Cholesterol synthase

In the laboratory of S.V. Ley, the total synthesis of the 3-lactone cholesterol synthase inhibitor 1233A was achieved by using the oxidative decompiexation of a (jt-allyl)tricarbonyliron lactone as the key step. " The (Z)-alkene present in the target was introduced using the S-G modified HWE olefination of an aldehyde with b/s(2,2,2-trifluoroethyl) (methoxycarbonylmethyl)phosphonate to give the desired a, 3-unsaturated methyl ester in excellent yield. 

Ketone body synthesis occurs only in the mitochondrial matrix. The reactions responsible for the formation of ketone bodies are shown in Figure 24.28. The first reaction—the condensation of two molecules of acetyl-CoA to form acetoacetyl-CoA—is catalyzed by thiolase, which is also known as acetoacetyl-CoA thiolase or acetyl-CoA acetyltransferase. This is the same enzyme that carries out the thiolase reaction in /3-oxidation, but here it runs in reverse. The second reaction adds another molecule of acetyl-CoA to give (i-hydroxy-(i-methyl-glutaryl-CoA, commonly abbreviated HMG-CoA. These two mitochondrial matrix reactions are analogous to the first two steps in cholesterol biosynthesis, a cytosolic process, as we shall see in Chapter 25. HMG-CoA is converted to acetoacetate and acetyl-CoA by the action of HMG-CoA lyase in a mixed aldol-Claisen ester cleavage reaction. This reaction is mechanistically similar to the reverse of the citrate synthase reaction in the TCA cycle. A membrane-bound enzyme, /3-hydroxybutyrate dehydrogenase, then can reduce acetoacetate to /3-hydroxybutyrate. 

The search for inhibitors of this pathway began with the first key regulatory enzyme, HMG CoA reductase. Several clinically useful inhibitors of HMG CoA reductase are now known. One of the most successful, Mevacor, produced by Merck, is one of the pharmaceutical industry s best selling products. However, the problem with inhibiting a branched biosynthetic pathway at an early point is that the biosynthesis of other crucial biomolecules may also be inhibited. Indeed, there is some evidence that levels of ubiquinone and the dolichols are affected by some HMG CoA reductase inhibitors. Consequently, efforts have recently been directed towards finding inhibitors of squalene synthase, the enzyme controlling the first step on the route to cholesterol after the FPP branch point. 

Medicinal chemistry has frequently drawn inspiration and important new leads from the examination of natural products, and this was proven to be the case once more. In 1992, researchers at Merck and Glaxo announced, almost simultaneously, the independent discovery of the same new class of natural products from two different fungi. As a consequence, the same family of natural products has two names - the zaragozic acids (Merck)4 or the squalestatins (Glaxo).5 A typical member of the family, zaragozic acid A (squa-lestatin SI) (1) was shown to have a tremendous affinity for squalene synthase (K, = 79 pM for rat microsomal squalene synthase) and could even lower serum cholesterol levels in vivo in a population of marmosets.6... 

Abbreviations TG, triglycerides LDL-C, low density lipoprotein cholesterol HDL-C, high density lipoprotein cholesterol FAS, fatty acid synthase VLDL, very low density lipoprotein. 

LDL (apo B-lOO, E) receptors occur on the cell surface in pits that are coated on the cytosolic side of the cell membrane with a protein called clathrin. The glycoprotein receptor spans the membrane, the B-lOO binding region being at the exposed amino terminal end. After binding, LDL is taken up intact by endocytosis. The apoprotein and cholesteryl ester are then hydrolyzed in the lysosomes, and cholesterol is translocated into the cell. The receptors are recycled to the cell surface. This influx of cholesterol inhibits in a coordinated manner HMG-CoA synthase, HMG-CoA reductase, and, therefore, cholesterol synthesis stimulates ACAT activ-... 

While the fluid mosaic model of membrane stmcture has stood up well to detailed scrutiny, additional features of membrane structure and function are constantly emerging. Two structures of particular current interest, located in surface membranes, are tipid rafts and caveolae. The former are dynamic areas of the exo-plasmic leaflet of the lipid bilayer enriched in cholesterol and sphingolipids they are involved in signal transduction and possibly other processes. Caveolae may derive from lipid rafts. Many if not all of them contain the protein caveolin-1, which may be involved in their formation from rafts. Caveolae are observable by electron microscopy as flask-shaped indentations of the cell membrane. Proteins detected in caveolae include various components of the signal-transduction system (eg, the insutin receptor and some G proteins), the folate receptor, and endothetial nitric oxide synthase (eNOS). Caveolae and lipid rafts are active areas of research, and ideas concerning them and their possible roles in various diseases are rapidly evolving. 

