Cholesterol Metabolism

Yıldırım Beyazıt University Medical Faculty Biochemistry Department Cholesterol Metabolism Prof. Dr. Fatma Meriç YILMAZ fatmamericyilmaz@hotmail.com

CHOLESTEROL • Characteristicsteroidalcohol of animaltissues • Not found in plants • Performs a number of essentialfunctions in the body

Functions of Cholesterol • Cholesterol is a structural component of all cell membranes • Modulates the fluidity of the cell membranes

Cholesterol is abundant in the tissue of the brain and nervous system. Myelin, which covers nerve axons to help conduct the electrical impulses that make movement, sensation, thinking, learning, and remembering possible, is over one fifth cholesterol by weight • Precursor to bile salts and steroid hormones • Cholesterol precursors converted to vitamine D

Cholesterol is an amphibaticstructure. Freecholesterol is found on thesurface of lipoproteins Cholesterolestersaretransported in thecore of thelipoproteins Hidrophilic Hidrophobic

Regulation of CholesterolLevelsareImportant • The balance between cholesterol influx and efflux is important • If the influx exceeds the efflux rate cholesterol accumulates in the tissues, particularly in the endothel linings of blood vessels. • This is a potentially life-threatening occurence when this deposition leads to plaque formation

Plaqueformationnarrowsthebloodvessels. • Theamount of oxygen, carriedbyblooddecreases • Tissue can not obtainenoughoxygenandthiscausesseriousproblems in theeffectedtissue. • Atherosclerosis can causecardio-, cerebro- andperipheralvasculardiseases • Mostimportantandseriousresultsoccur in coronaryarteries of heartandcausemiocardialinfarction.

CHOLESTEROL STRUCTURE • Fourfusedhydrocarbonrings (ABCD) • An eightcarbon, branchedhydrocarbonchainattachedto 17. carbon of D ring • A OH group at carbon 3 • A doublebondbetweencarbon 5 andcarbon 6 at ring B

CHOLESTEROL AND CHOLESTERYL ESTERS FA binding site • Mostcholesterol is in an esterified form whichmakesthestructuremorehydrophobic • A fattyacid is boundtothethirdcarboninstead of the OH group • Cholesterylestersare not found in membranesandhaveto be transportedwithlipoproteins in theblood.

CHOLESTEROL: Background • 1785: Fisrst discovery • 1910; Adolf Windaus ;Atherosclerosis-cholesterol association • 1985: LDL receptor determination, Joseph L. Goldstein- Michael Brown • More than 10 nobel prizes for the studies about cholesterol

De novo synthesis Cholesterol from Extrahepatic tissues Diet Liver cholesterol pool Secretion to the Bloodstream in lipoproteins Free cholesterol in bile Synthesis of Bile acids

DIET CHOLESTEROL

CHOLESTEROL SYNTHESIS • Synthesized in all tissues. • Liver, intestine, adrenal cortex and reproductive tissues make the largest contributions to the body’s cholesterol pool • In the cytoplasm • Carbons from Acetyl CoA • NADPH, ATP and O2 required

CHOLESTEROL SYNTHESIS • 4 steps; • One: HMG CoAformation • Two: 5 carbonisoprenoidformation • Three: Squalensynthesisfrom 6isoprenoid • Four:Cholesterolsynthesisfromsqualen + O2

CholesterolSynthesis1. Stage • HMG CoA formation • Analog with keton synthesis (mitochondria)

CHOLESTEROL SYNTHESIS • 4 steps; • One: HMG CoAformation • Two: 5 carbonisoprenoidformation • Three: Squalensynthesisfrom 6isoprenoid • Four:Cholesterolsynthesisfromsqualen + O2

CholesterolSynthesis2. Stage • Convertion of HMG CoA to 5 carbon isoprenoids

CHOLESTEROL SYNTHESIS • 4 steps; • One: HMG CoAformation • Two: 5 carbonisoprenoidformation • Three: Squalensynthesisfrom 6isoprenoids • Four:Cholesterolsynthesisfromsqualen + O2

IPP IPP DPP IPP GPP FPP FPP NADPH Squalen CholesterolSynthesis3. Stage • Head to tail attachment of isoprenes to form Geranyl pyrophosphate • Head to tail condensation of Geranyl pyrophosphate and isopentenylpyrophosphate to form Farnesyl pyrophosphate • Head to head fusion of two Farnesyl pyrophosphate to form squalene

CHOLESTEROL SYNTHESIS • 4 steps; • One: HMG CoAformation • Two: 5 carbonisoprenoidformation • Three: Squalensynthesisfrom 6isoprenoids • Four:Cholesterolsynthesisfromsqualen + O2

CholesterolSynthesis4. Stage NADPH • Squalenemonooxygenase adds oxygen to form an epoxide • Unsaturated carbons (double bonds) are aligned to allow cyclization and formation of lanosterol • After many reactions cholesterol is synthesized

Convertion of lanosterol to cholesterol requires many steps

Fates of Cholesterol Vitamin D Membranes CHOLESTEROL Cholesteryl Esters SteroidHormones Bile acidsandbiliaryfreecholesterol

Cholesteryl Esters • Acyl-CoA:cholesterol acyl transferase (ACAT) is an ER membrane protein • ACAT transfers fatty acid of CoA to C3 hydroxyl group of cholesterol • Excess cholesterol is stored as cholesterol esters in cytosolic lipid droplets Fig. 8

