LIPID SYNTHESIS

IF THE SHEEP IN THIS PICTURE COULD HEAR TODAY’S LECTURE, THEY MIGHT DECIDE TO RE-THINK THEIR PLANS!

HAVING LEARNED ABOUT THE DIFFERENT TYPES OF LIPIDS, THEIR INGESTION, TRANSPORT, STORAGE, SOME PATHOLOGY AND USE AS FUELS – WE NOW TURN TO LIPID BIOSYNTHESIS.

THIS IS A VAST AND COMPLEX FIELD, SO FOCUS WILL JUST BE ON FATTY ACID AND CHOLESTEROL SYNTHESES. OTHER AREAS THAT WILL NOT BE APPROACHEDARE SYNTHESES OF SPHINGOLIPIDS, BILE ACIDS AND STEROID HORMONES.

FATTY ACID SYNTHESIS – (GENERAL CONSIDERATIONS)

IF A LARGE VARIETY OF FATTY ACIDS ARE TAKEN IN, IN THE DIET,AND ARE STORED AND BROKEN DOWN FOR SPECIFIC CELL NEEDS – YOU MIGHT WONDER WHY CELLS GO THROUGH THE PROCESS OF MAKING NEW FATTY ACIDS.

IN FACT, IN OUR DIETS THAT ARE COMMON TO THE SO-CALLED“DEVELOPED” NATIONS, THE NEED FOR THE SYNTHESIS OF FATTYACIDS IS NOT SO CRITICAL, BUT IT DOES OCCUR AND DOES SO INTWO WAYS:

1) “DE NOVO” SYNTHESIS OF FATTY ACIDS2) CARBON LENGTHENING OF PALMITIC ACID & FORMATION OF DOUBLE-BONDS

“DE NOVO” SYNTHESIS MEANS THE SYNTHESIS OF FATTY ACIDSCOMES FROM 2-CARBON UNITS.

WHEN IS IT IMPORTANT? UNDER CONDITIONS WHEN THE DIETARYSUPPLY OF FATTY ACIDS IS LIMITED – SUCH AS THE DEVELOPING FETUS; IN COUNTRIES THAT HAVE DECREASED FAT IN THEIR DIET.

WHAT “SIDE-EFFECTS” ARE PRODUCED BY DE NOVO SYNTHESIS?SINCE GLUCOSE IS USED TO PRODUCE 2-CARBON UNITS (AS ACETYLCoA), THE 2-CARBON UNITS THAT ARE NOT USED TO MAKE ATPARE CONVERTED TO FATTY ACIDS BY THE DE NOVO PATHWAY.THIS IS A ONE-WAY PROCESS – MEANING THAT THE CONVERSIONOF FATTY ACIDS TO ACETYL CoA CANNOT BE USED TO MAKE GLUCOSE. IN PRACTICAL TERMS, A DIET THAT IS HIGH IN GLUCOSEWILL BE CONVERTED TO FAT IN A SEDENTARY INDIVIDUAL.

TISSUE “SITES” OF ACTION FOR DE NOVO SYNTHESIS:

LIVER TISSUE (primarily) FAT TISSUE (secondarily)

SUB-CELLULAR SITES OF SYNTHESIS: CYTOSOL MITOCHONDRIA CYTOSOL (ASSUMING THAT GLUCOSE IS THE SOURCE)

GLUCOSE PYRUVATE ACETYL-CoA CITRATE ACETYL-CoA FATTY ACIDS

THESE REACTIONS ARE ENZYMATICALLY CATALYZED THROUGH SEVERAL STEPS.OTHER FATTY ACIDS & AMINO ACIDS MAY ALSO ACT AS SOURCES OF SYNTHESIS.

PART I: WHAT IS NEEDED TO BEGIN SYNTHESIS AND WHEREIT COMES FROM

THE SYNTHETIC PATHWAY REQUIRES ACETYL CoA AND NADPH. ACETYL CoA MAY ORIGINATE FROM GLUCOSE (BY WAY OF PYRUVATE, SEE THE REDARROWS), AMINO ACID CATABOLISM (STARVATION CONDITIONS) AND EVEN OTHER FATTY ACID OXIDATION (STARVATION CONDITIONS?). NADPH, A PRODUCT OF THE PENTOSE SHUNT OR FROM MALATE DH ACTIVITY, IS ALSO NEEDED. (The rednumbers show what typically occurs.)

