24.2 How Are Complex Lipids Synthesized? 743 2% to 8% of the lipids in most animal membranes, but breakdown products of PI, in- cluding inositol-1,4,5-trisphosphate and diacylglycerol, are second messengers in a vast array of cellular signaling processes. Dihydroxyacetone Phosphate Is a Precursor to the Plasmalogens Certain glycerophospholipids possess alkyl or alkenyl ether groups at the 1-position in place of an acyl ester group. These glyceroether phospholipids are synthesized from dihydroxyacetone phosphate (Figure 24.23). Acylation of dihydroxyacetone phosphate (DHAP) is followed by an exchange reaction, in which the acyl group is removed as a carboxylic acid and a long-chain alcohol adds to the 1-position. This C O CH 2 H C OH O CH 2 O O P O – O – CoA S O C R C CH 2 O O CH 2 O O P O – O – O C R HO CH 2 CH 2 R 1 RC O – C CH 2 O O CH 2 O P O – O – R 1 CH 2 CH 2 R 1 CH 2 CH 2 R 1 CH 2 CH 2 O + HOCH CH 2 O CH 2 O P O – O – O CoAS O C R 2 O C R 2 O C CH 2 O CH 2 O P O – O – R 1 CH 2 CH 2 O OH CDP- ethanolamine O C R 2 O C CH 2 CH 2 O P O O – O H CH 2 CH 2 NH 3 + ++ 2 H 2 O + O C R 2 O C CH 2 O CH 2 O P O O – OH CH 2 CH 2 NH 3 + H C R 1 CDP NAD + NADH NADP + NADPH H + H + O 2 CoASH CoASH 1 2 3 4 5 6 Dihydroxyacetone phosphate 1-Acyldihydroxyacetone phosphate 1-Acyldihydroxyacetone phosphate synthase Dihydroxyacetone phosphate acyltransferase 1-Alkyldihydroxyacetone phosphate 1-Alkyldihydroxyacetone phosphate oxidoreductase 1-Alkylglycero-3-phosphate 1-Alkylglycerophosphate acyltransferase 1-Alk y l-2-ac y l g l y cero-3-phosphate CDP-ethanolamine transferase 1-Alkyl-2-acylglycero-3-phosphoethanolamine 1-Alkyl-2-acylglycero- phosphoethanolamine desaturase Plasmalogen FIGURE 24.23 Biosynthesis of plasmalogens in animals. (1) Acylation at C-1 is followed by (2) exchange of the acyl group for a long-chain alcohol. (3) Reduction of the keto group at C-2 is followed by (4 and 5) transferase reactions, which add an acyl group at C-2 and a polar head-group moiety (as shown here for phospho- ethanolamine), and a (6) desaturase reaction that forms a double bond in the alkyl chain.The first two enzymes are of cytoplasmic origin, and the last transferase is located at the endoplasmic reticulum. 744 Chapter 24 Lipid Biosynthesis long-chain alcohol is derived from the corresponding acyl-CoA by means of an acyl- CoA reductase reaction involving oxidation of two molecules of NADH. The 2-keto group of the DHAP backbone is then reduced to an alcohol, followed by acylation. The resulting 1-alkyl-2-acylglycero-3-phosphate can react in a manner similar to phosphatidic acid to produce ether analogs of phosphatidylcholine, phosphatidyl- ethanolamine, and so forth (Figure 24.23). In addition, specific desaturase enzymes associated with the ER can desaturate the alkyl ether chains of these lipids as shown. The products, which contain ␣,␤-unsaturated ether-linked chains at the C-1 posi- tion, are plasmalogens; they are abundant in cardiac tissue and in the central nervous system. The desaturases catalyzing these reactions are distinct from but sim- ilar to those that introduce unsaturations in fatty acyl-CoAs. Platelet-Activating Factor Is Formed by Acetylation of 1-Alkyl-2-Lysophosphatidylcholine A particularly interesting ether phospholipid with unusual physiological prop- erties, 1-alkyl-2-acetylglycerophosphocholine, also known as platelet-activating factor, possesses an alkyl ether at C-1 and an acetyl group at C-2 (Figure 24.24). The very short chain at C-2 makes this molecule much more water soluble than typical g lycerolipids. Platelet-activating factor displays a dramatic ability to dilate blood vessels (and thus reduce blood pressure in hypertensive animals) and to aggregate platelets. Sphingolipid Biosynthesis Begins with Condensation of Serine and Palmitoyl-CoA Sphingolipids, ubiquitous components of eukaryotic cell membranes, are present at high levels in neural tissues. The