Bile Salts Bile acids & salts are effective detergents Synthesized in the liver Stored & concentrated in the gallbladder Discharged into intestine and aides in absorption of intraluminal lipids, cholesterol, & fat soluble vitamines Bile acid refers to the protonated form while bile salts refers to the ionized form

Smith-Lemli-Opitz (SLO) syndrome • 7-dehydrocholesterol reductase deficiency • 3. common metabolism disorder in USA • Cystic fibrosis & PKU • Prevalance 1:20-60,000 • Multiple congenital abnormalities/mental retardation • Spontaneous abortus • multiorgan failure • congenital heart disease • Pathology • Very low plasma cholesterol • inability to correctly produce or synthesize cholesterol • Membranes, steroid hormons& bile acids, myelin sheath

Smith-Lemli-Opitz (SLO) syndrome • The most commonly observed features include: • Characteristic dysmorphic faces • Microcephaly • Syndactyly(most commonly of the second and third toes) • Polydactyly • Growth retardation • Intellectual disability • Cleft palate • Hypospadias (males only)

Cholesterol Degradation • The ring structure can not be metabolized to carbondioxide and water • Intact sterol nucleus is eliminated from the body by convertion to bile acids and bile salts • Bile is the only efficient way for the elimination • Some of the cholesterol in the intestine is modified by bacteria before excretion. The primary compounds made are coprostenol and cholestanol. • Together with cholesterol, these compounds make up the bulk of neutral fecal sterols.

REGULATION OF CHOLESTEROL SYNTHESIS • HMG CoAReductase is the rate limitingenzyme in cholesterolsynthesisandthemajorcontrolpoint. • It is subjecttodifferentkinds of metaboliccontrol: • Sterol dependentregulation of gene expression • Sterol-acceleratedenzymedegradation • Sterol-independentphosphorylation/dephosphorylation • Hormonalregulation • Inhibitionbydrugs

Sterol dependent regulation of gene expression • Feedback regulatory system • Rate of HMG-CoA reductase mRNA synthesis controlled by sterol regulatory element binding protein (SREBP) • When sterol levels are low, SERBP is cleaved in Golgi to release the transcription factor • This transcription factor binds to Sterol Regulatuar Element and activates the synthesis of HMG-CoA reductase

SREBP SRE DNA Proteolitik kesilme Transkripsiyon (-) mRNA SREBP mRNA Translasyon Fosfoprotein fosfataz HMG CoA REDÜKTAZ (aktif) HMG CoA redüktaz (inaktif) Mevalonik asit Protein kinaz AMP (+) Kolesterol HMG CoA Sterol dependent regulation of gene expression

Sterol-accelerated enzyme degradation • When sterol is present, enzyme undergoes sterol accelerated ERAD (ER associated degradation) • HMG-CoAreductaseis ubiquitinated and extracted from membrane where it is then degraded by proteosomes

Sterol-independent phosphorylation/dephosphorylation • HMGCoA enzyme is inactive when it is phosphorylated and active when it is dephosphorylated • AMP –activated protein kinase and a phosphoprotein phosphatase control the activity of HMG CoA Reductase • When AMP increases, kinase is activated and the enzyme is inactive. • Decrease in ATP concentrations decrease cholesterol synthesis

Hormonal regulation Insulin • Activates protein phosphatase • HMG-CoA reductase is activated Glucagon • Activates protein kinase • HMG-CoA reductase is inactivated

Inhibition by drugs • Statin drugs are structural analogs of HMG CoA • They are competetive inhibitors of HMG CoA Reductase enzyme

CHOLESTEROL-DIET INTERACTION • Cholesterol from diet %15 • Cholesterol synthesized in the body %85 • The effect of diatery restriction %10-20 • Monounsaturated fatty acid ; cholesterol level • Saturated fatty acid ; LDL receptor activity • Polyunsaturated fatty acid ; LDL receptor activity

LDL receptor-cholesterol interaction Cholesterol pool Blood Liver Liver Cholesterol pool

CHOLESTEROL-DIET INTERACTIONAND SITOSTEROLS • Cholesterol from diet %15 • Synthesized cholesterol %85

EFFECT OF SITOSTEROLS + Plant sterols

Bile Acid Synthesis from Cholesterol

Bile Acids are synthesized from Cholesterol • Primary bile acids are synthesized from cholesterol in the liver • Primary bile acids are Cholic Acid and Chenodeoxycholic Acid • Bile acids contain 24 carbons, with two or three hydroxyl groups and a side chain that terminates in a carboxyl group

First step is a hydroxylation from 7. carbon • Rate limiting step in bile acid synthesis • Enzyme 7-hydroxylase • Requires O2, NADPH and cytocrome P450

7-Hydroxycholesterol, is the precursor molecule for other bile acids • Cholic acid carries an additional OH group at the 12. carbon • 27 C hydrocarbon chain is shortened 3 carbons to produce 24 C bile acids • The double bond in cholesterol B ring is reduced

Functions of Bile Acids -Bile acidsdecreasethesurfacetensionwiththehelp of theiramphibaticstructure

Functions of Bile Acids • Helpstosolubilizethefreecholesterol in bile juice • Bile saltsprovidetheonlysignificantmechanismforcholesterolexcretion • Helpsthedigestion of lipidsandlipidsolublevitamins

Bile Salts • Bile acids are conjugated with glycine or taurine before excretion to intestine and form bile salts Glyco- and taurocholic acid or Glyco- and taurochenodeoxycholic acids

Cholesterol Metabolism