Biochemistry: Cholesterol and Steroid Metabolism PDF

BIOCHEMISTRY LIPIDS: Cholesterol and Steroid Metabolism Jandoc, M.D. B. Liver Cholesterol...

BIOCHEMISTRY LIPIDS: Cholesterol and Steroid Metabolism Jandoc, M.D. B. Liver Cholesterol 1. Central Role in the Regulation of the Body’s Cholesterol Balance TOPIC OUTLINE a. Cholesterol Enters the Liver’s Cholesterol Pool from Different I. Overview Sources II. Structure of cholesterol - dietary cholesterol III. Cholesterol Biosynthesis - synthesized cholesterol IV. Cholesterol Degradation · extrahepatic tissues V. Bile Acids and Biles Salts · hepatic de novo synthesis b. Cholesterol is Eliminated as I. OVERVIEW - unmodified cholesterol in the bile  Cholesterol, the characteristic steroid alcohol of animal tissues, - component of plasma performs a number of essential functions in the body. lipoproteins sent to the  As a typical product of animal metabolism, cholesterol occurs in peripheral tissues foods of animal origin such as egg yolk, meat, liver, and brain. - bile salts secreted into the  Cholesterol is an amphipathic lipid and as such is an essential intestinal lumen structural component of membranes, where it is important for 2. Cholesterol Influx and Efflux Imbalance the maintenance of the correct permeability and fluidity, and of → gradual cholesterol deposition the outer layer of plasma lipoproteins. in the endothelial linings of blood vessels → narrowing of A. Cholesterol blood vessels (atherosclerosis) - synthesized virtually by all tissues in humans → increased risk of coronary - embedded inside the lipid bilayer artery disease - most abundant sterol in humans - very hydrophobic 1. Largest Contribution to the Body’s Pool - liver - intestines II. STRUCTURE OF CHOLESTEROL - adrenal cortex - reproductive tissues  Cholesterol is an alicyclic compound whose basic structure · ovaries includes the perhydrocyclopentanophenanthrene nucleus · testes containing four fused rings. · placenta  In its “free” form, the cholesterol molecule contains 27 carbon 2. Essential Functions in the Body atoms. - structural component of cell membranes  Consists of 4 Fused Rings - modulate membrane fluidity - identified by the 1st 4 letters of the alphabet - precursor of: - carbons numbered in sequence · bile acids a. Ring A · steroid hormones · hydroxyl group at C3 · vitamin D b. Ring B - component of plasma lipoproteins · double bond between C5 and C6 c. Ring C Cholesterol steroid d. Ring D nucleus composed · 8-membered, branched hydrocarbon chain attached of 4 rings. to C17 Derivatives: · Cholic acid (Bile salt) · 17 β–estradiol (Steroid hormone) - a methyl group (carbon 19) attached to carbon 10, and a second methyl group (carbon 18) attached to carbon 13 - The structure of cholesterol suggests that its synthesis involves multimolecular interactions and significant reducing power. Trans FINALS 6 ABACO, ALDERITE, ASISTIN, BALANZA, BAYAS, BIANG 1 of 8  BIOCHEMISTRY LIPIDS: Cholesterol and Steroid Metabolism A. Sterols - balance the rate of cholesterol synthesis against the rate of 1. Steroids with cholesterol excretion - 8-10 carbon atoms in the side chain (aliphatic) at C17 - imbalance - alcohol hydroxyl group at C3 → elevated circulating cholesterol levels → possibility of 2. Cholesterol coronary artery disease (CAD) - major sterol in animal tissues → excessive cholesterol secretion into the bile → 3. β-Sitosterol cholesterol precipitation in the gallbladder - plant sterol (cholelithiasis) and bile duct (choledocholithiasis) - poorly absorbed in humans A. Synthesis of 3-hydroxy-3-methylglutaryl (HMG) CoA - commercially available as trans fatty acid-free margarine - A.K.A. – β -Hydroxy-β-Methylglutaryl CoA - can block the absorption of cholesterol by interfering - Two HMG-CoA Synthase Isoenzyme in the Liver Parenchymal Cells competitively with its binding to intestinal mucosal cell a. Cytosolic Enzyme membranes before uptake → clinically useful dietary · for cholesterol synthesis treatment for hypercholesterolemia b. Mitochondrial Enzyme B. Cholesterol Esters · for ketone body synthesis - fatty acid attached to C3 → more hydrophobic than free B. Mevalonic Acid (Mevalonate) Synthesis cholesterol → must be transported either - rate-limiting step in cholesterol synthesis · in association with protein as a component of a - occurs in the cytosol lipoprotein particle - uses 2 molecules of NADPH as reducing agents → release of CoA · solubilized by phospholipid and bile salts in the bile → irreversible reaction - not found in membranes 1. HMG CoA Reductase - low levels in most cells - intrinsic membrane protein of the ER - catalytic domain projects into the cytosol III. CHOLESTEROL BIOSYNTHESIS 2. Feeding of Cholesterol → reduced HMG CoA reductase activity → reduced hepatic  A little more than half the cholesterol of the body arises by cholesterol biosynthesis synthesis (about 700 mg/d), and the remainder is provided by - intestinal cholesterol biosynthesis does not respond to feeding the average diet. of high-cholesterol diets  The liver and intestine account for approximately 10% each of 3. Fasting total synthesis in humans. → limited acetyl CoA and NADPH availability → reduced HMG 1. Occur Virtually in All Tissues CoA reductase activity → decreased cholesterol synthesis  Major Site of Synthesis 4. High Fat or Carbohydrate Diets i. Liver → increased acetyl CoA and NADPH availability → increased ii. Other Organs cholesterol synthesis - intestines 5. Reversible HMG CoA Reductase Phosphorylation- - adrenal cortex Dephosphorylation - reproductive tissues - dephosphorylated form (more active) - ovaries - phosphorylated form (less active) - testes - placenta - skin - neural tissue - aorta 2. Acetate Moiety of Acetyl CoA - all 27 carbon atoms of cholesterol are derived from acetate 3. NADPH - provides the reducing equivalents 4. Pathway - driven by the hydrolysis of the - high-energy thioester bond of acetyl CoA - terminal phosphate bond of ATP 5. Synthesis a. Occurs in the · cytoplasm · endoplasmic reticulum membrane b. Enzymes A. (HMG) CoA Synthesis B. Mevalonate Synthesis · in the - cytosol · ER C. Major Stages of Cholesterol Biosynthesis 6. Regulatory Mechanisms Trans FINALS 6 ABACO, ALDERITE, ASISTIN, BALANZA, BAYAS, BIANG 2 of 8  BIOCHEMISTRY LIPIDS: Cholesterol and Steroid Metabolism 1. Acetate → Hydroxymethylglutaryl-CoA (HMG-CoA) → Mevalonate 2. Condensation of 6 Isoprene Units → Squalene → Isopentenyl Pyrophosphate  Squalene  Isopentenyl Pyrophosphate - formed from 6 isoprenoid units - isoprene unit - linear 30-carbon compound - precursor of isoprenoids: - 3 ATPs hydrolyzed per mevalonic acid residue converted a. Sterol Isoprenoid to isopentenyl pyrophosphate → 18 ATPs required to · cholesterol make the polyisoprenoid squalene · non-sterol isoprenoids · dolichol · ubiquinone b. Plant-Derived Isoprenoid Compounds · rubber · camphor · fat-soluble vitamins (A, D, E, K) · β-carotene (provitamin A) i. Polyprenoids - synthesized from 5 carbon isoprene units - include: a. Ubiquinone · member of mitochondrial respiratory chain b. Dolichol · takes part in glycoprotein synthesis by transferring carbohydrate residues to asparagine residues of the polypeptide ii. Prenylation The formation of squalene from six isoprene units. The activation of - covalent attachment of farnesyl to proteins the isoprene units drives their condensation to form geranyl - one mechanism for anchoring proteins to plasma pyrophosphate, farnesyl pyrophosphate, and squalene. membranes 3. Squalene Oxidation and Cyclization → Lanosterol - squalene hydroxylation triggers the cyclization of the structure of lanosterol - Steps: · squalene monooxygenase adds a single oxygen atom from O2 to the end of the → In the second stage of squalene molecule, forming cholesterol synthesis, three an epoxide phosphate groups are · NADPH then reduces the transferred from three other oxygen atom of O2 to molecules of ATP to H2O mevalonate. The purpose of · unsaturated carbons of the these phosphate transfers is squalene 2,3-epoxide are to activate both carbon 5 aligned in a way that allows and the hydroxyl group on conversion of the linear carbon 3 for further squalene epoxide into a cyclic reactions in which these structure groups will participate. · cyclization leads to the formation of lanosterol, a sterol with the four ring structure characteristic of the The formation of activated steroid nucleus. isoprene units ( Δ 3-isopentenyl · A series of complex reactions pyrophosphate anddimethylallyl containing many steps, leads