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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
Breakdown ofTriacylglycerols
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The glycerol is absorbed by the liverand converted to glycolyticintermediates.
Breakdown ofTriacylglycerols
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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
Transport into MitochondrialMatrix
T t i t Mit h d i l
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Transport into MitochondrialMatrix
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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Each round in fattyacid degradationinvolves four
reactions2. Hydration to L3Hydroxylacyl CoA
Fatty acid oxidation
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Each round in fattyacid degradationinvolves four
reactions3. Oxidation to 3Ketoacyl CoA
Fatty acid oxidation
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Each round in fattyacid degradationinvolves four
reactions4. Thiolysis to produceAcetylCoA
Fatty acid oxidation
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Each round in fattyacid degradationinvolves four
reactions The process repeats
itself
Fatty acid oxidation
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Each round in fatty acid degradationinvolves four reactions
Fatty acid oxidation
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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