Methylmalonyl CoA mutase, leucine aminomutase, and methionine synthase (Figure 45-14) are vitamin Bj2-dependent enzymes. Methylmalonyl CoA is formed as an intermediate in the catabolism of valine and by the carboxylation of propionyl CoA arising in the catabolism of isoleucine, cholesterol, and, rarely, fatty acids with an odd number of carbon atoms—or directly from propionate, a major product of microbial fer-... 

Besides cholesterol efflux from arterial wall and its role in RCT, additional properties of HDL have been proposed for its protective anti-atherogenic activities. HDL protects vascular function by a number of potential alternative mechanisms, including inhibition of LDL oxidation [8,9], platelet aggregation and coagulation [10], and endothelial monocyte adhesion [11], as well as promotion of endothelial nitric oxide synthase (eNOS) [12], and prostacyclin synthesis [13-15]. The proposed alternate protective mechanisms for HDL are attractive but many of them lack validation under in vivo conditions. 

D. J. S. Riley, E. M. Kyger, C. A. Spilburg, L. G. Lange, Pancreatic Cholesterol Esterases. 2. Purification and Characterization of Human Pancreatic Fatty Acid Ethyl Esterase Synthase , Biochemistry 1990, 29, 3848-3852. 

Figure 22.10 Reverse cholesterol transfer. High density lipoprotein (HDL) collects cholesterol from cells in various tissues/ organs the complex is then transported in the blood to the liver where it binds to a receptor on the hepatocyte, is internalised and the cholesterolis released into the hepatocyte. This increases the concentration in the liver cells which then decreases the synthesis of cholesterol by inhibition of the rate-limiting enzyme in cholesterol synthesis, HMG-CoA synthase. The cholesterol is also secreted into the bile or converted to bile acids which are also secreted into the bile, some of which is lost in the faeces (Chapter A).

This enzyme [EC 2.3.1.26], also known as sterol O-acyl-transferase, sterol-ester synthase, and cholesterol acyl-transferase, catalyzes the reaction of an acyl-coenzyme A derivative with cholesterol to produce coenzyme A and the cholesterol ester. The animal enzyme is highly specific for transfer of acyl groups having a single cis double bond at C9. 

This enzyme [EC 3.1.1.13] (also known as cholesterol esterase, sterol esterase, cholesterol ester synthase, and triterpenol esterase) catalyzes the hydrolysis of a steryl ester to produce a sterol and a fatty acid anion. This class represents a group of enzymes exhibiting broad specificity. They act on esters of sterols and long-chain fatty acids, and may also bring about the esterification of sterols. These enzymes are typically activated by bile salts. See also Esterases D. P. Hajjar (1994) Adv. Enzymol. 69, 45. 

Zaragozic acid or the squalastatins were discovered as fungal metabolites and have been identified as potent inhibitors of squalene synthase. These compounds have presented themselves as potential cholesterol lowering agents. 

AcetylCoA carboxylase Glycogen synthase HMGCoA reductase NO synthase Biosynthesis of fatty adds Glycogen synthesis Biosynthesis of cholesterol Biosynthesis of NO ... 

Scheme 7.6 Stereoselective acetylation of the hydroxyl diethyl ester (9) to the (S)-acetate as an intermediate for the synthesis of a squalene synthase inhibitor ( ) of cholesterol synthesis.

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. 

Fig. 5.1.2 Cholesterol biosynthesis branch of the isoprenoid biosynthetic pathway. Enzymes are numbered as follows 1 squalene synthase 2 squalene epoxidase 3 2,3-oxidosqua-lene sterol cyclase 4 sterol A24-reductase (desmosterolosis) 5 sterol C-14 demethylase 6 sterol A14-reductase (hydrops-ectopic calcification-moth-eaten, HEM, dysplasia) 7 sterol C-4 demethylase complex (including a 3/ -hydroxysteroid dehydrogenase defective in congenital hemidyspla-sia with ichthyosiform nevus and limb defects, CHILD, syndrome) 8 sterol A8-A7 isomerase (Conradi-Hunermann syndrome CDPX2) 9 sterol A5-desaturase (lathosterolosis) 10 sterol A7-reductase (Smith-Lemli-Opitz syndrome). Enzyme deficiencies are indicated by solid bars across the arrows...

Stage Synthesis of Mevalonate from Acetate The first stage in cholesterol biosynthesis leads to the intermediate mevalonate (Fig. 21-34). Two molecules of acetyl-CoA condense to form acetoacetyl-CoA, which condenses with a third molecule of acetyl-CoA to yield the six-carbon compound /3-hydroxy-/3-methylglu-taryl-CoA (HMG-CoA). These first two reactions are catalyzed by thiolase and HMG-CoA synthase, respectively. The cytosolic HMG-CoA synthase in this pathway is distinct from the mitochondrial isozyme that catalyzes HMG-CoA synthesis in ketone body formation (see Fig. 17-18). 

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