Metab Lipid

8/3/2019 Metab Lipid

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Lipids

Metabolism


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AN OVERVIEW OF FATTY ACIDMETABOLISM

Fatty acid degradation andsynthesis are relatively simple

processes that are essentially the

reverse of each other.


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principally in adipose tissue and liver

Requires acetyl CoA and ATP

adipose: FA stored as triacylglycerols via esterification

liver: produces TAG packaged into VLDL and exported

compounds metabolized to acetyl CoA can serve as a

fat precursor

glucose = primary source of carbons for fat synthesis.

LIPOGENESIS


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CYTOPLASM

MITOCHONDRIALMATRIX

Pyruvate

Citrate

CSOxaloacetate

PC

ATP,CO2

ADP,

Pi

PP

PPyruvate

Glucose

Glycolys

is

FAS

Fat

tyAcids

Citrate

AcetylCoA

CL

ATP,

CoA

ADP+Pi

Oxaloacetate

ACC

ADP, Pi

CO2,ATP

Malonyl

CoA

AcetylCoA

NAD,CoA

NADH,CO2

PDH

MDHNADH

NA

D+

Malate

ME NAD

P+

NADPH

CO2

Figure 1. Export of acetyl CoA as citrate for fatty acidbiosynthesis, generation of NADPH and pathway of lipogenesis.


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Rule:

Fatty acid biosynthesis is a stepwise assemblyof acetyl-CoA units (mostly as malonyl-CoA)ending with palmitate (C16 saturated)

ActivationElongation

Terminati

on

3Phases


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Acetyl-CoA Activation: MakingMalonyl

This third subunit, atranscarboxylase,does exactly that:

transfers the CO2 toacetyl-CoA,converting it into

malonyl-CoA, to beused in the nextstep of the reaction


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H3C C SCoA

O

CH2 C SCoA

O

OOC

acetyl-CoA

malonyl-CoA


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Acetyl-CoACarboxylasecatalyzes the

2-stepreaction bywhich acetyl-CoA iscarboxylated to

form malonyl-CoA.ll

Enzyme-biotinHCO3

-+ ATP

ADP + PiEnzyme-biotin-CO2

-

O

CH3-C-SCoA

acetyl-CoAO

-O2C-CH2-C-SCoA

malonyl-CoA

ll

Enzyme-biotin

1

2

As with other carboxylation reactions, theenzyme prosthetic group is biotin.


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The overall reaction, which is spontaneous, may besummarized as:

HCO3 + ATP + acetyl-CoA ADP + Pi + malonyl-CoA

ll

Enzyme-biotinHCO3

-+ ATP

ADP + PiEnzyme-biotin-CO2

-

O

CH3-C-SCoA

acetyl-CoAO

-O2C-CH2-C-SCoA

malonyl-CoA

ll

Enzyme-biotin

1

2


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Acetyl-CoA Carboxylase, which convertsacetyl-CoA to malonyl-CoA, is thecommitted step of the fatty acid synthesispathway.

The mammalian enzyme is regulated, byw phosphorylationw allosteric control by local metabolites.


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Fatty acid synthesis from acetyl-CoA &malonyl-CoA occurs by a series of reactions

that are:w in bacteria catalyzed by 6 different

enzymes plus a separate acyl carrier protein(ACP)

w in mammals catalyzed by individualdomains of a very large polypeptide thatincludes an ACP domain.

w

NADPH serves as electron donor in thetwo reactions involving substrate reduction.

The NADPH is produced mainly by the PentosePhosphate Pathway.


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To start an elongation cycle, AcetylCoAand MalonylCoA are each transferredto an acyl carrier protein

Elongation


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Acyl Carrier Protein (ACP)


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Acyl-malonyl ACP

condensing enzymeforms Acetoacetyl-ACP.

Elongation


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The next three reactions are similarto the reverse of fatty aciddegradation, except

The NADPH is used instead of NADH and FADH2 The Denantiomer of Hydroxybutarate is formed instead of the L

enantiomer

4.3 Elongation


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The elongation cycle is repeated sixmore times, using malonylCoA eachtime, to produce palmitylACP.

Elongation


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CH3C-

O

CH2C~S-

O

ACP

NADPH

CH3CH2CH2C~S-

O

ACP

CH3C-CH2C~S-

O

ACP

HO

H

CH3C-= C- C~S-

O

ACPH

H

-H2O

NADPH

-Carbon Elongation

D isomer

Reduction

Dehydration

Reduction

-Ketoacyl-ACP reductase

-Hydroxyacyl-ACP dehydrase

Enoyl-ACP reductase


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-KS

CO2

-S-ACP

TERMINATION KetoacylACP

Synthase

Free to bindMalonyl-CoA

Transfer toKS

Split outCO2

Transfer to Malonyl-CoA

-CH2CH2CH2C~S-

O

ACP

When C16 stage is reached, instead of transferring to KS,the transfer is to H2O and the fatty acid is released