pyrophosphate) from mevalonic to the formation of acid. Note the large ATP cholesterol requirement for these steps. Trans FINALS 6 ABACO, ALDERITE, ASISTIN, BALANZA, BAYAS, BIANG 3 of 8  BIOCHEMISTRY LIPIDS: Cholesterol and Steroid Metabolism 4. Further Modification → Cholesterol · transmembrane protein  Conversion of Lanosterol to Cholesterol Results to · remains localized to the ER when bound to cholesterol · shortening of the carbon chain from 30 to 27 (as occurs when the cellular cholesterol concentration · removal of the 2 methyl groups at C4 is high) · migration of the double bond from C8 to C5 ii. Active SREBP · reduction of the double bond between C24 and C25 - inactive SREBP → proteolytic cleavage by proteases → - all of the enzymes are located in the ER active form (Nterminal fragment) migrates to the  Smith-Lemli-Opitz Syndrome (SLOS) nucleus → binding with available SRE sites on DNA → - relatively common increased reductase gene expression - autosomal recessive iii. Feedback Inhibition - partial deficiency in 7-dehydrocholesterol-7-reductase  Low Cholesterol Level · involved in the migration of the double bond → cholesterol dissociation from SCAP → conformational - one of several multisystem, embryonic malformation change in SCAP → migration of the SCAP-SREBP syndromes associated with impaired cholesterol synthesis complex into the Golgi apparatus → SREBP activation by proteolytic cleavage → increased HMG CoA reductase → increased cholesterol synthesis  High Cholesterol Level → induce the binding of SCAP to other ER membrane proteins (insig proteins) → retention of the SCAP- SREBP in the ER → activation of SREBP prevented → down-regulation of cholesterol synthesis c. Cholesterol Content - affects the stability of · HMG CoA reductase protein · HMG CoA reductase mRNA - increased cholesterol → decreased stability → increased degradation of the protein and its mRNA 2. Sterol-Accelerated Enzyme Degradation a. Reductase Enzyme → Formation of Cholesterol: The formation of cholesterol from - integral protein of the ER membrane lanosterol takes place in the membranes of the endoplasmic b. High Cellular Sterol Levels reticulum and involves changes in the steroid nucleus and the - reductase binds to proteins → ubiquitination and proteasomal side chain. The methyl groups on C14 and C4 are removed to degradation of the reductase form 14-desmethyl lanosterol and then zymosterol. The double bond at C8—C9 is subsequently moved to C5—C6 in two steps, forming desmosterol. Finally, the double bond of the side chain is reduced, producing cholesterol. D. Cholesterol Synthesis Regulation  HMG CoA Reductase - intrinsic membrane protein of the ER - active site extends into the cytosol - rate-limiting enzyme of cholesterol synthesis - subject to different kinds of metabolic control 1. Sterol-Dependent Regulation of Gene Expression a. HMG CoA Reductase Gene - expression controlled by sterol regulatory element-binding protein (SREBP) b. SREBP The Proteasome and cap proteins. The cap proteins (PA700 and PA28) - transcription factor regulate the activity of this proteolytic complex by recruiting to the - binds to the cis-acting sterol regulatory element (SRE) of the complex the substrates for proteolysis. The ATP requirement is to reductase gene unfold and denature the proteins targeted for destruction. o SRE - located upstream of the reductase gene i. Inactive SREBP 3. Sterol-Independent Phosphorylation/Dephosphorylation - transmembrane protein of the ER - HMG CoA reductase activity controlled covalently through the - noncovalently bound to SREBP cleavage-activating actions of protein (SCAP) o SREBP Cleavage – Activating Protein (SCAP) Trans FINALS 6 ABACO, ALDERITE, ASISTIN, BALANZA, BAYAS, BIANG 4 of 8  BIOCHEMISTRY LIPIDS: Cholesterol and Steroid Metabolism i. Adenosine Monophosphate (AMP) - Activated Protein Kinase E. Cholesterol Esterification (AMPK) 1. Esterified with Long Chain Fatty Acids at C3 - activated by AMP → cholesterol synthesis is decreased - bulk of tissue cholesterol when ATP availability is decreased - 65% of plasma cholesterol ii. Phosphoprotein Phosphatase 2. Cellular Cholesterol Ester Synthesis a. Active Form - require ATP to form fatty acyl CoA derivative → transferred to · dephosphorylated 