(1)

(2)

(3)

(4)

(5)

(8)

(7)

(6)

IN ORDER TO BEGINFATTY ACID SYNTHESIS,SOME ACETYL-CoAMUST BE CONVERTED TOMALONYL-CoA. THIS ISDONE BY CONDENSINGBICARBONATE (AS CO2) ONTO ACETYL CoA (SEE ) USING ACETYL CoACARBOXYLASE. BIOTIN, ACO-ENZYME (AKA VITAMIN B7) IS USED TO TRANSFER THE CO2 FORM TO ACETYL CoA. THE ILLUSTRATIONSHOWS A DIAGRAM OFTHE ACETYL CoA CARBOXY-LASE ENZYME THAT HAS 3 POLYPEPTIDE CHAINS. IN THE DIAGRAM, BIOTIN (GREEN ARROW 1)PICKS UP CO2 FROM THE CARBOXYLASE (LEFT, GREEN ARROW 2) & TRANSFERS IT TO ACETYL CoA (GREEN ARROW 3) BY A SWINGING “TETHER” OF THE BIOTIN CARRIER PROTEIN (RIGHT).

PART II: FORMATION OF MALONYL-CoA (3CARBON FA FORM)

2

1 3

NOTES ON MALONYL CoA FORMATION:

MALONYL CoA FORMATION AND ALL THE FATTY ACID SYNTHESIS STEPS ARE COMMITTED AND POWERED BY THE HYDROLYSIS OF ATP (SEE PREVIOUS SLIDE ATARROW 2).

THE ACTIVITY OF ACETYL-CoA CARBOXYLASE (THE ENZYME THAT MAKES MALONYLCoA) IS THE RATE LIMITING ENZYME OF FATTY ACID SYNTHESIS AND IS NOT A PARTOF THE FATTY ACID SYNTHASE MEGAENZYME (TO BE SHOWN).

ACTIVATION AND INHIBITION OF ACETYL-CoA CABOXYLASE IS TIGHTLY CONTROLLEDAND COMPLEX. THE ENZYME IS AN ALLOSTEIC ENZYME WITH MANY SITES FOR ACTI-VATION AND INHIBITION (SEE TEXT pp 726-727).

THE HYDROLYSIS OF ATP, PLACES A PHOSPHATE GROUP ONTO A CRITICAL POSITION ON RESIDUE 1200 (LYS) OF THE BIOTIN CARBOXYLASE SUBUNIT (ARROW 2). THE PHOSPHATEBECOMES BOUND TO BICARBONATE, THE TRANSFER FORM OF CO2. INSULIN PROMOTESWHILE EPINEPHRINE AND GLUCAGON INHIBIT THE ENZYME. ALSO THE RELATIVE SUPPLYOF ENERGY (CITRATE = ACETYL CoA) ACTIVATES THE ENZYME WHILE PALMITATE TENDS TOINACTIVATE THE ENZYME, AS DOES EXCESS PHOSPHATE FROM ATP. GENERALLY, THE ACTIVITY OF THE ENZYME IS DETERMINED BY THE SUM OF THE BOUND ACTIVATORS &INHIBITORS AT ANY GIVEN TIME.

PART III GENERAL SCHEME OF DE NOVO FATTY ACID SYNTHESIS: THE BIG PICTURE (OPERATION OF THE MEGAENZYME*)

PART V: THE FATTY ACID SYNTHASE STRUCTURE (AKA THE “MEGASYNTHASE”)

DH= DEHYDRATASEER= b-ENOYL REDUCTASEKR= b-KETOACYL REDUCTASEKS= b-KETOACYL SYNTHASEMAT= MALONYL-CoA- ACETYL CoA-ACP TRANSACYLASETE= THIOESTERASEACP= ACYL CARRIER PROTEIN