myelin sheath that insulates nerve axons is particu- larly rich in sphingomyelin and other related lipids. Prokaryotic organisms normally do not contain sphingolipids. Sphingolipids are built upon sphingosine backbones rather than glycerol. The initial reaction, which involves condensation of serine and palmitoyl-CoA with release of bicarbonate, is catalyzed by 3-ketosphinganine synthase, HO C CH 2 O CH 2 O O P O O – RCH 2 CH 2 H CH 2 CH 2 N(CH 3 ) 3 + O CH 3 C SCoA O CH 3 CO – O C CH 2 O CH 2 O O P O O – RCH 2 CH 2 H CH 2 CH 2 N(CH 3 ) 3 + O CH 3 C 1-Alkyl-2-lysophosphatidylcholine Acetyl-CoA: 1-alkyl-2-lysoglycero- phosphocholine transferase Acetylhydrolase 1-Alkyl-2-acetylglycerophosphocholine (platelet-activatin g factor, PAF) CoASH H 2 O FIGURE 24.24 Platelet-activating factor, formed from 1-alkyl-2-lysophosphatidylcholine by acetylation at C-2, is degraded by the action of acetylhydrolase. 24.2 How Are Complex Lipids Synthesized? 745 a PLP-dependent enzyme (Figure 24.25). Reduction of the ketone product to form sphinganine is catalyzed by 3-keto-sphinganine reductase, with NADPH as a reactant. In the next step, sphinganine is acylated to form N-acyl sphinganine, which is then desaturated to form ceramide. Sphingosine itself does not appear to be an interme- diate in this pathway in mammals. CH 3 (CH 2 ) 14 O CSCoA – OOC C CH 2 OH + NH 3 H CH 3 (CH 2 ) 14 O C C CH 2 OH + NH 3 H CH 3 (CH 2 ) 14 C C CH 2 OH + NH 3 H H CH 3 (CH 2 ) 12 CH 3 (CH 2 ) n C C CH 2 OH OH OH O NH H C H H C C H Acyl- Palmitoyl-CoA Serine 3-Ketosphinganine synthase 2S-3-Ketosphinganine N-acyl-sphinganine Ceramide X XH 2 2S,3R-Sphinganine + 3-Ketosphinganine reductase HCO 3 – NADP + NADPH H + CoASH H 2 O SCoA CoASH FIGURE 24.25 Biosynthesis of sphingolipids in animals begins with the 3-ketosphinganine synthase reaction, a PLP-dependent condensation of palmitoyl-CoA and serine. Subsequent reduction of the keto group, acyla- tion, and desaturation (via reduction of an electron acceptor, X) form ceramide, the precursor of other sphingolipids. 746 Chapter 24 Lipid Biosynthesis Ceramide Is the Precursor for Other Sphingolipids and Cerebrosides Ceramide is the building block for all other sphingolipids. Sphingomyelin, for example, is produced by transfer of phosphocholine from phosphatidylcholine (Figure 24.26). Glycosylation of ceramide by sugar nucleotides yields cerebrosides, such as galactosylceramide, which makes up about 15% of the lipids of myelin sheath CH 2 OH O O HO OH OH H N O C H C C H (CH 2 ) 12 CH 2 CH CHOH R 1 UDP-Gal UDP CH 2 OH O H H H H N OH H OH O N H O HC C OH C H R 1 CH 2 R 2 O CH 2 OH O HO H H H OH H OH H CH 2 OH O H H H H OH H OH O N H O HC C OH C H R 1 CH 2 R 2 O CH 2 OH O H H H OH H OH H CMP- sialic acid O CH 2 OH O H H OH H H H NH COCH 3 HO O H H H H OH H OH O N H O HC C OH C H R 1 CH 2 R 2 O CH 2 OH O H H H O H OH H O CH 2 OH O H H OH H H H NH COCH 3 HO O H H COH H H HN OH H COO – CO CH 3 H HCOH CH 2 OH CH 2 OH UDP- Glu UDP UDP- Gal UDP CH 3 H N O C H C C H (CH 2 ) 12 CH 2 CH 2 OH CH CHOH R 1 CH 3 CH 3 H N O O O OP – O C H C C H (CH 2 ) 12 CH 2 CH 2 CH 3 CH 3 + H 3 C C CHOH R 1 1,2-Diacylglycerol Phosphatidyl- choline ␤- D-Galactosylceramide UDP- galacto- syltransferase ␤- D-Galactosyl-(1 4)-␤-D-glucosylceramide UDP- N-Acetylgalactosamine UDP N-Acetylgalactosaminyltransferase ␤- D-N-Acetylgalactosamine-(1 4)- ␤-D-galactosyl-(1 4)-␤-D-glucosylceramide CMP Sialytransferase Ganglioside GM 2 Ceramide Sphingomyelin FIGURE 24.26 Glycosylceramides (such as galactosyl- ceramide), gangliosides, and sphingomyelins are synthe- sized from ceramide in animals. 24.3 How Are Eicosanoids Synthesized, and What Are Their Functions? 747 structures. Cerebrosides that contain one or more sialic acid (N-acetylneuraminic acid) moieties are called gangliosides. Several dozen gangliosides have been charac- terized, and the general form of the biosynthetic pathway is illustrated for the case of ganglioside GM 2 (Figure 24.26). Sugar units are added to the developing ganglioside from nucleotide derivatives, including UDP–N-acetylglucosamine, UDP–galactose, and UDP–glucose. 