product, cholesterol, and the intermediates of the pathway participate in diverse cellular functions. The isoprenoid units give rise to dolichol, CoQ, heme A, isopentenyl-tRNA, famesylated proteins, and vitamin D (in the presence of sunlight and 7-dehydrocholesterol). Dolichol is used in the synthesis of glycoproteins, CoQ in the mitochondrial electron transport chain, famesylation and geranylgeranylation by posttrans-lational lipid modification that is required for membrane association and function of proteins such as p2V and G-protein subunits. 

The fat-soluble vitamins share many properties despite their limited chemical similarity. They are absorbed into the intestinal lymphatics, along with other dietary lipids, after emulsification by bile salts. Lipid malabsorption accompanied by steatorrhea usually results in poor uptake of all the fat-soluble vitamins. Deficiency disease (except in the case of vitamin K) is difficult to produce in adults because large amounts of most fat-soluble vitamins are stored in the liver and in adipose tissue. The fat-soluble vitamins are assembled from isoprenoid units this fact is apparent from examination of the structures of vitamins A, E, and K cholesterol, the precursor of vitamin D, is derived from six isoprenoid units (Chapter 18). Specific biochemical functions for vitamins A, D, and K are known, but a role for vitamin E, other than as a relatively nonspecific antioxidant, remains elusive. 

The answer is a. (Murray, pp 505—626. Scriver, pp 4029-4240. Sack, pp 121-138. Wilson, pp 287-320.) In mammals, p-carotene is the precursor of retinal, which is the basic chromophore of all visual pigments. Isopen-tenyl pyrophosphate and dimethylallyl pyrophosphate are isoprenoid isomers formed from the repeated condensation of acetyl CoA units. By continued condensation in mammalian systems, cholesterol can be formed. In plant systems, carotenoids are formed. In addition to producing the color of tomatoes and carrots, carotenoids serve as the light-absorbing molecules of photosynthesis. Ketone bodies are derived from condensation of acetyl CoA units but not from isoprenoid units. Vitamin C (ascorbic acid), carnitine, and thiamine (vitamin BO are not derived from isoprenoid units. 

Terpenoids do not necessarily contain exact multiples of five carbons and allowance has to be made for the loss or addition of one or more fragments and possible molecular rearrangements during biosynthesis. In reality the terpenoids are biosynthesized from acetate units derived from the primary metabolism of fatty acids, carbohydrates and some amino acids (see Fig. 2.10). Acetate has been shown to be the sole primary precursor of the terpenoid cholesterol. The major route for terpenoid biosynthesis, the mevalonate pathway, is summarized in Fig. 2.16. Acetyl-CoA is involved in the generation of the C6 mevalonate unit, a process that involves reduction by NADPH. Subsequent decarboxylation during phosphorylation (i.e. addition of phosphate) in the presence of ATP yields the fundamental isoprenoid unit, isopentenyl pyrophosphate (IPP), from which the terpenoids are synthesized by enzymatic condensation reactions. Recently, an alternative pathway has been discovered for the formation of IPP in various eubacteria and plants, which involves the condensation of glyceraldehyde 3-phosphate and pyruvate to form the intermediate 1-deoxy-D-xylulose 5-phosphate (Fig. 2.16 e.g. Eisenreich et al. 1998). We consider some of the more common examples of the main classes of terpenoids below. 

Dolichol is an isoprenoid compound synthesized by the same metabolic route as cholesterol (see here). In vertebrate tissues, dolichol contains 18-20 isoprenoid units (90-100 carbons total). 

The long side chain of CoQ has 10 of the 5-carbon isoprenoid units, and is sometimes called CoQk,. It is also called ubiquinone (the quinone found everywhere) because quinones with similar structures are found in all plants and animals. CoQ can be synthesized in the human from precursors derived from carbohydrates and fat. The long isoprenoid side chain is formed in the pathway that produces the isoprenoid precursors of cholesterol. CoQio is sometimes prescribed for patients recovering from a myocardial infarction, in an effort to increase their exercise capacity. 

Condensation of isoprenoid units then leads to the production of squalene and, ultimately, cholesterol. Both of the isoprenoid derivatives we have met so far are required. Two further condensation reactions take place. As a result, famesyl pyrophosphate, a 15-carbon compound, is produced. Two molecules of famesyl pyrophosphate condense to form squalene, a 30-carbon compound. The reaction is catalyzed by squalene synthase, and NADPH is required for the reaction. 

The biosynthesis of cholesterol proceeds by the condensation of five-carbon isoprenoid units. 

In steroid biosynthesis, three acetyl-CoA molecules condense to form the six-carbon mevalonate, which then gives rise to a five-carbon isoprenoid unit. A second and then a third isoprenoid unit condense, giving rise to a 10carbon unit. Two of the 15units condense, forming a SOcarbon precursor of cholesterol. 

