Transcript LIPID SYNTHESIS

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 APPROACHED
ARE 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 FATTY
ACIDS IS NOT SO CRITICAL, BUT IT DOES OCCUR AND DOES SO IN
TWO WAYS:
1) “DE NOVO” SYNTHESIS OF FATTY ACIDS
2) CARBON LENGTHENING OF PALMITIC ACID &
FORMATION OF DOUBLE-BONDS
“DE NOVO” SYNTHESIS MEANS THE SYNTHESIS OF FATTY ACIDS
COMES FROM 2-CARBON UNITS.
WHEN IS IT IMPORTANT? UNDER CONDITIONS WHEN THE DIETARY
SUPPLY 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 ACETYL
CoA), THE 2-CARBON UNITS THAT ARE NOT USED TO MAKE ATP
ARE CONVERTED TO FATTY ACIDS BY THE DE NOVO PATHWAY.
THIS IS A ONE-WAY PROCESS – MEANING THAT THE CONVERSION
OF FATTY ACIDS TO ACETYL CoA CANNOT BE USED TO MAKE
GLUCOSE. IN PRACTICAL TERMS, A DIET THAT IS HIGH IN GLUCOSE
WILL 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 WHERE
IT COMES FROM
THE SYNTHETIC PATHWAY
REQUIRES ACETYL CoA
AND NADPH. ACETYL CoA
MAY ORIGINATE FROM
GLUCOSE (BY WAY OF
PYRUVATE, SEE THE RED
ARROWS), 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 red
numbers show what
typically occurs.)
(6)
(4)
(5)
(7)
(2)
(3)
(8)
(1)
PART II: FORMATION OF MALONYL-CoA (3CARBON FA FORM)
IN ORDER TO BEGIN
FATTY ACID SYNTHESIS,
SOME ACETYL-CoA
MUST BE CONVERTED TO
MALONYL-CoA. THIS IS
DONE BY CONDENSING
BICARBONATE (AS CO2) ON
TO ACETYL CoA (SEE
)
USING ACETYL CoA
CARBOXYLASE. BIOTIN, A
CO-ENZYME (AKA
VITAMIN B7) IS USED TO
TRANSFER THE CO2
FORM TO ACETYL
1 3
CoA. THE ILLUSTRATION
SHOWS A DIAGRAM OF
2
THE ACETYL CoA CARBOXYLASE 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).
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 AT
ARROW 2).
THE ACTIVITY OF ACETYL-CoA CARBOXYLASE (THE ENZYME THAT MAKES MALONYL
CoA) IS THE RATE LIMITING ENZYME OF FATTY ACID SYNTHESIS AND IS NOT A PART
OF THE FATTY ACID SYNTHASE MEGAENZYME (TO BE SHOWN).
ACTIVATION AND INHIBITION OF ACETYL-CoA CABOXYLASE IS TIGHTLY CONTROLLED
AND COMPLEX. THE ENZYME IS AN ALLOSTEIC ENZYME WITH MANY SITES FOR ACTIVATION 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 PHOSPHATE
BECOMES BOUND TO BICARBONATE, THE TRANSFER FORM OF CO2. INSULIN PROMOTES
WHILE EPINEPHRINE AND GLUCAGON INHIBIT THE ENZYME. ALSO THE RELATIVE SUPPLY
OF ENERGY (CITRATE = ACETYL CoA) ACTIVATES THE ENZYME WHILE PALMITATE TENDS TO
INACTIVATE 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= DEHYDRATASE
ACP
ER= b-ENOYL REDUCTASE
KR= b-KETOACYL
REDUCTASE
TE
KS= b-KETOACYL SYNTHASE
MAT= MALONYL-CoAACETYL CoA-ACP
TRANSACYLASE
TE= THIOESTERASE
ACP= ACYL CARRIER
PROTEIN
ACP
ACP
ACP
TE
TE TE
Reaction
chamber
Reaction
chamber
NOTES: ON THE MAMMALIAN ENZYME, THE COMPLETE STRUCTURE IS FORMED AS
TWO 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 ACTIVE
SITES. PICTURE THE GROWING FATTY ACYL GROUPS BEING MOVED FROM REACTIVE SITE
TO REACTIVE SITE BY THE TETHERED ACYL CARRIER PROTEINS.
PART IV: PRIMING THE PUMP –THE ACYL CARRIER PROTEIN DOMAIN
& TRANSFER OF ACETYL
& MALONYL GROUPS.
=
-C-CH3
O
-C-CH3
=
NOTE: THE ACYL
CARRIER PROTEIN +
THE PHOSPHOPANTETHEINE
GROUP WILL BE NOTED IN
SUBSEQUENT SLIDES AS
“ACP” AS IN ACETYL-ACP OR
MALONYL-ACP
MALONYL-CoAACETYL-CoA—ACP
TRANSACYLASE
(MAT)
O
Acetyl CoA
PART V: THE 1st CYCLE OF
SYNTHESIS
MAT
Malonyl CoA
KS
KR
ACP
MAT= malonylCoA-acetylCoA-ACP transacylase
KS=b-ketoacyl synthase
KR =b-ketoacyl reductase
DH=dehydratase
ER=b-enoyl reductase
ACP
DH
ER
Recycle
reactions
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 THE
WOMAN WHO KISSED
THE FROG PRINCE?