24.3 How Are Eicosanoids Synthesized, and What Are Their Functions? Eicosanoids, so named because they are all derived from 20-carbon fatty acids, are ubiquitous breakdown products of phospholipids. In response to appropriate stimuli, cells activate the breakdown of selected phospholipids (Figure 24.27). Phospholipase A 2 (see Chapter 8) selectively cleaves fatty acids from the C-2 po- sition of phospholipids. Often these are unsaturated fatty acids, among which is arachidonic acid. Arachidonic acid may also be released from phospholipids by the combined actions of phospholipase C (which yields diacylglycerols) and dia- cylglycerol lipase (which releases fatty acids). Eicosanoids Are Local Hormones Animal cells can modify arachidonic acid and other polyunsaturated fatty acids, in processes often involving cyclization and oxygenation, to produce so-called local hor- mones that (1) exert their effects at very low concentrations and (2) usually act near their sites of synthesis. These substances include the prostaglandins (PG) (Figure 24.27) as well as thromboxanes (Tx), leukotrienes, and other hydroxyeicosanoic acids. Thromboxanes, discovered in blood platelets (thrombocytes), are cyclic ethers (TxB 2 is actually a hemiacetal; see Figure 24.27) with a hydroxyl group at C-15. Prostaglandins Are Formed from Arachidonate by Oxidation and Cyclization All prostaglandins are cyclopentanoic acids derived from arachidonic acid. The biosynthesis of prostaglandins is initiated by an enzyme associated with the ER, called prostaglandin endoperoxide H synthase (PGHS), also known as cyclooxygenase (COX). The enzyme catalyzes simultaneous oxidation and cyclization of arachidonic acid. The enzyme is viewed as having two distinct activities, COX and peroxidase (POX), as shown in Figure 24.28. A DEEPER LOOK The Discovery of Prostaglandins The name prostaglandin was given to this class of compounds by Ulf von Euler, their discoverer, in Sweden in the 1930s. He extracted fluids containing these components from human semen. Because he thought they originated in the prostate gland, he named them prostaglandins. Actually, they were synthesized in the seminal vesi- cles, and it is now known that similar substances are synthesized in most animal tissues (both male and female). Von Euler observed that injection of these substances into animals caused smooth mus- cle contraction and dramatic lowering of blood pressure. Von Euler (and others) soon found that it is difficult to analyze and characterize these obviously interesting compounds because they are present at extremely low levels. Prostaglandin E 2 ␣, or PGE 2 ␣, is present in human serum at a level of less than 10 Ϫ14 M! In addition, they often have half-lives of only 30 seconds to a few minutes, not lasting long enough to be easily identified. Moreover, most animal tissues upon dissection and homogenization rapidly synthesize and degrade a variety of these substances, so the amounts obtained in isolation procedures are extremely sensitive to the methods used and highly variable even when procedures are carefully controlled. Sune Bergström, Bengt Samuelsson, and their colleagues described the first structural determinations of prostaglandins in the late 1950s. In the early 1960s, dramatic ad- vances in laboratory techniques, such as NMR spectroscopy and mass spectrometry, made further characterization possible. Von Euler received the Nobel Prize for Physiology or Medicine in 1970, and Bergström, Samuelsson, and John Vane shared the Nobel for Physiology or Medicine in 1982. 