3-β hydroxyl group of cholesterol b. Inactive Form 3. Acyl CoA: Cholesterol Acyltransferase · Phosphorylated - esterifies cholesterol associated with plasma lipoproteins IV. CHOLESTEROL DEGRADATION  Cholesterol is excreted from the body via the bile either in the unesterified form or after conversion into bile acids in the liver. A. Ring Structure of Cholesterol - cannot be metabolized to CO2 and H2O by humans - Rather, the intact sterol nucleus is eliminated from the body by conversion to bile acids and bile salts, which are excreted in the feces, and by secretion of cholesterol into the bile, which 4. Hormonal Regulation transports it to the intestine for elimination a. Glucagon - favors - formation of the inactive (phosphorylated) form of B. Intact Sterol Ring HMG-CoA reductase → decreased rate of cholesterol - eliminated from the body by synthesis 1. Conversion to Bile Acids and Bile Salts - downregulation of HMG CoA reductase gene expression → excreted in the feces b. Insulin 2. Secretion of Cholesterol into the Bile - favors - formation of the active (dephosphorylated) form of → intestines → elimination HMG-CoA reductase → increased rate of cholesterol synthesis C. Some of the Cholesterol in the intestines are modified by bacteria - upregulation of HMG CoA reductase gene expression before elimination c. Thyroid Hormone  Coprostanol and Cholestanol - stimulates HMG CoA reductase activity → increased - primary compounds made cholesterol synthesis - reduced derivatives of cholesterol 5. Inhibition by Drugs - isomers differing in the orientation of the hydrogen atom  Statin Drugs between the A and B rings - are structural analogues of HMG CoA, and are (or are - principal sterol in the feces metabolized to) reversible, competitive inhibitors of HMG CoA reductase and are used to decrease plasma cholesterol levels in patients with hypercholesterolemia. Examples · lovastatin · mevastatin · simvastatin · atorvastatin · fluvastatin · pravastatin · rosuvastatin D. Bulk of Neutral Fecal Sterols Structural similarity of HMG - cholesterol and pravastatin, a clinically - coprostanol useful cholesterol-lowering - cholestanol drug of the“statin” family. V. BILE ACIDS AND BILE SALTS  The primary bile acids are synthesized in the liver from cholesterol.  These are cholic acid (found in the largest amount in most mammals) andchenodeoxycholic acid Trans FINALS 6 ABACO, ALDERITE, ASISTIN, BALANZA, BAYAS, BIANG 5 of 8  BIOCHEMISTRY LIPIDS: Cholesterol and Steroid Metabolism 1. Predominant Bile Acids in Humans Structures of the primary and secondary bile salts. Primary bile salts form - cholic conjugates with taurine or glycine in the liver. After secretion into the intestine, - chenodeoxycholic they may be deconjugated and dehydroxylated by the bacterial flora, forming - deoxycholic secondary bile salts. Note that dehydroxylation occurs at position 7, forming the deoxy family of bile salts. Dehydroxylation at position 12 also leads to - lithocholic excretion of the bile salt. 2. Bile - watery mixture of organic and inorganic compounds a. Most Important Organic Components 2. Rate-Limiting Step - phosphatidylcholine - introduction of a hydroxyl group at carbon 7 of the steroid ring - bile salt - catalyzed by cholesterol-7-α-hydroxylase b. Most Common Bile Acids - ER-associated cytochrome P450 enzyme - cholic acid - found only in the liver - chenodeoxycholic acid - downregulated by cholic acid A. Structure of the Bile Acids - upregulated by cholesterol - derived from cholesterol (C-27 steroid) by scission of the side chain which leaves - C-24 carboxyl group with the loss of 3 carbons - saturation of the Δ5 double bond of cholesterol - hydroxylation of the steroid nucleus - contain 24 carbons with 2 or 3 hydroxyl groups and a side chain that terminates in a carboxyl group (pKa of about 6) → not fully ionized at physiologic pH 1. Amphipathic - all of the hydroxyl groups are in β orientation (lie above the plane of the rings) - methyl groups are α (lie below the plane of the rings) - have polar and nonpolar face → act as emulsifying agents in the intestines → prepare dietary TAG and other complex The reaction catalyzed by 7-α-hydroxylase. An α-hydroxyl group is formed at lipids for degradation by pancreatic digestive