Cholesterol:• Importance: -• It is the most important sterol in animal tissues as freealcohol or in an esterified form (with linoleic, oleic, palmiticacids or other fatty acids).• Steroid hormones, bile salts and vitamin D are derivativesfrom it.• Tissues contain different amounts of it that serve astructural and metabolic role, e.g., adrenal cortex content is10%, whereas, brain is 2%, others 0.2-0.3%.• Source: - It is synthesized in the body from acetyl-CoA(1gm/day, cholesterol does not exist in plants) and is alsotaken in the diet (0.3 gm/day as in, butter, milk, egg yolk,brain, meat and animal fat).
Physical propeties:It has a hydroxyl group on C3, a doublebond between C5 and C6, 8 asymmetric carbon atoms and aside chain of 8 carbon atoms.• It is found in all animal cells, corpus luteum and adrenalcortex, human brain (17% of the solids).• In the blood (the total cholesterol amounts about 200 mg/dLof which 2/3 is esterified, chiefly to unsaturated fatty acidswhile the remainder occurs as the free cholesterol.CH3CH3HOCH3CH3CH3Cholesterol
• Chemical properties Intestinal bacteria reducecholesterol into coprosterol and dihydrocholesterol.• - It is also oxidized into 7-Dehydrocholesterol and furtherunsaturated cholesterol with a second double bond betweenC7 and C8. When the skin is irradiated with ultraviolet light7-dehydrocholesterol is converted to vitamin D3. Thisexplains the value of sun light in preventing rickets.CH3CH3HOCH3CH3CH3Coprosterol,in fecesHCH3CH3HOCH3CH3CH3Dihydrocholesterol,in blood and other tissuesH
Synthesis of Cholesterol• Cholesterol is synthesized by virtually all tissues inhumans, although liver, intestine, adrenal cortex, andreproductive tissues, including ovaries, testes, andplacenta, make the largest contributions to the bodyscholesterol pool.• As with fatty acids, all the carbon atoms in cholesterolare provided by acetate, and NADPH provides thereducing equivalents. The pathway is endergonic, beingdriven by hydrolysis of the high-energy thioester bondof acetyl coenzyme A (CoA) and the terminalphosphate bond of adenosine triphosphate (ATP).
• Synthesis occurs in the cytoplasm, with enzymesin both the cytosol and the membrane of theendoplasmic reticulum.• The pathway is responsive to changes incholesterol concentration, and regulatorymechanisms exist to balance the rate ofcholesterol synthesis within the body against therate of cholesterol excretion. An imbalance in thisregulation can lead to an elevation in circulatinglevels of plasma cholesterol, with the potentialfor coronary artery disease.
Synthesis of 3-hydroxy-3-methylglutaryl (HMG) CoA• The first two reactions in the cholesterol synthetic pathwayare similar to those in the pathway that produces ketonebodies . They result in the production of HMG CoA .• First, two acetyl CoA molecules condense to formacetoacetyl CoA.• Next, a third molecule of acetyl CoA is added, producingHMG CoA, a six-carbon compound. [Note: Liverparenchymal cells contain two isoenzymes of HMG CoAsynthase. The cytosolic enzyme participates in cholesterolsynthesis, whereas the mitochondrial enzyme functions inthe pathway for ketone body synthesis.]
Synthesis of mevalonic acid(mevalonate)• The next step, the reduction of HMG CoA to mevalonicacid, is catalyzed by HMG CoA reductase, and is therate-limiting and key regulated step in cholesterolsynthesis.• It occurs in the cytosol, uses two molecules of NADPHas the reducing agent, and releases CoA, making thereaction irreversible . [Note: HMG CoA reductase is anintrinsic membrane protein of the endoplasmicreticulum (ER), with the enzymes catalytic domainprojecting into the cytosol. Regulation of HMG CoAreductase activity is discussed below.]
Synthesis of cholesterol• The reactions and enzymes involved in the synthesis ofcholesterol from mevalonate are illustrated as•  Mevalonic acid is converted to 5-pyrophosphomevalonate in two steps, each of whichtransfers a phosphate group from ATP.•  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 diversefunctions, the isoprenoids. Cholesterol is a sterolisoprenoid.•  IPP is isomerized to 3,3-dimethylallyl pyrophosphate(DPP).•  IPP and DPP condense to form ten-carbon geranylpyrophosphate (GPP).
