694:407/115:5121 - Molecular Biology and Biochemistry Lecture#3, Part 2 -- Lipid Biosynthesis Nov. 11, 2008 1. Fatty acid biosynthesis

a.) Sources of acetyl-CoA and NADPH for fatty acid biosynthesis b.) Acetyl-CoA activated to malonyl-CoA via biotin-linked

carboxylation c.) Multifunctional fatty acid synthase catalyzes:

-- Transacylation of malonyl-CoA and acetyl-CoA to ACP derivatives -- Condensation to acetoacetyl-ACP -- NADPH-linked reduction to D--hydroxy derivative -- Dehydration to trans-2-derivative -- NADPH-linked reduction to butyryl-ACP -- Condensation with malonyl-ACP and 6 more cycles -- Palmitoyl-ACP as product

d.) Elongation via CoA derivatives e.) Structure of the multifunctional fatty acid synthase complex --

intertwined dimer with two lateral semicircular reaction chambers, each with a full set of catalytic domains

f.) Desaturation driven by 02 and electron transfer from NADPH which, together with e- derived from desaturation of substrate 2H2O

g.) How fatty acid levels are regulated Acetly-CoA carboxylase as first committed step in FA biosynthesis subjected to feed-back inhibition by Palmitoyl-CoA product, allosteric activation by citrate and hormone dependent phosphorylation/dephosphoryl-ation

2. Phospholipid biosynthesis a.) Basic phospholipid biosynthetic patterns b.) Important eukaryotic pathways in ER and mitochondria

-- Synthesis from CDP-diacylglycerol as activated species: phosphatidylinositol, phosphatidylglycerol and cardiolipin -- Synthesis from CDP-head-group and diacylglycerol: phosphatidylcholine and phosphatidylethanolamine

-- Head-group exchange: Interconversion of phosphatidylserine and phosphatidylethanol

-- Methylation of attached headgroup: phosphatidylcholine formation from phosphatidylethanolamine in liver

3. Cholesterol biosynthesis a.) Synthesized in four stages: -- Conversion of acetyl-CoA to mevalonate (C6) -- Conversion of mevalonate to activated isoprene unit -- Polymerization of isoprenes to from squalene (C30) -- Squalene cyclization and conversion to cholesterol (C27) b.) Esterified with fatty acid by ACAT or LCAT c.) Cholesterol transport and utilization:

-- Transported by plasma lipoproteins (chylomicrons, VLDL, LDL, HDL) that are synthesized mainly in ER -- Cellular uptake of LDL by receptor-mediated endocytosis -- LDL is released from LDL receptors into acidic endosomes by interaction between N-terminal LDL receptor repeats and -propeller region of extracellular receptor domain -- LDL receptors are recycled -- LDL apoprotein B-100 is degraded, cholesterol esters are stored in cell -- Free cholesterol from LDL monolayer is released and esterif- ied as needed, or acts as inhibitor of HMG-CoA reductase and LDL receptor synthesis

Reading: Nelson, D.L. and Cox, M.M. (2008) Lehninger Principles of Biochemistry 5th Edition, Chapter 21 (p. 805-815; 820-844.

1Lipid BiosynthesisLipid Biosynthesis

Fatty acid biosynthesis Phospholipid biosynthesis Cholesterol biosynthesis Cholesterol transport and

utilization

Subcellular Subcellular Localization of Lipid Metabolic EventsLocalization of Lipid Metabolic Events

Fatty acid, isoprenoid,and sterol synthesistake place in cytosolwhere NADPH con-centration is high

Early steps of phos-pholipid synthesisusing diacylglycerolalso occur in thecytoplasm

Mitochondria

Phospholipid synthesis

Phospholipid synthesis (early stages)

Fatty Acid Biosynthesis is Not Merely a ReversalFatty Acid Biosynthesis is Not Merely a Reversalof Fatty Acid Oxidationof Fatty Acid Oxidation

Occurs in cytosol rather than in the mitochondrial matrix Reductases are NADPH linked After first step (acetyl-CoA carboxylase, ACC):

-- Intermediates are activated by linkage to acyl carrier protein (ACP) rather than to CoA-- Reactions are catalyzed in mammals by a single multi- functional fatty acid synthase protein containing ACP and all requisite catalytic sites (ACP acts as a flexible arm delivering substrates to enzyme active sites during modification reactions)

Acetyl-CoA Acetyl-CoA is Delivered to the is Delivered to the Cytosol Cytosol For Fatty AcidFor Fatty AcidBiosynthesis by the Biosynthesis by the Citrate-Malate-Pyruvate Citrate-Malate-Pyruvate ShuttleShuttle

1. Acetyl-CoA (arisingfrom oxidation of pyruv-ate and amino acids) +oxaloacetate are conden-sed to citrate by citratesynthase in matrix

2. Inner membrane tricar-boxylate transporter carr-ies citrate to cytosol

3. Citrate lyase convertscytosolic citrate back tooxaloacetate + acetyl-CoA for FA biosynthesis

4. Oxaloacetate is unableto return and is convertedto malate and finally backto oxaloacetate to com-plete the shuttle (5.)