SHE’S TALKING TO HER
FRIEND ON THE PHONE –
“OH, YES MARTHA, NOW
YOU 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, THE
TERM “ACTIVATION”
MEANS CARRIER
BINDING TO ALLOW
ENZYMATIC REACTIONS
TO 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 THE
SYNTHESIS OF FATTY ACIDS, BUT THE SYNTHESIS STILL OCCURS
IN 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 DOUBLEBONDS.
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)
STEARATE (18:0)
OLEATE (18:1D9)
palmitoleic acid
steric acid
oleic acid
IN FACT, THESE THREE FATTY ACIDS COMMONLY OCCUR IN THEIR
ESTER FORMS IN ADIPOCYTES. THE ENZYMES THAT CARRY OUT THESE
ACTIVITIES ARE CALLED FA ELONGASES AND FA DESATURASES.
GETTING LONGER CHAIN, POLYUNSATURATED FATTY ACIDS REQUIRES
ESSENTIAL FATTY ACIDS FROM THE DIET. THIS IS THE POLYUNSATURATED 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) AND
a-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 the
methyl end of the fatty acid – a practice used by nutritional
biochemists).
WHAT ARE SOME OTHER PUFAs AND WHAT ARE SOME OF THEIR PRODUCTS?
(EICOSANOID SYNTHESIS)
LINOLEATE
me
ARACHIDONATE
me
PROSTAGLANDINS
me
me
THROMBOXANES
me
LEUCOTRIENES
AN ABBREVIATED FORM OF EICOSANOID SYNTHESIS FROM ARACHIDONATE. ARACHIDONATE
IS RELEASED FROM PHOSPHOLIPIDS ON PLASMA MEMBRANES BY PHOSPHOLIPASE A2.
MULTIPLE ENZYMES (me) CONVERT ARACHIDONATE TO PROSTAGLANDINS (PG), THROMBOXANES (TX) OR LEUCOTRIENES (LT). EACH EICOSANOID SERVES AS A SHORT ACTING, LOCAL
HORMONE. E.g., PGD2 HAS ANTI-INFLAMMATORY ACTIVITY WHILE LEUKOTRIENE C SUSTAINS
INFLAMMATORY REACTIONS. STEROIDS AND “NSAIDS” INHIBIT EICOSANOID SYNTHESIS.
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 SHOWS
AA 33-583 OF COX-1. A BLUE MOLECULE OF
IBUPROFEN IS BOUND AT THE ACTIVE SITE OF THE
ENZYME AT WHAT IS KNOWN AS THE TUNNEL
PORTION OF THE ENZYME. THIS ACTION
PREVENTS THE ARACHIDONATE FROM
COMPLETING ITS CONVERSION TO PROSTAGLANDIN 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 IS
SYNTHESIZED FROM a-LENOLENATE IN THE ENDOPLASMIC RETICULUM
AND 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 GREATER
FREEDOM TO MOVE WITHIN THE MEMBRANE.
APPOXIMATELY 22% OF ALL THE FATTY ACIDS FOUND
IN THE RETINA IS CERVONIC ACID.
AT THE RIGHT ARE SHOWN 2 POSSIBLE CONFORMATIONS OF CERVONIC ACID IN A MEMBRANE (see arrow).
THEREFORE - POINTS TO NOTE ABOUT FATTY ACID SYNTHESIS:
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 STARTING
POINTS 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 VIRTUALLY
NO EXCEPTONS IN ANIMAL TISSUE, A PUFA HAS A MINIMAL CARBON LENGTH OF 18.
DHA = DOCOSAHEAENOIC ACID.
THE SYNTHESIS OF CHOLESTEROL
CHOLESTEROL
HAS GOTTEN A
BAD REPUTATION
IN MORE RECENT
YEARS DUE TO ITS
ASSOCIATION WITH
BLOOD VESSEL
BLOCKAGE. IN FACT,
THIS MOLECULE IS
QUITE IMPORTANT
AS 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 RIGHT
A van der WAALS REPRESENTATION OF HOW THE MOLECULE MIGHT LOOK IN 3-DIMENSIONAL
SPACE. IF YOU WERE TO TURN THE MOLECULE AROUND ON ITS Z-AXIS (VERTICAL AXIS) YOU
WOULD 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 INDIVIDUAL
HAS 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 WHICH
WAS PREVIOUSLY DESCRIBED.
SITE & ORIGINS OF SYNTHESIS
WITH AN OVERALL DIAGRAM.
CHOLESTEROL IS
MADE INITIALLY IN THE LIVER
CELLS’ CYTOSOL AND THEN IN
ITS ENDOPLASMIC RETICULUM.