ACPACP ACPACP

NOTES: ON THE MAMMALIAN ENZYME, THE COMPLETE STRUCTURE IS FORMED ASTWO POLYPEPTIDE CHAINS (ONLY ONE IS SHOWN HERE). THE LOCATIONS OFTHE ACP AND THE THIOESTERASE ARE NOT KNOWN WITH CERTAINTY, BUT THE ACP ISTHOUGHT TO HAVE FLEXIBLE LOCATIONS IN ORDER TO “DELIVER” THE SUBSTRATE TO VARIOUS CATALYTIC SITES (*). THE LABELS CALLED “REACTION CHAMBERS” REALLY INCLUDE ALL THE ACTIVESITES. PICTURE THE GROWING FATTY ACYL GROUPS BEING MOVED FROM REACTIVE SITETO REACTIVE SITE BY THE TETHERED ACYL CARRIER PROTEINS.

Reactionchamber

Reactionchamber

TETETETE

PART IV: PRIMING THE PUMP –THE ACYL CARRIER PROTEIN DOMAIN & TRANSFER OF ACETYL & MALONYL GROUPS.

-C-CH3=

O

-C-CH3

O

=

NOTE: THE ACYLCARRIER PROTEIN +THE PHOSPHOPANTETHEINEGROUP WILL BE NOTED INSUBSEQUENT SLIDES AS“ACP” AS IN ACETYL-ACP ORMALONYL-ACP

MALONYL-CoA- ACETYL-CoA—ACP TRANSACYLASE (MAT)

Acetyl CoA

Malonyl CoA

MAT

KS

KR

DH

ER

Recycle reactions

ACPACP

KR

MAT= malonylCoA-acetylCoA-ACP transacylaseKS=b-ketoacyl synthase =b-ketoacyl reductaseDH=dehydrataseER=b-enoyl reductase

PART V: THE 1st CYCLE OFSYNTHESIS

PART VI: SUBSEQUENT CYCLES AND RELEASE

FIRST CYCLE TO HERE*

SECOND CYCLE TO HERE, ETC.

*SEE PREVIOUS SLIDE FOR DETAILED REACTIONS.THERE ARE A TOTAL OF 7 CYCLES EACH ADDING 2 MORE CARBONS. SINCE THE SYNTHESIS BEGINS WITH 2 CARBONS, THE TOTAL PRODUCED IS 16 CARBONS AS PALMITATE. PALMITATE IS RELEASED FROM THE ACP IN THE ENZYME BY THE CATALYTIC ACTIVITY OF THIOESTERASE (TE) LOCATED ON THE FATTY ACID SYNTHASE.THIS OCCURS SINCE THE MEGASYNTHASE CANNOT ACCOMMODATE ANY MORE THAN 16 CARBON FATTY ACIDS.

TIME OUT!

DID YOU EVER WONDER WHAT HAPPENED TO THEWOMAN WHO KISSED THE FROG PRINCE?

SHE’S TALKING TO HER FRIEND ON THE PHONE –“OH, YES MARTHA, NOWYOU SHOULD SEE ALL THE TADPOLES I’M STUCK WITH.”

A HELPFUL COMPARISON BETWEEN FATTY ACID BASIC METABOLIC MECHANISMS:FATTY ACID b-OXIDATION or (DEGRADATION…last lecture) vs. FATTY ACID CONDENSATION or (SYNTHESIS…this lecture)

IN EACH PATHWAY, THETERM “ACTIVATION”MEANS CARRIER BINDING TO ALLOWENZYMATIC REACTIONSTO TAKE PLACE:ACETYL CoA FOR DEGRADATION;ACYL CARRIER PROTEIN (ACP) FOR SYNTHESIS.

b

co2

FATTY ACID SYNTHESIS – (CONTINUATION OF GENERAL CONSIDERATIONS)

RECALL THAT IT WAS SAID THAT DIETS COMMON TO THE SO-CALLED“DEVELOPED” NATIONS DO NOT HAVE A CRITICAL NEED FOR THESYNTHESIS OF FATTY ACIDS, BUT THE SYNTHESIS STILL OCCURSIN TWO WAYS:

1) “DE NOVO” SYNTHESIS OF FATTY ACIDS (JUST DISCUSSED)2) CARBON LENGTHENING OF EXISTING PALMITIC ACID & FORMATION OF DOUBLE-BONDS

NOW LETS LOOK AT THE SECOND PART OF THAT SYNTHESIS: FURTHER CHAIN LENGTHENING AND THE FORMATION OF DOUBLE-BONDS.