Theories on the in vivo synthesis of phytanic acid concentrate mainly on two possibilities. One of them deals with the possible formation of phytanic acid from isopren units or four molecules of mevalonic acid (or mevalonate), (Kahlke 1964 a, Kahlke and Richterich 1965). Instead of an end-to-end condensation of two molecules of famesyl pyrophosphate which results in the formation of squalene and finally cholesterol, a fourth active isoprenoid unit might be attached to famesyl pyrophosphate. From this intermediate several steps of hydrogenation and oxidation would be required for the formation of phytanic acid. This hypothesis now appears unlikely since Steinberg (1965) was unable to detect any activity in the phytanic acid fraction after administration of labeled mevalonate to a patient with HAP (case T.E. of Refsum). 

As mentioned in section 6.4, cholesterol is a major and important consituent of animal membranes. Different sterols may be important in other organisms. Sterols are, however, a relatively small group of a very large class of biogenetically related substances - the polyisoprenoids (or terpenoids). These compounds are all derived from a common precursor 3-methylbut-3-enyl(isopentenyl)diphosphateisoprene. Polyisoprenoids can be open-chain, partly cyclized or fully cyclized substances. They contain a basic structure of a branched-chain C5 (isoprenoid) unit. 

D-Mevalonic acid is the fundamental intermediate in the biosynthesis of the terpenoids and steroids, together classed as poly-isoprenoids. The biogenetic isoprene unit is isopentenyl pyrophosphate which arises by enzymic decarboxylation-dehydration of mevalonic acid pyrophosphate. D-Mevalonic acid is almost quantitatively incorporated into cholesterol synthesized by rat liver homogenates. 

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-... 

We begin with an account of the main steps in the biosynthesis of cholesterol from acetate, then discuss the transport of cholesterol in the blood, its uptake by cells, the normal regulation of cholesterol synthesis, and its regulation in those with defects in cholesterol uptake or transport. We next consider other cellular components derived from cholesterol, such as bile acids and steroid hormones. Finally, an outline of the biosynthetic pathways to some of the many compounds derived from isoprene units, which share early steps with the pathway to cholesterol, illustrates the extraordinary versatility of isoprenoid condensations in biosynthesis. 

A five-carbon isoprene unit—isopentenyl pyrophosphate (IPP)— is formed by the decarboxylation of 5-pyrophosphomevalonate. The reaction requires ATP. [Note IPP is the precursor of a family of molecules with diverse functions, the isoprenoids. Cholesterol is a sterol isoprenoid. Non-sterol isoprenoids include dolichol (see p. 165) and ubiquinone (see p. 75).]... 

This proposal supported, with an earlier prediction of Sir Robert Robinson, that cholesterol was a cyclization product of squalene, a 30-carbon polymer of isoprene units. In 1953, Robert Bums Woodward and Bloch postulated a cyclization scheme for squalene (fig. 20.2) that was later shown to be correct. In 1956, the unknown isoprenoid precursor was identified as mevalonic acid by Karl Folkers and others at Merck, Sharpe, and Dohme Laboratories. The discovery of mevalonate provided the missing link in the basic outline of cholesterol biosynthesis. Since that time, the sequence and the stereochemical course for the biosynthesis of cholesterol have been defined in detail. 

Many natural products are derived from terpenes, even though they do not have carbon skeletons composed exclusively of C5 isoprene units. These terpene-like compounds are called terpenoids. They may have been altered through rearrangements, loss of carbon atoms, or introduction of additional carbon atoms. Cholesterol is an example of a terpenoid that has lost some of the isoprenoid carbon atoms. 

Cholesterol is synthesized mainly in the liver by a three-stage process. All 27 carbon atoms in the cholesterol molecule are derived from acetyl-CoA. The first stage is the synthesis of the activated five-carbon isoprene unit, isopentenyl pyrophosphate. Six molecules of isopentenyl pyrophosphate then condense to form squalene in a sequence of reactions that also synthesize isoprenoid intermediates that are important in protein isoprenylation modifications. The characteristic four-ring structure of cholesterol is then formed by cycUzing of the linear squalene molecule. Several demethylations, the reduction of a double bond, and the migration of another double bond result in the formation of cholesterol. Figure 34-1 provides an overview of cholesterol biosynthesis. 

Comforth and his colleagues have investigated the stereochemistry of isomerization of isopentenyl to dimethylallyl pyrophosphate in isoprenoid biosynthesis. They find that the prototropic change involved is stereochemically different from the superficially analogous association of C5 units. Bisabolene appears to be excluded as an intermediate in the biosynthesis of helicobasidin and trichothecin by recent labelling studies (see also Vol. 1, p. 232, ref. 81) and a 1,4-hydride shift in the initially formed intermediate is indicated. The loss of the C-14 methyl group in cholesterol biosynthesis differs from loss of the C-4 methyl groups. The 32-carbon atom is released at the aldehyde oxidation level as formic acid. 

Cholesterol (and other sterols) is derived from acetyl-CoA. By a series of reactions the 5C-isoprene unit is formed and this can then self-condense to give a series of IOC, 15C, 30C etc. isoprenoid molecules. Reduction of hydroxymethylglutaryl-CoA is the key regulatory step in the overall process. To form sterols from the open-chain isoprenes requires cyclization... 

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