Mevalonate-5-pyrophosphate cholesterol biosynthesis

FTase catalyzes the covalent attachment of a farnesyl moiety via a thioether Unkage to the proteins bearing a C-terminal amino acid sequence known as the CAAX motif (Fig. 2) [12,21]. The farnesyl moiety is derived from farnesyl pyrophosphate (FPP), a 15-carbon isoprenyl intermediate in the mevalonate pathway of cholesterol biosynthesis. The binding of FPP to the enzyme has relatively high affinity (K = 1-lOnM), and FPP binding must precede the binding of the peptide substrate for successful catalysis [22,23]. 

The first stage in the synthesis of cholesterol is the formation of isopentenyl pyrophosphate Fig. 1). Acetyl CoA and acetoacetyl CoA combine to form 3-hydroxy-3-methylglutaryl CoA (HMG CoA). This process takes place in the liver, where the HMG CoA in the mitochondria is used to form ketone bodies during starvation (see Topic K2), whereas that in the cytosol is used to synthesize cholesterol in the fed state (under the influence of cholesterol). HMG CoA is then reduced to mevalonate by HMG CoA reductase Fig. 1). This is the committed step in cholesterol biosynthesis and is a key control point. Mevalonate is converted into 3-isopentenyl pyrophosphate by three consecutive reactions each involving ATP, with C02 being released in the last reaction Fig. 1). 

Key intermediates in cholesterol biosynthesis are HMG CoA, mevalonic acid, isopentenyl pyrophosphate, and squalene. 

Cholesterol is primarily restricted to eukaryotic cells where it plays a number of roles. Undoubtedly, the most primitive function is as a structural component of membranes. Its metabolism to bile acids and the steroid hormones is relatively recent in the evolutionary sense. In this chapter, the pathway of cholesterol biosynthesis will be divided into segments which correspond to the chemical and biochemical divisions of this biosynthetic route. The initial part of the pathway is the 3-step conversion of acetyl-CoA to 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA). The next is the reduction of this molecule to mevalonate, considered to be the rate-controlling step in the biosynthesis of polyisoprenoids. From thence, a series of phosphorylation reactions both activate and decarboxylate mevalonate to isopen tenyl pyrophosphate, the true isoprenoid precursor. After a rearrangement to the allylic pyrophosphate, dimethylallyl pyrophosphate, a sequence of l -4 con-... 

Not all investigators are convinced that peroxisomes are involved in cholesterol biosynthesis. In contrast to Olivier and Krisans [4], Hogenboom and colleagues have found no deficiency in cholesterol biosynthesis in fibroblasts from patients with a peroxisomal biogenesis disorder (S. Hogenboom, 2003). In addition, using a variety of biochemical and microscopic techniques, they found that mevalonate kinase, phospho-mevalonate kinase, and mevalonate pyrophosphate decarboxylase are cytosolic, not peroxisomal enzymes (S. Hogenboom, 2004). 

HMG-CoA reductase is the major regulatory enzyme in cholesterol biosynthesis. HMG-CoA reductase is controlled hormonally by insulin and glucagon and transcription and translation of the enzyme can be suppressed by the presence of cholesterol in cells. Mevalonate is converted in the cytosol to the five carbon building blocks of isoprene synthesis-isopentenyl pyrophosphate (IPP) and dimethylallyl pyrophosphate (DPP)-in the reactions shown in Figure 19.19. Subsequently, IPP and DPP form famesyl pyrophosphate in the cytosol (Figure 19.20)... 

F4MG-CoA is then reduced into mevalonate via a reaction catalyzed by HMG-CoA reductase, a key enzyme of cholesterol biosynthesis. In fact, HMG-CoA reductase is the major rate-limiting enzyme of de novo cholesterol synthesis. Consistently, inhibitors of HMG-CoA reductase (statins) block the whole pathway of cholesterol synthesis by preventing the formation of mevalonate. The following steps in the cholesterol synthetic pathway include the ATP-dependent transformation of mevalonate into isopentenyl pyrophosphate (Fig. 3.5). 

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. 

Elimination usually involves loss of a proton together with a nucleophilic group such as -OH, -NH3+, phosphate, or pyrophosphate. However, as in Eq. 13-18, step c, electrophilic groups such as -COO-can replace the proton. Another example is the conversion of mevalonic acid-5-pyrophosphate to isopentenyl pyrophosphate (Eq. 13-19) This is a key reaction in the biosynthesis of isoprenoid compounds such as cholesterol and vitamin A (Chapter 22). The phosphate ester formed in step a is a probable intermediate and the reaction probably involves a carbo-cationic intermediate generated by the loss of phosphate prior to the decarboxylation. 

The biosynthesis of steroids begins with the conversion of three molecules of acetyl-CoA into mevalon-ate, the decarboxylation of mevalonate, and its conversion to isopentenyl pyrophosphate. Six molecules of isopentenyl pyrophosphate are polymerized into squalene, which is cyclized to yield lanosterol. Lanos-terol is converted to cholesterol, which is the precursor of bile acids and steroid hormones. 

At first glance, it is not at all clear that steroids are terpenoid in origin. The 5n numbers are absent— cholesterol is a C27 compound while the others variously have 20,21, or 23 carbon atoms. Studies with labelled mevalonic acid showed that cholesterol is terpenoid, and that it is formed from two molecules of farnesyl pyrophosphate (2 x C45 = C30 so three carbon atoms must be lost). Labelling of one or other of the methyl groups (two experiments combined in one diagram ) showed that two of the green carbon atoms and one of the black carbon atoms were lost during the biosynthesis. 

Goodman, D.S., and Popjak, G. (1960). Studies on the biosynthesis of cholesterol Xll. Synthesis of allyl pyrophosphates from mevalonate and their conversion into squalene with liver enzymes. J Lipid Res 1 286-300. 

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. 

Statins are the newest class of cholesterol-reducing drugs. Statins reduce serum cholesterol levels by inhibiting the enzyme that catalyzes the reduction of hydroxymethylglu-taryl-CoA to mevalonic acid (Section 26.8). Decreasing the mevalonic acid concentration decreases the isopentenyl pyrophosphate concentration, so the biosynthesis of all terpenes, including cholesterol, is diminished. As a consequence of diminished cholesterol synthesis in the liver, the liver expresses more LDL receptors— the receptors that help clear LDL from the bloodstream. Studies show that for every 10% that choles-... 

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