CHAIN LENGTHENING AND DOUBLE-BOND FORMATION(AND ITS LIMITATIONS):

PALMITATE LENGTHENING OCCURS IN THE MITOCHONDRIA AND SMOOTH ENDOPLASMIC RETICULUM BY SEPARATE, BUT SIMILAR MECHANISMS. THE LENGTHENING IS USUALLY LIMITED TO 2 ADDITIONAL CARBONS. ALSO: ONLY SINGLE UNSATURATED FATTY ACIDS CAN BE FORMED FROM THE SATURATED FATTY ACIDS. SO, THE PRODUCTS SYNTHESIZED ARE ONLY:

PALMITOLEATE (16:1D9) palmitoleic acidSTEARATE (18:0) steric acidOLEATE (18:1D9) oleic acid

IN FACT, THESE THREE FATTY ACIDS COMMONLY OCCUR IN THEIRESTER FORMS IN ADIPOCYTES. THE ENZYMES THAT CARRY OUT THESEACTIVITIES ARE CALLED FA ELONGASES AND FA DESATURASES.

GETTING LONGER CHAIN, POLYUNSATURATED FATTY ACIDS REQUIRESESSENTIAL FATTY ACIDS FROM THE DIET. THIS IS THE POLY-UNSATURATED FATTY ACID DISCONNECT FOR HIGHER LIFE FORMS.

IN ORDER TO GET LONGER CHAIN, POLYUNSATURATED FATTY ACIDS(PUFAs) THEN, THERE IS A NEED FOR LINOLEATE (18:2D9,12) ANDa-LINOLENATE (18:3D9,12,15) AS STARTING FAs. THESE FAs ARE ESSENTIAL FOR HIGHER ANIMALS SINCE THEY CANNOT MAKE(SYNTHESIZE) THESE FATTY ACIDS FROM PALMITATE. LINOLEATE AND a-LINOLENATE MUST BE USED TO MAKE POLYUNSATURATED FATTY ACIDS (e.g. ARACHIDONATE (20:4D5,8,11,14).

NOTE: LINOLEATE & ARACHIDONATE ARE AKA w6 FATTY ACIDS WHILE a-LINOLENATE IS AKA w3 FATTY ACID (counting from themethyl end of the fatty acid – a practice used by nutritional biochemists).

LINOLEATE

ARACHIDONATE

PROSTAGLANDINS

THROMBOXANES

LEUCOTRIENES

WHAT ARE SOME OTHER PUFAs AND WHAT ARE SOME OF THEIR PRODUCTS? (EICOSANOID SYNTHESIS)

me

me

me

me

AN ABBREVIATED FORM OF EICOSANOID SYNTHESIS FROM ARACHIDONATE. ARACHIDONATEIS RELEASED FROM PHOSPHOLIPIDS ON PLASMA MEMBRANES BY PHOSPHOLIPASE A2. MULTIPLE ENZYMES (me) CONVERT ARACHIDONATE TO PROSTAGLANDINS (PG), THROMBOX-ANES (TX) OR LEUCOTRIENES (LT). EACH EICOSANOID SERVES AS A SHORT ACTING, LOCAL HORMONE. E.g., PGD2 HAS ANTI-INFLAMMATORY ACTIVITY WHILE LEUKOTRIENE C SUSTAINSINFLAMMATORY REACTIONS. STEROIDS AND “NSAIDS” INHIBIT EICOSANOID SYNTHESIS.

me

NOTES ON THE PATHWAYS TO EICOSANOID SYNTHESIS:

1) LINOLEATE IS CONVERTED TO ARACHIDONATE BY ENZYMES STORED IN THE ENDOPLASMIC RETICULUM BEFORE INCORPORATION INTO PHOSPHOLIPID PLASMA MEMBRANES.