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ACP

KS -SH

HSAcetyl-CoA

CoA-SH

-C-CH3

O

S

KS S-C-CH3

O

KS -SH

SH

CoA-SH

Malonyl-CoA

S -C-CH2-COO-

O

CO2C=O

CH2

C=O

CH

S

O

CH3-CH -CH2-C-S

OH

O

CH3-CH=CH-C-S

OCH3-CH2-CH2-C-S

S-C-CH2-CH2-CH3

O

KS

KS

NADP+ NADPH

H+

NADPHH+

NADP+

H2O

Initiationor

priming

Elongatio

n

Fatty AcidSynthase

-Ketoacyl-ACP

reductase

-Hydroxyacyl-ACP

dehydrase

Enoyl-ACP

reductase

-Keto-ACPsynthase (condensingenzyme)

Malonyl-CoA-ACPtransacylase

Acetyl-CoA-ACPtransacylase

-Ketoacyl-ACPsynthase


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The stoichiometry of palmitate

synthesis: Synythesis of palmitate from Malonyl

CoA

Synthesis of MalonylCoA from AcetylCoA

Overall synthesis

8


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PROBLEM:

Fatty acid biosynthesis takes place in the

cytosol. Acetyl-CoA is mainly in theMitochondria

How is acetyl-CoA made available to the cytosolicfatty acyl synthase?

SOLUTION:Acetyl-CoA is delivered to cytosol from the

mitochondria as CITRATE

acetyl-

CoA


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Citrate Shuttle


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Post-Synthesis Modifications

C16 satd fatty acid (Palmitate) is theproduct

Elongation

Unsaturation

Incorporation into triacylglycerols

Incorporation into acylglycerolphosphates


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Elongation beyond the 16-C length of the palmitateproduct of Fatty Acid Synthase is mainly catalyzed byenzymes associated with the endoplasmic reticulum

(ER).

ER enzymes lengthen fatty acids produced by Fatty AcylSynthase as well as dietary polyunsaturated fatty acids.

Fatty acids esterified to coenzyme A serve as

substrates.

Malonyl-CoA is the donor of 2-carbon units in areaction sequence similar to that of Fatty AcidSynthase except that individual steps are catalyzed by

separate proteins.A family of enzymes designated Fatty Acid Elongasesor ELOVL (elongation of very long chain fatty acid)catalyze the initial condensation step.


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Desaturases introduce double bonds at

specific positions in a fatty acid chain.Mammalian cells are unable to produce

double bonds at certain locations, e.g., 12.

Thus some polyunsaturated fatty acids aredietary essentials, e.g., linoleic acid, 18:2

cis 9,12 (18 C atoms long, with cis double

bonds at carbons 9-10 & 12-13).

C

O

OH

910

oleate 18:1 cis9


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Formation of a double bond in a fatty acidinvolves the following endoplasmic

reticulum membrane proteins inmammalian cells:

w NADH-cyt b5 Reductase, a flavoproteinwith FAD as prosthetic group.

w Cytochrome b5, which may be aseparate protein or a domain at one endof the desaturase.

w Desaturase, with an active site thatcontains two iron atoms complexed by

C

O

OH

910

oleate 18:1 cis

9


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The desaturase catalyzes a mixed function oxidationreaction.

There is a 4-electron reduction ofO2 2 H2O as a fatty acid is

oxidized to form a double bond.w 2e pass from NADH to the desaturase via the FAD-

containing reductase & cytochrome b5, the order ofelectron transfer being:

NADHFADcyt b5desaturase

w

2eare extracted from the fatty acid as the double bondis formed.E.g., the overall reaction for desaturation of stearate (18:0) toform oleate (18:1 cis 9) is:

stearate + NADH + H+ + O2oleate + NAD+ + 2H2O


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FATTY ACID DEGRADATION


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Three stages of processing

Triglycerols are degraded to fatty acidsand glycerol in the adipose tissue andtransported to other tissues.

Fatty acids are activated andtransported into the mitochondria.

Fatty acids are broken down into two-

carbon acetylCoA units and fed into thecitric acid cycle.Utilization of Fatty Acids as

Fuel


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In the adipose tissue, lipases areactivated by hormone signaledphosphorylation

Breakdown ofTriacylglycerols


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The lipases break thetriacylglycerols downto fatty acids and

glycerol The fatty acids are

transportred in theblood by serum

albumin



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The glycerol is absorbed by the liverand converted to glycolyticintermediates.



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Acyl CoA synthetase reaction occursin the on the mitochondrialmembrane.

Activation of Fatty Acids

T t i t Mit h d i l


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Carnitine carrieslong-chainactivated fatty

acids into themitochondrialmatrix

Transport into MitochondrialMatrix

i i h d i l


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Carnitine carries long-chain activatedfatty acids into the mitochondrialmatrix


T t i t Mit h d i l


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AMiracle???

Fatty acid oxidation


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Each round in fattyacid degradationinvolves four

reactions1. oxidation to trans-2-Enoly-CoA

Fatty acid oxidation( -oxidation pathway)


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reactions2. Hydration to L3Hydroxylacyl CoA



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reactions3. Oxidation to 3Ketoacyl CoA



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reactions4. Thiolysis to produceAcetylCoA



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reactions The process repeats

itself



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Each round in fatty acid degradationinvolves four reactions



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The complete oxidation of the 16carbon palmitoylCoA produces 106ATP's

ATP Yield

B t O id ti


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Beta Oxidation


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Unsaturated fattyacids(monounsaturated)

Special Cases


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Unsaturated fatty acids(polyunsaturated)

Special Cases

dd h i


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Odd-Chain

Metab Lipid

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