6. In addition to malicenzyme, pentose-Ppathway is source ofNADPH

The mitochondrial inner membrane is impermeable to Acetyl-CoA and this indirect route is used to shuttle acetyl groups outto the cytoplasm, disguised in citrate.

12

65

4

3

2The The Acetyl-CoA Carboxylase Acetyl-CoA Carboxylase ReactionReaction Three functional regions of muli-

functional mammalian acetyl-CoAcarboxylase catalyze malonyl-CoA formation

First committed and rate-limit-ing step in fatty acid biosyn-thesis, and its control point

C-terminal domain struct-ure of E. coli biotincarrier protein

Biotinylprostheticgroup

Structure of E. coli biotincarboxlase; active site pocketis in C-terminal domain (pink)

ATP-dependent activ-ation of CO2 byattachment to N inbiotin ring

Transfer of acti-vated CO2 toAcetyl-CoA

Fatty Acid Synthesis ProceedsFatty Acid Synthesis Proceedsby Addition of Two Carbons toby Addition of Two Carbons toa Growing Fatty a Growing Fatty Acyl Acyl ChainChain

Two thiol groupson complex arecharged with acylgroups:

1. In reaction calalyzed by bifuctionalMalonyl-Acetyl Transferase(MAT), acetyl group of acetyl-CoAis transferred to a Cys-SH group inthe -Ketoacyl Synthase (KS)catalytic site

2. Malonyl group of malonyl-CoA istransferred to the -SH group ofACP in a reaction also catalyzedby Malonyl-Acetyl Transferase(MAT)

3. KS catalyzes condensation andreleases CO2, which pulls thereaction

4. -Ketoacyl-ACP Reductase (KR)reduces -keto group to an alcoh-ol in an NADPH linked reaction

1

2

4

3

MAT

KS

KR

ACP-- or longerfatty acylchain

MAT

Fatty Acid Synthesis --Fatty Acid Synthesis -- LastLastTwo StepsTwo Steps of First Roundof First Round

Structure of E. coli ACP

5. -Hydroxyacyl-ACP dehydrase (DH)creates double bond through H2elimination

6. Enoyl-ACP reductase (ER) reducesdouble bond to form saturated fatty acylgroup (butyryl-ACP) in an NADPH linkedreaction to complete first cycle(Steps 3-6 are essential a reversal of -oxidation)

6

5

DH

ER

Butyrl-ACP

The Overall Process of The Overall Process of Palmitate Palmitate SynthesisSynthesis

The fatty acyl chain grows by two-carbon units donated byactivated malonate, with loss of CO2 at each step

After first round, butyrl-ACP is translocated to KS Cys-SHOverall reaction: 8 Acetyl-CoA + 7ATP + 14(NADPH + H+) palmitate + 7(ADP + Pi) + 14NADP+ + 6H2O

3Architecture of Mammalian Fatty Acid Synthase Architecture of Mammalian Fatty Acid Synthase DeterminedDeterminedby X-ray Crystallography at 3.2- Resolutionby X-ray Crystallography at 3.2- Resolution

Substrate shuttling is facilitated by flexible tethering of acyl carrier proteindomain and by limited contact between condensing and modifying portionsof FAS multienzyme

Active sites are mainly connected by linkers rather than direct interaction

ACP and Thioesterase(TE), that releases final FAproduct, are missing fromthe structure because oftheir apparent inherentmobility

LD = linker domains

KR /KR and ME / ME = noncatalytic pseudo-ketoreductase and pseudo-methyltransferase, probablya remnants of ancestralmethyltransferase domains

Pseudo 2-fold dimer axis (Maier et al. Science321:1315, 2008)Bound NADP+ is

shown in blueAdvantages of Multifunctional FAS ComplexAdvantages of Multifunctional FAS Complex