2) PHOSPHOLIPASE A2 BREAKS ARACHIDONATE AWAY FROM PM PHOSPHOLIPIDS. THE ENZYME IS ACTIVATED BY EITHER A HORMONE OR BY MECHANICAL CONTACT & IS INHIBITED BY CORTICOSTEROIDS.

3) PROSTAGLANDIN SYNTHASE IS A COMPLEX OF TWO ENZYMES: AN OXIDASE AND A CYCLASE. IT IS AKA CYCLOOXYGENASE (COX). COX INHIBITORS ARE CALLED NON-STEROIDAL, ANTI-INFLAMMATORY DRUGS (NSAIDS). THE ENZYME IS BOUND TO THE CELL PLASMA MEMBRANE.

THE ILLUSTRATION ON THE RIGHT SHOWSAA 33-583 OF COX-1. A BLUE MOLECULE OFIBUPROFEN IS BOUND AT THE ACTIVE SITE OF THEENZYME AT WHAT IS KNOWN AS THE TUNNELPORTION OF THE ENZYME. THIS ACTION PREVENTS THE ARACHIDONATE FROM COMPLETING ITS CONVERSION TO PROSTA-GLANDIN H2.

ANOTHER PUFA WORTHY OF OUR ATTENTION:AN w-3 FATTY ACID THAT HAS GAINED RECENT ATTENTION IS DHA(DOCOSAHEXANENOIC ACID aka CERVONIC ACID). THIS FATTY ACID ISSYNTHESIZED FROM a-LENOLENATE IN THE ENDOPLASMIC RETICULUMAND HAS THE STRUCTURE 22:6D4,7,10,13,16,19). THE HIGH DEGREE OF

UNSATURATION (6 DOUBLE BONDS!) MAKES THIS FATTY ACID A DESIRABLE COMPONENT IN THOSE MEMBRANES THAT REQUIRE CONSIDERABLE FLEXIBILITY AS BIOLOGICAL LIQUID CRYSTALS. THIS FLEXIBILITY ALLOWS FOR THE COMPLEX CONDUCTION PROCESSES REQUIRED IN NERVOUS TISSUES. THAT IS, PROTEINS IN THESE MEMBRANES HAVE GREATERFREEDOM TO MOVE WITHIN THE MEMBRANE. APPOXIMATELY 22% OF ALL THE FATTY ACIDS FOUNDIN THE RETINA IS CERVONIC ACID. AT THE RIGHT ARE SHOWN 2 POSSIBLE CONFORM-ATIONS OF CERVONIC ACID IN A MEMBRANE (see arrow).

FATTY ACID SYNTHESIS (DE NOVO) AND FATTY ACID ELONGATION, ALONG WITH UNSATURATION, DO NOT SEEM TO SERVE HUMANS WITH THE BEST EFFICIENCY SINCE:

1) CARBOHYDRATES ARE SYNTHESIZED TO EXCESS FATTY ACIDS WHEN THEY ARE NOT USED TO MAKE ATP.2) THE PROCESSES OF ELONGATION & UNSATURATION OF C-16 FATTY ACIDS IS LIMITED SINCE CONVERSION TO PUFAs CANNOT OCCUR.

ESSENTIAL FATTY ACIDS, WHICH WE CANNOT MAKE, ARE NEEDED AS STARTINGPOINTS FOR THE SYNTHESIS OF A VARIETY OF USEFUL, POLYUNSATURATED FATTY ACIDS (AKA PUFAs). SOME EXAMPLES OF PUFAs ARE PROSTAGLANDINS (SHORT ACTING HORMONES) AND DHA (A FATTY ACID THAT MAY BE CARDIOPROTECTIVE, ANTI-INFLAMMATORY AS WELL AS A COMPONENT IN NERVOUS TISSUE FUNCTIONS).

A PUFA IS ANY FATTY ACID WITH TWO OR MORE DOUBLE BONDS. WITH VIRTUALLYNO EXCEPTONS IN ANIMAL TISSUE, A PUFA HAS A MINIMAL CARBON LENGTH OF 18.