•  A second molecule of IPP then condenses with GPP to form 15-carbon farnesyl pyrophosphate (FPP). [Note: Covalent attachmentof farnesyl to proteins, a process known as “prenylation,” is onemechanism for anchoring proteins to plasma membranes.]•  Two molecules of FPP combine, releasing pyrophosphate, andare reduced, forming the 30-carbon compound squalene. [Note:Squalene is formed from six isoprenoid units. Because three ATP arehydrolyzed per mevalonic acid residue converted to IPP, a total of18 ATP are required to make the polyisoprenoid squalene.]•  Squalene is converted to the sterol lanosterol by a sequence ofreactions that use molecular oxygen and NADPH. The hydroxylationof squalene triggers the cyclization of the structure to lanosterol.
•  The conversion of lanosterol to cholesterol is amultistep process, resulting in the shortening of the carbonchain from 30 to 27 carbons, removal of the two methylgroups at C-4, migration of the double bond from C-8 to C-5, and reduction of the double bond between C-24 and C-25.• [Note: This pathway includes more than 19 differentenzymatic reactions. Smith-Lemli-Opitz syndrome (SLOS), arelatively common autosomal recessive disorder ofcholesterol biosynthesis, is caused by a partial deficiency in7-dehydrocholesterol-7-reductase—an enzyme involved inthe migration of the double bond. SLOS is one of severalmultisystem, embryonic malformation syndromesassociated with impaired cholesterol synthesis.]
Regulation of cholesterol synthesis• HMG CoA reductase, the rate-limiting enzyme,is the major control point for cholesterolbiosynthesis, and is subject to different kindsof metabolic control.
Sterol-dependent regulation of geneexpression:• Expression of the HMG CoA reductase gene is controlled bythe transcription factor, SREBP (sterol regulatory element–binding protein) that binds DNA at the cis-acting sterolregulatory element (SRE) of the reductase gene. SREBP isan integral protein of the ER membrane, and associateswith a second ER membrane protein,SCAP (SREBP cleavage–activating protein).• When sterol levels in the cell are low, the SREBP-SCAPcomplex is sent out of the ER to the Golgi. In the Golgi,SREBP is acted upon by proteases which generate a solublefragment that enters the nucleus and functions as atranscription factor. This results in increased synthesis ofHMG CoA reductase and, therefore, increased cholesterolsynthesis.
• If sterols are abundant, however, they inducethe binding of SCAP to yet other ERmembrane proteins (insigs). This results in theretention of the SCAP-SREBP in the ER, thuspreventing the activation of SREBP, andleading to down-regulation of cholesterolsynthesis.
Sterol-accelerated enzymedegradation:• The reductase itself is an integral protein ofthe ER membrane. When sterol levels in thecell are high, the reductase binds to insigproteins. This binding leads to ubiquitinationand proteasomal degradation of the reductase
Sterol-independentphosphorylation/dephosphorylation:• HMG CoA reductase activity is controlledcovalently through the actions of adenosinemonophosphate (AMP)–activated protein kinase(AMPK), and a phosphoprotein phosphatase .• The phosphorylated form of the enzyme isinactive, whereas the dephosphorylated form isactive. [Note: AMPK is activated by AMP, socholesterol synthesis, like fatty acid synthesis, isdecreased when ATP availability is decreased.]
Hormonal regulation:• The amount (and, therefore, the activity) ofHMG CoA reductase is controlled hormonally.An increase in insulin favors up-regulation ofthe expression of the HMG CoA reductasegene. Glucagon has the opposite effect.
Inhibition by drugs:• The statin drugs (atorvastatin, fluvastatin,lovastatin, pravastatin, rosuvastatin, andsimvastatin) are structural analogs of HMGCoA, and are (or are metabolized to)reversible, competitive inhibitors of HMG CoAreductase. They are used to decrease plasmacholesterol levels in patients withhypercholesterolemia.