748 Chapter 24 Lipid Biosynthesis A Variety of Stimuli Trigger Arachidonate Release and Eicosanoid Synthesis The release of arachidonate and the synthesis or interconversion of eicosanoids can be initiated by a variety of stimuli, including histamine, hormones such as epineph- rine and bradykinin, proteases such as thrombin, and even serum albumin. An im- portant mechanism of arachidonate release and eicosanoid synthesis involves tissue injury and inflammation. When tissue damage or injury occurs, special inflammatory OH O COO – HO OH OH COO – HO – OOC O HO OH COO – O OH HO H COO – OH O O COO – OH HO O HO H COO – HSCH 2 CH C O N H CH 2 COO – NH C O CH 2 CH 2 CH COO – H 3 + N 2 O 2 PGE 2 PGF 2a PGI 2 TxB 2 PGH 2 PGD 2 Leukotriene C Arachidonate Activation of PLC and diacylglycerol lipase Activation of PLA 2 Receptor Hormone (or other stimulus) Phospholipids FIGURE 24.27 Arachidonic acid, derived from breakdown of phospholipids (PL), is the precursor of prostaglandins, thromboxanes, and leukotrienes.The letters used to name the prostaglandins are assigned on the basis of similar- ities in structure and physical properties.The class denoted PGE, for example, consists of ␤-hydroxyketones that are soluble in ether, whereas PGF denotes 1,3-diols that are soluble in phosphate buffer.PGA denotes prosta- glandins possessing ␣,␤-unsaturated ketones.The number following the letters refers to the number of carbon– carbon double bonds.Thus, PGE 2 contains two double bonds. 24.3 How Are Eicosanoids Synthesized, and What Are Their Functions? 749 cells, monocytes and neutrophils, invade the injured tissue and interact with the resi- dent cells (such as smooth muscle cells and fibroblasts). This interaction typically leads to arachidonate release and eicosanoid synthesis. Examples of tissue injury in which eicosanoid synthesis has been characterized include heart attack (myocardial infarc- tion), rheumatoid arthritis, and ulcerative colitis. “Take Two Aspirin and…” Inhibit Your Prostaglandin Synthesis In 1971, biochemist John Vane was the first to show that aspirin (acetylsalicylate; Fig- ure 24.29) exerts most of its effects by inhibiting the biosynthesis of prostaglandins. Its site of action is PGHS. COX activity is destroyed when aspirin O -acetylates Ser 530 HH 85 11 14 O H COOH COX POX COOH . O COOH O OH H H H H COOH HH H H O O O O H O O O 5, 8,11,14-Eicosatetraenoic acid (arachidonic acid) Peroxide radical PGG 2 PGH 2 OO FIGURE 24.28 Prostaglandin endoperoxide H synthase (PGHS), the enzyme that converts arachidonic acid to prostaglandin PGH 2 , possesses two distinct activities: cyclooxygenase (COX) and a glutathione-dependent hydroperoxidase (POX).The mechanism of the reaction begins with hydrogen atom abstraction by a tyrosine radical on the enzyme, followed by rearrangement to cyclize and incorporate two oxygen molecules. Reduc- tion of the peroxide at C15 completes the reaction. COX is the site of action of aspirin and other analgesic agents. O C CH 3 O Ser COO – O HO NH C C CH 3 CHCH CH 3 CH 2 CH 3 O O COO – COOH OH OHSer H 3 C H 3 C (b)(a) Salicylate Active cyclooxygenase Acetaminophen Ibuprofen Inactive cyclooxygenase Acetylsalicylate (aspirin) FIGURE 24.29 (a) The structures of several common analgesic agents.Acetaminophen is marketed under the trade name Tylenol. Ibuprofen is sold as Motrin, Nuprin, and Advil. (b) Acetylsalicylate (aspirin) inhibits the COX activity of endoperoxide synthase via acetylation (covalent modification) of Ser 530 . 750 Chapter 24 Lipid Biosynthesis on the enzyme. From this you may begin to infer something about how prosta- glandins (and aspirin) function. Prostaglandins are known to enhance inflammation in animal tissues. Aspirin exerts its powerful anti-inflammatory effect by inhibiting this first step in their synthesis. Aspirin does not have any measurable effect on the peroxidase activity of the synthase. Other nonsteroidal anti-inflammatory agents, such as ibuprofen (Figure 24.29) and phenylbutazone, inhibit COX by competing at the active site with arachidonate or with the peroxyacid intermediate (PGG 2 , as in Figure 24.28). See A Deeper Look above. 