DHA = DOCOSAHEAENOIC ACID.

THEREFORE - POINTS TO NOTE ABOUT FATTY ACID SYNTHESIS:

THE SYNTHESIS OF CHOLESTEROL

CHOLESTEROLHAS GOTTEN ABAD REPUTATIONIN MORE RECENTYEARS DUE TO ITSASSOCIATION WITHBLOOD VESSELBLOCKAGE. IN FACT,THIS MOLECULE ISQUITE IMPORTANTAS A COMPONENT OF PLASMA MEMBRANE STRUCTURE; AS WELL AS A PRECURSOR NECESSARY FOR LIPID DIGESTION (BILE SALTS), LIPID SOLUBLE VITAMINS; AND STEROID HORMONES. WE COULD NOT LIVE WITHOUT THIS MOLECULE.

THE FIGURES ON THIS SLIDE SHOW YOU DIFFERENT WAYS OF REPRESENTING THIS CHOLESTEROL. ON THE LEFT IS THE FAMILIAR 2-DIMENSIONAL, STRUCTURAL FORMULA.IN THE MIDDLE, IS A DIAGRAM OF ITS THREE MAIN COMPONENTS, AND ON THE RIGHTA van der WAALS REPRESENTATION OF HOW THE MOLECULE MIGHT LOOK IN 3-DIMENSIONALSPACE. IF YOU WERE TO TURN THE MOLECULE AROUND ON ITS Z-AXIS (VERTICAL AXIS) YOUWOULD SEE THAT IT IS BOTH FLAT AND SOMEWHAT BULKY.

WE OBTAIN CHOLESTEROL FROM 2 SOURCES: DIET AND SYNTHESIS.

DIETARY CHOLESTEROL INTAKE IS AN AMOUNT THAT CAN BE CONTROLLED (AND IN SOME CASES MUST BE CONTROLLED *) TO AVOID HIGH LDL/HDL LEVELS & POSSIBLE BLOOD VESSEL BLOCKAGE.

CHOLESTEROL SYNTHESIS IS A PROCESS OVER WHICH AN INDIVIDUALHAS NO (?) CONTROL. IN THIS PART OF THE LECTURE, WE WILL CONSIDER HOW THIS PROCESS OCCURS AND WHAT CONTROL MECHANISMS ARE ASSOCIATED WITH CHOLESTEROL SYNTHESIS.

*IN CASES OF OVERWEIGHT AND HYPERCHOLESTEROLEMIA WHICHWAS PREVIOUSLY DESCRIBED.

SITE & ORIGINS OF SYNTHESISWITH AN OVERALL DIAGRAM. CHOLESTEROL IS MADE INITIALLY IN THE LIVER CELLS’ CYTOSOL AND THEN INITS ENDOPLASMIC RETICULUM. THE 1ST MOLECULES ARE ACETYL CoA, JUST AS WITH FATTY ACID SYNTHESIS. THERE THE SIMILAR-ITY ENDS. EACH STAGE IN THEDIAGRAM TO THE RIGHT REPRESENTS SOME MAJOR INTERMEDIATES: MEVALONATE,ACTIVATED ISOPRENE, AND SQUALENE. THE CIRCLED NUMBERSARE FOR MULTIPLE-STEP, ENZYMECATALYZED REACTIONS.

COMMITMENT

BUILDING BLOCKFORMATION

CONDENSATION

CYCLIZATION

THE STEPS IN CHOLESTEROL BIOSYNTHESIS ARE LONG AND COMPLEX.WHY SHOULD THEY BE STUDIED?

AS WITH MANY THINGS IN LIFE, IT’S ALL ABOUT CONTROL AND FUNDING TO RESEARCH THE MECHANISM OF SYNTHESIS WAS “SOLD” ON THE BASIS OF EVENTUALLY FINDING WAYS TO PREVENT THE EXCESSIVE SYNTHESIS OF CHOLESTEROL IN THE BODY -- TO PREVENT, FOR EXAMPLE, HEART ATTACKS. HISTORICALLY, THE INVESTIGATIONS TO DETERMINE HOW CHOLESTEROL IS MADE HAVE EXTENDED FROM THE 1940’S EVEN UP TO THE PRESENT DAYSINCE SOME MINOR DETAILS OF ENZYMATIC REACTIONS HAVE YET TO BE DESCRIBED.