Coordinates sequence of synthetic activities Enhanced protein stability when compared to a

complex of different enzyme proteins Intermediates can traverse from one active site to

another without leaving the complex The multifunctional FAS complex is an evolution-

ary product of exon shuffling, since each comp-onent enzyme is recognizably homologous to aseparate bacterial enzyme protein counterpart

Fatty Acid Biosynthesis is Tightly RegulatedFatty Acid Biosynthesis is Tightly Regulated

Fatty acid levels in vertebrates are controlled by allosteric regulation ofacetyl-CoA carboxylase and by hormone-dependent phosphorylation/dephosphorylation

EM showing filaments of acetyl-CoA carboxylase in active, dephosphor-ylated form -- it is converted to inactive monomers in phosphorylated form

Inhibition of carnitine acyltransferase I by malonyl-CoA prevents fatty acid-oxidation

Feedback inhibition

Allosteric activation

Vertebrate Fatty Vertebrate Fatty Acyl-CoA Desaturase Acyl-CoA Desaturase RequiresRequiresElectron Transfer From NADPHElectron Transfer From NADPH

The two desaturase substrates, NADPH and fatty acyl-CoA, undergooxidation by molecular oxygen

Power of O2 drives cis-double bond formation -- when desaturase is inreduced (Fe2+) state, interacts with O2 and the saturated fatty acyl-CoAsubstrate

Two e- are passed from NADPH through chain involving cytochrome b5and its reductase

Two e- are then derived from fatty acyl substrate to give rise to the 2H2O This mechanism is used in synthesis of palmitoleate (16:1-9) and

oleate (18:1-9), the 2 most common monounsaturated FAs, frompalmitate and stearate

Electron transfer pathwayElectron transfer pathway

4Phospholipid Phospholipid BiosynthesisBiosynthesisPhospholipids functions: -- Structural elements of membranes and plasma lipoproteins -- Second messengers in signal transduction pathwaysBasic pattern of phospholipid biosynthetic pathways: -- Synthesis of the glycerol backbone molecule -- Fatty acid attachment to glycerol backbone thru ester linkages -- Addition of hydrophilic head-group through phosphodiester linkages -- Alterations and exchange of some head-groupsImportant eukaryotic pathways -- Synthesis of phosphatidylinositol, phosphatidylglycerol and cardio-

lipin from CDP-diacylglycerol-- Synthesis of phosphatidylcholine and phosphatidylethanolamine from diacylglycerol and salvaged head-groups-- Interconversion of phosphatidylserine and phosphatidylethanol- amine by head group exchange-- Methylation of phosphatidylethanolamine to phosphatidylcholine in liver

Strategies For Attachment of Head-Groups byStrategies For Attachment of Head-Groups byPhosphodiester Phosphodiester Bond FormationBond Formation

Synthesis of phosphatidylinositol (PI)in ER; phosphatidylglycerol (PG) andcardiolipin (CL) in mitochondria

Synthesis of phosphatidylcholine(PC) and phosphatidylehtanolamine(PE) in ER

Synthesis of Synthesis of Cardiolipin Cardiolipin and and PhosphatidylinositolPhosphatidylinositolin Eukaryotesin Eukaryotes

Specific PI kinases convert PI to phos-phorylated derivatives that activatesignal transduction pathways

Phosphatidlycholine Phosphatidlycholine is is Synthe-Synthe-sized sized in the Mammalian ER byin the Mammalian ER bySalvaging Salvaging CholineCholine

PE is synthesized by a parallelpathway from salvagedethanolamine

5Summary of the Summary of the Phosphatidly-Phosphatidly-choline choline and and Phosphatidyl-Phosphatidyl-ethanolamine ethanolamine PathwaysPathways

In liver, PC is synthesized by 3 meth-ylations of PE using S-adenosylmeth-ionine (adoMet) as the methyl donorwhich gives rise to S-adenoshomo-cysteine (adoHcy) -- final steps ofmajor path from PS to PE (decarb-oxylation) to PC in eukaryotes

Salvage pathways

Head-groupexchange

Cholesterol MetabolismCholesterol Metabolism Component of cell membrane bilayers (fluidity buffer) Component of plasma lipoproteins (outer layer) Precursor of bile acids, steroid hormones, vitamin D3 (7-

dehydrocholesterol) All the Cs of cholesterol arise from acetate Isoprene units are the essential intermediates in the

pathway from acetate to cholesterol

Cholesterol is Synthesized in Four StagesCholesterol is Synthesized in Four Stages (Stages 1 to 3 in cytosol, stage 4 on ER)