24.4 How Is Cholesterol Synthesized? The most prevalent steroid in animal cells is cholesterol (Figure 24.30). Plants con- tain no cholesterol, but they do contain other steroids very similar to cholesterol in structure (see page 236). Cholesterol serves as a crucial component of cell mem- branes and as a precursor to bile acids (such as cholate, glycocholate, taurocholate) and steroid hormones (such as testosterone, estradiol, progesterone). Also, vitamin D 3 is derived from 7-dehydrocholesterol, the immediate precursor of cholesterol. Liver is the primary site of cholesterol biosynthesis. A DEEPER LOOK The Molecular Basis for the Action of Nonsteroidal Anti-inflammatory Drugs Prostaglandins are potent mediators of inflammation. The first and committed step in the production of prostaglandins from arachi- donic acid is the bis-oxygenation of arachidonate to prostaglandin PGG 2 . This is followed by reduction to PGH 2 in a peroxidase reac- tion. Both these reactions are catalyzed by PGHS or COX. This en- zyme is inhibited by the family of drugs known as nonsteroidal anti- inflammatory drugs, or NSAIDs. Aspirin, ibuprofen, flurbiprofen, and acetaminophen (trade name Tylenol) are all NSAIDs. There are two isoforms of COX in animals: COX-1, which car- ries out normal, physiological production of prostaglandins, and COX-2, which is induced by cytokines, mitogens, and endotoxins in inflammatory cells and is responsible for the production of prostaglandins in inflammation. The enzyme structure shown in panel a is that of residues 33 to 583 of COX-1 from sheep, inactivated by ibuprofen (cyan). These 551 residues comprise three distinct domains. The first of these, (a) pdb id ϭ 1EQG (b) Superposition of pdb id ϭ 1EQG and 1CX2 24.4 How Is Cholesterol Synthesized? 751 Mevalonate Is Synthesized from Acetyl-CoA Via HMG-CoA Synthase The cholesterol biosynthetic pathway begins in the cytosol with the synthesis of mevalonate from acetyl-CoA (Figure 24.31). The first step is the ␤-ketothiolase– catalyzed Claisen condensation of two molecules of acetyl-CoA to form acetoacetyl- CoA. In the next reaction, acetyl-CoA and acetoacetyl-CoA join to form 3-hydroxy- 3-methylglutaryl-CoA, which is abbreviated HMG-CoA. The reaction, a second Claisen condensation, is catalyzed by HMG-CoA synthase. The third step in the pathway is residues 33 to 72, is a small, compact module that is similar to epi- dermal growth factor. The second domain, composed of residues 73 to 116, forms a right-handed spiral of four ␣-helical segments. These ␣-helical segments form a membrane-binding motif. The helical segments are amphipathic, with most of the hydrophobic residues facing away from the protein, where they can interact with a lipid bilayer. The third domain of the COX enzyme, the cat- alytic domain, is a globular structure that contains both the COX and the peroxidase active sites. The COX active site lies at the end of a long, narrow, hydro- phobic tunnel or channel. Three of the ␣-helices of the membrane- binding domain lie at the entrance to this tunnel. The walls of the tunnel are defined by four ␣-helices, formed by residues 106 to 123, 325 to 353, 379 to 384, and 520 to 535. The COX-1 structure shown in panel a has a molecule of ibupro- fen bound in the tunnel. Deep in the tunnel, at the far end, lies Tyr 385 , a catalytically important residue. Heme-dependent peroxi- dase activity is implicated in the formation of a proposed Tyr 385 rad- ical, which is required for COX activity. Aspirin and other NSAIDs block the synthesis of prostaglandins by filling and blocking the tun- nel, preventing the migration of arachidonic acid to Tyr 385 in the ac- tive site at the back of the tunnel. Why do the new “COX-2 inhibitors” bind to (and inhibit) COX-2 but not COX-1? A single amino acid substitution makes all the dif- ference. Panel b shows an overlay of COX-1 (1EQG) and COX-2 (1CX2) structures. COX-2 has a valine (blue) at position 523, which leaves room for binding of a Celebrex-like inhibitor (orange). On the other hand, COX-1 has bulkier isoleucine (red) at position 523, which prevents binding of the inhibitor. COX-2 inhibitors were introduced as pain medications in 1997, and by 2004 nearly half of the 100 million prescriptions written an- nually for NSAIDs in the United States were COX-2 inhibitors. However, several COX-2 inhibitors were taken off the U.S. market in late 2004 and early 2005, when their use was linked to heart at- tacks and strokes in a small percentage of users. Since that time, prescriptions for COX-2 inhibitors have dropped by 65%. Inter- estingly, although COX-2 inhibitors were originally intended to al- leviate pain without the risk of adverse gastrointestinal effects, less than 5% of patients that used COX-2 prescriptions at the peak of their popularity were at high risk for these adverse effects. COOH COOH O O CH 2 Br CH 3 CH 3 H 3 N CHF 2 NH 2 CF 3 O O S F O SO O N N N N Ibuprofen Bromoaspirin Celebrex Deramaxx for dogs* * Abby Garrett took this. (c) H HO AB CD 1 2 3 4 5 CH 3 19 10 9 8 6 7 11 13 14 12 16 17 15 H 3 C 18 C H 3 C 21 20 22 24 23 C CH 3 CH 3 H 25 26 27 HO A B C D 2 1 3 4 5 6 7 8 9 10 11 13 14 15 16 17 CH 3 18 H C H 3 C 21 20 22 24 23 C CH 3 CH 3 H 25 26 27 CH 3 19 12 (a) (b) FIGURE 24.30 The structure of cholesterol, drawn (a) in the traditional planar motif and (b) in a form that more accurately describes the conformation of the ring system. 752 Chapter 24 Lipid Biosynthesis the rate-limiting step in cholesterol biosynthesis. Here, HMG-CoA undergoes two NADPH-dependent reductions to produce 3R-mevalonate (Figure 24.32). The reac- tion is catalyzed by HMG-CoA reductase, a 97-kD glycoprotein that spans the ER membrane with its active site facing the cytosol. As the rate-limiting step, HMG-CoA reductase is the principal site of regulation in cholesterol synthesis. Three different regulatory mechanisms are involved: 1. Phosphorylation by cAMP-dependent protein kinases inactivates the reductase. This inactivation can be reversed by two specific phosphatases (Figure 24.33). 2. Degradation of HMG-CoA reductase. This enzyme has a half-life of only 3 hours, and the half-life itself depends on cholesterol levels: High [cholesterol] means a short half-life for HMG-CoA reductase. 3. Gene expression. Cholesterol levels control the amount of mRNA. If [choles- terol] is high, levels of mRNA coding for the reductase are reduced. If [choles- terol] is low, more mRNA is made. (Regulation of gene expression is discussed in Chapter 29.) A Thiolase Brainteaser Asks Why Thiolase Can’t Be Used in Fatty Acid Synthesis If acetate units can be condensed by the thiolase reaction to yield acetoacetate in the first step of cholesterol synthesis, why couldn’t this same reaction also be used in fatty acid synthesis, avoiding all the complexity of the fatty acyl synthase? The answer is O – OOC CH 3 C SCoA O CH 3 C SCoA O CH 3 C O CH 2 C SCoA O CH 2 C SCoA CH 3 C OH CH 2 – OOC CH 2 C CH 3 C OH CH 2 H H 2 H + 2 + 2 O CH 3 C SCoA Acetyl-CoA OH NADP + NADPH CoASH CoASH CoASH Acetyl-CoA Acetyl-CoA Acetoacetyl-CoA 3-Hydroxy-3-methylglutaryl-CoA (HMG-CoA) HMG-CoA reductase 3R -Mevalonate Thiolase HMG-CoA synthase FIGURE 24.31 The biosynthesis of 3R-mevalonate from acetyl-CoA. – OOC O CH 2 C S CH 3 C OH CH 2 (a) 3-Hydroxy-3- methylglutaryl- CoA (HMG-CoA) – OOC CH 2 C CH 3 C OH CH 2 H H 3R -Mevalonate HH H + N C NH 2 R First Second – OOC O CH 2 C S CH 3 C OH CH 2 Enzyme-bound intermediate HH N C NH 2 R O H O H OH NADPH NADPH CoASH CoA CoA (b) HMG-CoA Reductase (pdb id = 1DQA) ANIMATED FIGURE 24.32 (a) A reaction mechanism for HMG-CoA reductase.Two successive NADPH-dependent reductions convert the thioester, HMG-CoA, to a primary alcohol. (b) HMG-CoA reductase structure. See this figure animated at www.cengage.com/login.