HERE WE ARE GOING TO TAKE A CAREFUL AND ABBREVIATED LOOKAT THOSE PARTS OF THE SYNTHETIC PATHWAY THAT HAVE YIELDEDUSEFUL INFORMATION.

COMMITMENT

BUILDING BLOCKFORMATION

CONDENSATION

CYCLIZATION

The commitment stage 1 causes mevalonateto be synthesized from 3 acetyl CoAs by 3 enzymes:thiolase, OH-methyl glutaryl-CoA synthase and OH-methyl glutaryl reductase:

Rx 1 2 acetyl CoA acetoacetyl CoA

Rx 2 acetyl CoA + acetoacetyl CoA 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA)

Rx3 HMG-CoA + 2 NADPH + 2 H+

3-mevalonate

USEFUL INFORMATION: THE 3RD ENZYME: OH-METHYL GLUTARYL REDUCTASE IS THE RATE LIMITING ENZYME FORTHE ENTIRE PATHWAY OF CHOLESTEROL SYNTHESIS. THE ENZYME IS CONTROLLED BY KINASES/PHOSPHATASES; CHOLESTEROL LEVELS ON ½ LIFE OFENZYME AND mRNA LEVELS OF THE ENZYME.

thiolase

synthase

reductase

INHIBITORS FOR HMG-CoA REDUCTASE – A CLINICAL PAYOFF FOR CHOLESTEROL SYNTHESIS STUDIES

=3-OH-3-METHYLGLUTARYL-CoA

MEVALONATE PRODUCT

OH-METHYL GLUTARYL-CoA SUBSTRATE

-E

STATINS

LIPITOR, HAVING A STRUCTURE SIMILAR TO THE OH-METHYL-GLUTARYL INTERMEDIATE,BINDS TO THE ACTIVE SITE OF THE REDUCTASE AND INHIBITS ITS CATALYTIC ACTION

OH-METHYL-GLUTARYL INTERMEDIATE

EXAMPLE OF AN INHIBITOR:

COMMITMENT

BUILDING BLOCKFORMATION

CONDENSATION

CYCLIZATION

Building block and condensation stages2 and 3 proceed through four enzymaticreactions that require ATP to drive the reactionsinitially to the formation of activated isoprene (seen on the right) AKA isopentenyl pyrophosphate.Squalene formation formation then takes place by enzymatic condensation of six molecules of Isopentenyl pyrophosphate such that:

C5 C10 C15 C30 is formed (5x6=30)

The cyclization stage 4 is a multi-step, enzymaticprocess that adds an OH group to ring A, closes thefour rings, adds a double bond to ring B, preservesthe methyl group between rings A and B, and formsa new methyl group between rings C and D (shortblue arrows).

Enzymes in stages 2-4 are not concerned with the rates of cholesterol synthesis. However, regulationof the rate of LDL receptor synthesis & theesterification of cholesterol are also important.