1. Condensation of three acetate unitsto form mevalonate

2. Conversion of mevalonate to activ-ated isoprene units

3. Polymerization of 6 five C isoprenesto form 30 C linear squalene

4. Cyclization of C30 squalene to formthe steroid ring nucleus with furtheroxidations, methyl group removalsand migrations to produce the C27cholesterol product

Formation of Formation of MevalonateMevalonateFrom From Acetyl-CoAAcetyl-CoA

The first 2 enzymes are cytosolic isozymesof the mitochondrial enzymes that initiateketone body formation

HMG-CoA reductase (97-kDa integral mem-brane glycoprotein) is 1st committed, rate-limiting step in cholesterol biosynthesis-- Regulatory site (feedback inhibition of act- ivity by cholesterol)-- Inactivated in signal transduction pathway by cAMP-dependent protein kinase phosphorlation-- Undergoes cholesterol-stimulated protein degradation, and cholesterol-mediated trancriptional control of mRNA levels-- Site of inhibition of cholesterol biosyn- thesis by statins

Structure of extramem-brane fragment of tetra-meric HMG-CoA reductase

6Conversion of Conversion of Mevalonate Mevalonate to Activated Isoprene Unitsto Activated Isoprene Units

Six of these activated units combine to form squalene Subsequent pathway to squalene involves condensation of 3 dimethylallyl-

PP to yield C15 farnesyl-PP Two farnesyl-PP are condensed with PP release to drive C30 squalene

formation Squalene undergoes oxygenase reaction to epoxide, complex cyclization

and conversion to C27 cholesterol product in many steps

Note pink highlightedleaving groups fromhypothetical intermediate

Cholesterol Esters Are FormedCholesterol Esters Are Formedin Liver For Storage and Trans-in Liver For Storage and Trans-port on the HDL Surfaceport on the HDL Surface

Reaction in liver is catalyzed by ACAT, where the cholesterol esters are stored, ortransported in secreted lipoprotein particles to other tissues

LCAT -- part of high density lipoprotein (HDL) apolipoprotein, located on surfaceof nascent HDL particles, stimulated by Apo-1 of HDL

Cholesterol esters synthesized by LCAT form an HDL core which trans- forms the nascent particles into mature spherical HDL

The cholesterol-rich particles return to liver from extrahepatic tissues where the cholesterol is unloaded and is largely excreted in bile salts

Cholesterol and other Lipids are Carried inCholesterol and other Lipids are Carried inPlasma LipoproteinsPlasma Lipoproteins

Low-density lipoprotein (LDL) issynthesized in liver and is majortransporter of cholesterol and chol-esterol esters from liver to extra-hepatic tissues

Apolipoprotein B-100 is a 513-kDapolypeptide recognized by LDLreceptors on plasma membrane ofextrahepatic tissues

Beside chylomicrons, very low-density lipoprotein (VLDL) is anadditional major lipoprotein

VLDL delivers endogenously synth-esized triacylglycerol from liver toadipose and other tissues-- Breakdown of VLDL remnants by lipoprotein lipase intermediate- density lipoproteins (IDL) LDL Structural cartoon of low density

lipoprotein (LDL)

LDL is Released from Receptor by Interactions in LDL is Released from Receptor by Interactions in Extracellular Extracellular ReceptorReceptorDomain Between Receptor Repeats and the Domain Between Receptor Repeats and the --Propeller RegionPropeller Region

Second extracellular domain (red) is homologous to epidermal growth factor (EGF)precursor

Note that LDL interacts with receptor repeats R4 and R5 The -propeller regions displaces bound LDL by acting as alternative substrate for

ligand-binding domain in a Ca2+-dependent LDL release

Structure of 6-blad-ed propeller domainand adjacent EGF-like domain(Rudenko et al., Nature298:2353, 2002)

7Cholesterol Uptake by Receptor-Mediated Cholesterol Uptake by Receptor-Mediated EndocytosisEndocytosis

Receptor recycling

Re-esterificationby ACAT

Incorporation intoER membrane

Regulation of Cholesterol Formation BalancesRegulation of Cholesterol Formation BalancesSynthesis with Dietary UptakeSynthesis with Dietary Uptake

Glucagon promotes phosphoryl-ative inactivation of HMG-CoAreductase

Insulin promotes dephosphoryla-tive activation

High concentrations of cholester-ol activate ACAT, increasingcholesterol esterification forstorage

High cellular cholesterol levelsalso diminish transcription of thegene encoding LDL receptors,resulting in less cholesteroluptake