enzymes position 7 of cholesterol. This reaction, which is inhibited by bile salts, is the 2. Bile Salts rate-limiting step in bile salt synthesis. - provide the only mechanism for cholesterol excretion both as C. Synthesis of Bile Salts - metabolic product of cholesterol  Bile salts are synthesized in the liver from cholesterol by reactions - essential solubilizer for cholesterol excretion in bile that hydroxylate the steroid nucleus and cleave the side chain. B. Synthesis of Bile Acids 1. Includes 1. Multistep, Multiorganelle Process that Includes - glycocholic acid - insertion of hydroxyl groups at specific positions on the steroid - glycochenodeoxycholic acid structure - taurocholic acid - reduction of the double bond of the cholesterol B ring - taurochenodeoxycholic acid - shortening of the hydrocarbon chain by 3 carbons 2. Bile Acids Conjugated to Either Glycine or Taurine - introduction of a carboxyl group at the end of the chain → bile salt formation → leave the liver → result to formation of primary bile acids - by an amide bond between the carboxyl group of the bile - cholic acid (triol) acid and the amino group of the added compound - chenodeoxycholic acid (diol) - glycine to taurine forms are 3:1 - stored in the gallbladder  Addition of Glycine or Taurine → presence of a carboxyl group with a lower pKa (from glycine) or of a sulfate group (from taurine) → glycocholate or taurocholate both are fully ionized (negatively charged) at physiologic pH  Taurine - end product of cysteine catabolism - abundant in the · retina · central nervous system - also found in other tissues (liver) 3. Enhanced Amphipathic Nature of Bile Salts → more effective detergents than bile acids 4. Provide the Only Significant Mechanism for Cholesterol Excretion - both as - metabolic cholesterol product - essential solubilizer for cholesterol excretion in bile 5. Exogenous Chenodeoxycholic Acid Trans FINALS 6 ABACO, ALDERITE, ASISTIN, BALANZA, BAYAS, BIANG 6 of 8  BIOCHEMISTRY LIPIDS: Cholesterol and Steroid Metabolism - treatment for genetic deficiencies in the conversion of - steroid nucleus cannot be oxidized to carbon dioxide and cholesterol to bile acids water by human tissues 2. Cholesterol Solubility in Bile - cholesterol is insoluble except for its association with · bile salts · phospholipids (chiefly phosphatidylcholine) 3. Bile Salts Secreted into the Intestines - efficiently reabsorbed and reused 4. Mixture of Primary and Secondary Bile Acids and Bile Acids D. Action of Intestinal Flora on Bile Salts - absorbed primarily in the ileum by active transport - remove glycine or taurine from bile salts → regenerating bile acids - actively moved from the intestinal mucosal cells into the - remove a hydroxyl group from some of the primary bile acids portal blood →secondary bile acids - efficiently removed by the liver parenchymal cells - deoxycholic from cholic acid 5. Bile Acids - lithocholic from chenodeoxycholic acid - hydrophobic Primary bile acids are further metabolized in the intestine by the activity of the - require a carrier (albumin) in the portal blood intestinal bacteria. Thus, deconjugation and 7α- dehydroxylation occur, 6. Liver - converts both primary and secondary bile acids to bile producing the secondary bile acids, deoxycholic acid, and lithocholic acid. salts by conjugation with glycine and taurine → ready for secretion into the bile 7. Quantity of Bile Salts Secreted - between 15 and 30 g of bile salts are secreted from the liver into the duodenum/day · 0.5 g is lost/day in the feces · 0.5 g is synthesized by the liver/day → cholesterol (liver) → bile acids → storage of bile in the gallbladder → intestines (primary bile acids, cholic and chenodeoxycholic, are converted to deoxycholate and lithocholate by intestinal bacteria) → deconjugation of primary and secondary bile acids → reabsorption via the portal vein bound to albumin → reconjugation of bile acids with taurine and glycine by the liver → all 4 bile acids into the bile 8. Cholestyramine - bile acid sequestrant - bind bile acids in the gut → prevent reabsorption → promote excretion - for the treatment of hypercholesterolemia 9. Bile Acid Removal → relieves the inhibition on bile acid synthesis in the liver divert additional cholesterol