A B

C D

20 Rx’s!

THE LDL RECEPTOR, ITS DEFECTS & BLOOD CHOLESTEROL – A REVIEWWHEN CHOLESTEROL HAS BEEN SYNTHESIZED IN THE LIVER AND ESTERIFIED FOR INCORPORATION INTO LDLs, RECALL THAT IT MUST BE REMOVED FOR CELLULAR DELIVERY BY AN LDL RECEPTOR.THIS REMOVAL IS ONE WAY IN WHICH BLOOD CHOLESTEROL IS CONTROLLED (LOWERED). THE DISEASECALLLED FAMILIAL HYPERCHOLESTEROLEMIA (mentioned in the Last lecture) INVOLVES AN ABSENCE OR DEFECT INTHE LDL RECEPTOR MOLECULE (several possibilities canoccur): a homozygous defect when no receptor is made[>600 mg/dL of blood cholesterol] while a heterozygousdefect results in ~1/2 of the receptor being made [>300mg/dL of blood cholesterol]. IN ADDITION, SEQUENCEMISTAKES IN THE LDL BINDING DOMAIN FAIL TO CAUSETHE RELEASE OF CHOLESTEROL ( ) OR SEQUENCEANOMALIES IN THE C-TERMINAL DOMAIN ( ) CAUSEA FAILURE IN PIT FORMATION TO INVAGINATE [TAKE UP]THE CHOLESTEROL. IN ADDITION, IT IS ALSO POSSIBLETHAT THE RECEPTOR PROTEIN DELIVERY MECHANISM FROM THE GOLGI APPARATUS TO THE CELL PLASMA MEMBRANE MAY BE DEFICIENT (see green arrow). THISIS HOW DRUGS LIKE LIPITOR COMBAT THESE PROBLEMS.

SUMMARYWE HAVE JUST LOOKED AT 2 PROCESSES ABOUT LIPID METABOLISM: THE SYNTHESISOF FATTY ACIDS AND THE SYNTHESIS OF CHOLESTEROL. IN ADDITION, THE DEFICIENCIESAND POSSIBLE PATHOLOGIES ASSOCIATED WITH THESE PATHWAYS HAVE BEEN POINTED OUT.

WHAT IS IMPORTANT?

The hazards ofspace travel!

FATTY ACID SYNTHESIS –

1) THE CHARACTERISTICS OF “DE NOVO” SYNTHESIS ** ITS IMPORTANCE ** ITS “SIDE” EFFECTS (EXCESS FAT FROM GLUCOSE) ** WHERE IT OCCURS ** WHAT IS NEEDED FOR SYNTHESIS ** MALONYL CoA FORMATION & GENERAL SCHEME ** THE MEGASYNTHASE MOLECULE ** THE ACYL CARRIER PROTEIN ** THE 1st CYCLE OF SYNTHESIS (the devil is in the details) ** SUBSEQUENT CYCLES (where does it stop)

CAN YOU COMPARE FATTY ACID SYNTHESIS WITH BETA-OXIDATION?

2) THE CHARACTERISTICS OF CARBON LENGTHENING & DESATURATION ** LIMITATIONS OF THIS PROCESS ** THE NEED FOR PUFAs – ESSENTIAL FATTY ACIDS IN THE DIET ** SYNTHESIS OF PROSTAGLANDINS, THROMBOXANES & LEUCOTRIENES -- WHAT ARE THEY & HOW ARE THEY MADE (the devil is in the details)? ** THE IMPORTANCE OF PLA2 & PROSTAGLANDIN SYNTHASE INHIBITORS ** SYNTHESIS OF CERVONIC ACID AND ITS FUNCTION (yes, the devil is here too).

THE SYNTHESIS OF CHOLESTEROL –

1) THE IMPORTANCE OF CHOLESTEROL ** IT CONTRIBUTES TO ------ ** IT IS A SOMEWHAT FLAT MOLECULE WITH THREE PARTS. WHY IS THAT IMPORTANT?2) THE “EVILS” OF CHOLESTEROL ** HYPERCHOLESTEROLEMIA 3) HOW THE CELLULAR SYNTHETIC PROCESS WORKS ** ACETYL CoA ** IMPORTANT INTERMEDIATES: MEVALONATE, ISOPRENE, SQUALENE ** COMMITMENT STAGE AND CONTROL OF SYNTHESIS. ** INHIBITORS OF HMG-CoA REDUCTASE & THEIR IMPORTANCE -- LIPITOR AS AN EXAMPLE. WHAT DOES IT DO? ** BUILDING BLOCK, CONDENSATION & CYCLIZATION STAGES 4) THE ROLE OF THE LDL RECEPTOR AND CONTROL OF BLOOD CHOLESTEROL ** THE RECEPTOR’S NORMAL ROLE ** BACK TO HYPERCHOLESTEROLEMIA – VARIATIONS IN DEFECTS OF THE LDL RECEPTOR.