THE 1ST MOLECULES ARE ACETYL
CoA, JUST AS WITH FATTY ACID
SYNTHESIS. THERE THE SIMILARITY ENDS. EACH STAGE IN THE
DIAGRAM TO THE RIGHT
REPRESENTS SOME MAJOR
INTERMEDIATES: MEVALONATE,
ACTIVATED ISOPRENE, AND
SQUALENE. THE CIRCLED NUMBERS
ARE FOR MULTIPLE-STEP, ENZYME
CATALYZED REACTIONS.
COMMITMENT
BUILDING BLOCK
FORMATION
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 DAY
SINCE SOME MINOR DETAILS OF ENZYMATIC REACTIONS HAVE YET
TO BE DESCRIBED.
HERE WE ARE GOING TO TAKE A CAREFUL AND ABBREVIATED LOOK
AT THOSE PARTS OF THE SYNTHETIC PATHWAY THAT HAVE YIELDED
USEFUL INFORMATION.
The commitment stage 1 causes mevalonate
to be synthesized from 3 acetyl CoAs by 3 enzymes:
thiolase, OH-methyl glutaryl-CoA synthase and OHmethyl glutaryl reductase:
Rx 1 2 acetyl CoA
thiolase
acetoacetyl CoA
COMMITMENT
BUILDING BLOCK
FORMATION
synthase
Rx 2 acetyl CoA + acetoacetyl CoA
3-hydroxy-3-methylglutaryl-CoA (HMG-CoA)
CONDENSATION
reductase
Rx3 HMG-CoA + 2 NADPH + 2
3-mevalonate
H+
USEFUL INFORMATION: THE 3RD ENZYME: OH-METHYL
GLUTARYL REDUCTASE IS THE RATE LIMITING ENZYME FOR
THE ENTIRE PATHWAY OF CHOLESTEROL SYNTHESIS.
THE ENZYME IS CONTROLLED BY KINASES/
PHOSPHATASES; CHOLESTEROL LEVELS ON ½ LIFE OF
ENZYME AND mRNA LEVELS OF THE ENZYME.
CYCLIZATION
INHIBITORS FOR HMG-CoA REDUCTASE – A CLINICAL
PAYOFF FOR CHOLESTEROL SYNTHESIS STUDIES
3-OH-3-METHYL
GLUTARYL-CoA
=
-E
OH-METHYL GLUTARYL-CoA
SUBSTRATE
STATINS
MEVALONATE
PRODUCT
EXAMPLE OF AN INHIBITOR:
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
Building block and condensation stages
2 and 3 proceed through four enzymatic
reactions that require ATP to drive the reactions
initially 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
COMMITMENT
BUILDING BLOCK
FORMATION
C30 is formed (5x6=30)
The cyclization stage 4 is a multi-step, enzymatic
process that adds an OH group to ring A, closes the
four rings, adds a double bond to ring B, preserves
the methyl group between rings A and B, and forms
a new methyl group between rings C and D (short
blue arrows).
Enzymes in stages 2-4 are not concerned with the
rates of cholesterol synthesis. However, regulation
of the rate of LDL receptor synthesis & the
esterification of cholesterol are also important.
CONDENSATION
CYCLIZATION
20 Rx’s!
C
A
B
D
THE LDL RECEPTOR, ITS DEFECTS & BLOOD CHOLESTEROL – A REVIEW
WHEN 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 DISEASE
CALLLED FAMILIAL HYPERCHOLESTEROLEMIA (mentioned
in the Last lecture) INVOLVES AN ABSENCE OR DEFECT IN
THE LDL RECEPTOR MOLECULE (several possibilities can
occur): a homozygous defect when no receptor is made
[>600 mg/dL of blood cholesterol] while a heterozygous
defect results in ~1/2 of the receptor being made [>300
mg/dL of blood cholesterol]. IN ADDITION, SEQUENCE
MISTAKES IN THE LDL BINDING DOMAIN FAIL TO CAUSE
THE RELEASE OF CHOLESTEROL (
) OR SEQUENCE
ANOMALIES IN THE C-TERMINAL DOMAIN (
) CAUSE
A FAILURE IN PIT FORMATION TO INVAGINATE [TAKE UP]
THE CHOLESTEROL. IN ADDITION, IT IS ALSO POSSIBLE
THAT THE RECEPTOR PROTEIN DELIVERY MECHANISM
FROM THE GOLGI APPARATUS TO THE CELL PLASMA
MEMBRANE MAY BE DEFICIENT (see green arrow). THIS
IS HOW DRUGS LIKE LIPITOR COMBAT THESE PROBLEMS.
SUMMARY
WE HAVE JUST LOOKED AT 2 PROCESSES ABOUT LIPID METABOLISM: THE SYNTHESIS
OF FATTY ACIDS AND THE SYNTHESIS OF CHOLESTEROL. IN ADDITION, THE DEFICIENCIES
AND POSSIBLE PATHOLOGIES ASSOCIATED WITH THESE PATHWAYS HAVE BEEN
POINTED OUT.
WHAT IS IMPORTANT?
The hazards of
space 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.