CholesterolCholesterol SynthesisSynthesis ThomasThomas Dayspring,Dayspring, MD,MD, FACPFACP ClinicalClinical AssistantAssistant ProfessorProfessor ofof MedicineMedicine UniversityUniversity ofof MedicineMedicine andand DentistryDentistry ofof NewNew Jersey,Jersey, NewNew JerseyJersey MedicalMedical SchoolSchool DiplomateDiplomate ofof thethe AmericanAmerican BoardBoard ofof ClinicalClinical LipidologyLipidology CertifiedCertified MenopauseMenopause Practitioner:Practitioner: NorthNorth AmericanAmerican MenopauseMenopause SocietySociety NorthNorth JerseyJersey InstituteInstitute ofof MenopausalMenopausal LipiLipidologydology Wayne,Wayne, NewNew JerseyJersey StSt JosephJoseph’s’s RegionalRegional MedicalMedical CenterCenter Paterson,Paterson, NJNJ 19641964 NobelNobel PrizePrize PhysiologyPhysiology && MedicineMedicine CholesterolCholesterol SynthesisSynthesis PathwayPathway FeodorFeodor LynenLynen KonradKonrad BlochBloch A sterol with 27 carbon molecules with an -OH group at the # 3 position Hydrocarbon tail 22 24 21 26 25 20 18 23 CH3 12 17 27 11 13 16 19 C27H45OH CH3 14 15 1 9 2 10 8 4 Hydrocarbon Rings 3 5 7 4 Hydrocarbon Rings OH 4 6 3 Hydroxy cholesterol Sterols are waxy insoluble substances or lipids synthesized from acetyl coenzyme A (CoA). They are steroid-based alcohols having a hydrocarbon (aliphatic) side chain of 8–10 carbons at the 17 position and a hydroxyl group (-OH) at the 3 position (making it an alcohol). Because of the hydrophilicity at the -OH end and hydrophobicity at the hydrocarbon side chain, sterols can be incorporated into the lipid bilayers of the cytoplasmic membrane. FourFour StagesStages ofof 1 CholesterolCholesterol BiosynthesisBiosynthesis 2 The liver is responsible for about 15% of cholesterol synthesis and the remainder is extrahepatic. 3 Cholesterol synthesis is a four-step process starting from its precursor acetate: CH3-COO− 4 CholesterolCholesterol Synthesis:Synthesis: StageStage 11 Thiolase is an oxidant enzyme (removes electrons) InIn thethe firstfirst step,step, twotwo moleculesmolecules ofof acetylacetyl--CoACoA uponupon thethe actionaction ofof thiolasethiolase reversiblyreversibly formsforms acetoacetylacetoacetyl--CoACoA CholesterolCholesterol Synthesis:Synthesis: StageStage 11 Acetyl-CoA condenses with acetoacetyl-Co upon the action of cytosolic hydroxymethylglutaryl (HMG) synthase and becomes HMG-CoA. CholesterolCholesterol SSynthesis:ynthesis: StaStagege 11 HMG-CoA β-hydroxy-β-methylglutaryl-CoA The catalytic action (rate-limiting state) of HMG-CoA reductase, an integral part of the smooth endoplasmic reticulum, irreversibly forms mevalonate (a five- carbon intermediate) with NADPH serving as the reductant. HMG-CoA reductase transcription is tightly regulated by a membrane-bound transcription factor designated as sterol regulatory element-binding protein-2 (SREBP-2) Different from mitochondrial HMG-CoA which plays a role in ketosis CholesterolCholesterol Synthesis:Synthesis: StageStage 22 5 Carbon units 5 Carbon units In the second step, mevalonate is phosphorylated from ATP to isoprene units or isoprenoids, namely isopentyl pyrophosphate, which can isomerize or interconvert to dimethylallyl pyrophosphate CholesterolCholesterol ++ Synthesis:Synthesis: StageStage 33 10 carbon intermediate Isoprenoids react with each other to form geranyl ++ pyrophosphate. Condensation with another isopentyl-PP yields farnesyl 15 carbon intermediate pyrophosphate. Squalene synthase catalyzes ++ the condensation of two molecules of farnesyl-PP with reduction by NADPH to make squalene. 30 carbon intermediate Squalene CholesterolCholesterol Synthesis:Synthesis: StageStage 44 A series of reactions - oxidation, cyclization, and loss of three methyl groups - results in conversion of squalene to cholesterol. The fourth step involves conversion of the linear squalene molecule to the four-ringed steroid nucleus. Lathosterol 27carbon molecule Cholesterol CholesterolCholesterol SynthesisSynthesis Acetate Acetoacetyl-CoA HMG synthase Dimethylallyl-PP HMGCoA HMG CoA reductase isomerization Mevalonate Isopentyl-pyrophosphate condensation Farnesyl-pyrophosphate Squalene Squalene synthase Isoprenoids Lanosterol Cholesterol 4 ringed structure 19-20 reactions including Desmosterol, Lathosterol Dayspring T in Chap 14 Davidson, Toth, Maki Therapeutic Lipidology 2008 NaturalNatural ProductsProducts DerivedDerived fromfrom ActivatedActivated IsopreneIsoprene UnitsUnits Isoprene units or Isoprenoids are a class of largely hydrophobic or nonpolar compounds related by being constructed biosynthetically from five-carbon units..