into the excretory pathway 10. Dietary Fiber - bind bile acids → increased excretion Biosynthesis and degradation of bile acids. A second pathway in mitochondria F. Function involves hydroxylation of cholesterol by sterol 27-hydroxylase. *Catalyzed by  continuous conversion of cholesterol to bile acids → prevent microbial enzymes. excessive accumulation of cholesterol in tissues → excretion in the E. Enterohepatic Circulation feces G. Bile Salt Deficiency: Cholelithiasis  movement of cholesterol from the liver into the bile must be accompanied by the simultaneous secretion of phospholipid and bile salts - if this process is disrupted → more cholesterol enters the bile than can be solubilized by bile salts and lecithin → cholesterol may precipitate in the gallbladder → cholelithiasis 1. Causes of Cholelithiasis a. Gross Bile Acid Malabsorption - from the intestines (patients with severe ileal disease) 1. Main Route of Cholesterol Excretion b. Biliary Tract Obstruction Trans FINALS 6 ABACO, ALDERITE, ASISTIN, BALANZA, BAYAS, BIANG 7 of 8  BIOCHEMISTRY LIPIDS: Cholesterol and Steroid Metabolism → interruption of the enterohepatic circulation produces the steroid ring system (parent steroid lanosterol), which, c. Severe Hepatic Dysfunction after the loss of three methyl groups and other changes generate → decreased bile salt synthesis cholesterol. The adrenal cortex and the gonads also synthesize other abnormalities of bile production cholesterol in significant amounts and use it as a precursor for steroid d. Excessive Feedback Suppression of Bile Acid Synthesis hormone synthesis. - as a result of an accelerated rate of bile acid recycling Although the fecal excretion of bile salts is relatively low, it is a e. Increased Biliary Cholesterol Excretion major means by which the body disposes of the steroid nucleus of - as with the use of fibrates cholesterol. Because the ring structure of cholesterol cannot be  Fibrates degraded in the body, it is excreted mainly in the bile as free · derivative of fibric acid cholesterol and bile salts. · used to decrease blood TAG level through up- regulation of fatty acid β-oxidation REFERENCES: · ex: gemfibrozil · Harvey RA. Lippincott’s Illustrated Reviews: Biochemistry. 5th ed. A major target of the fibrates is peroxisome proliferator-activated Philadelphia: Lippincott Williams & Wilkins, 2011. receptor-α (PPARα). Fibrate binding to PPARα activates this · Rodwell VW., et.al. Harper’s Illustrated Biochemistry. 13th ed. The transcription factor, which then leads to the transcription of a McGraw-Hill Education, 2015. multitude of genes that degrade lipids. · Lieberman M., et.al. Mark’s Basic Medical Biochemistry: A Clinical f. Cholesterol Precipitation Approach. 4th ed. Lippincott Williams & Wilkins, 2013 - out of solution around a core of protein and bilirubin 2. Treatment for Cholelithiasis a. Surgery b. Chenodiol - chenodeoxycholic acid - supplements the body’s supply of bile acids → gradual (months to years) dissolution of gallstones - for patients with gallstones primarily composed of precipitated cholesterol (about 80% of cholelithiasis population) H. Coprosterol (Coprostanol)  occurs in the feces  reduction of double bond of cholesterol between C5 and C6 by intestinal bacteria SUMMARY Cholesterol is transported in the blood in lipoproteins because of its absolute insolubility in water, serves as a stabilizing component of cell membranes and as a precursor of the bile salts, steroid hormones (corticosteroids, sex hormones), and vitamin D. The precursor for cholesterol synthesis is acetyl coenzyme A (acetyl-CoA), which can be produced from glucose, fatty acids, or amino acids. Precursors of cholesterol are converted to ubiquinone, dolichol, and, in the skin, to cholecalciferol, the active form of vitamin D. Two molecules of acetyl-CoA form acetoacetyl-CoA, which condenses with another molecule of acetyl-CoA to form hydroxymethylglutaryl-CoA (HMG-CoA). Reduction of HMG-CoA produces mevalonate. This reaction, catalyzed by HMG-CoA reductase, is the major rate-limiting step of cholesterol synthesis. Mevalonate produces isoprene units that condense, eventually forming squalene. Cyclization of squalene Trans FINALS 6 ABACO, ALDERITE, ASISTIN, BALANZA, BAYAS, BIANG 8 of 8