Fatty Acid Catabolism: A Major Energy Source

17|
Fa'y
Acid
Catabolism

© 2013 W. H. Freeman and Company
21|
Lipid
Biosynthesis

Oxida=on
of
fa'y
acids
is
a
major
energy

source
in
many
organisms

•  About
one-‐third
of
our
energy
needs
comes
from

dietary
triacylglycerols

•  About
80%
of
energy
needs
of
mammalian
heart
and

liver
are
met
by
oxida<on
of
fa=y
acids!!

•  Many
hiberna<ng
animals,
such
as
grizzly
bears,
rely

almost
exclusively
on
fats
as
their
source
of
energy

(and
water
during
their
long-‐term
sleep)

Fats
provide
eﬃcient
fuel
storage

•  The
advantage
of
fats
over
polysaccharides:

–  Fa=y
acids
carry
more
energy
per
carbon
because
they
are

more
reduced

–  Fa=y
acids
carry
less
water
along
because
they
are
nonpolar

(aggregate
in
lipid
droplets
and
are
unsolvated)

•  Glucose
and
glycogen
are
for
short-‐term
energy
needs,
quick

delivery

•  Fats
are
for
long-‐term
(months)
energy
needs,
good
storage,

slow
delivery

Fat
Storage
in
White
Adipose
Tissue

Nuclei
“Squeezed”

Dietary
fa'y
acids
are
absorbed
in
the

vertebrate
small
intes=ne

Emulsification
by biological
detergents
(bile)
Breakdown of
TAG to DAG,
MAG, FFA
and glycerol
Uptake by intestinal
cells
Chylomicrons
(lipoproteins)
Bloodstream to
target tissues
2nd
breakdown
of TAG
Used for
energy
(muscles)
or
reesterified
for energy
(adipose)
Remaining chylomicrons go to liver and enter by RME à used for ketone bodies synthesis.
When diet contains more f.a. than needed, liver converts them to TAG and packages
them into VLDL to be transported to adipocytes

Lipids
are
transported

in
the
blood
as
chylomicrons

Apoliporpotein + lipids particles = lipoprotein
Lipoproteins range in density: VLDL to VHDL

Hormones
trigger
mobiliza=on

of
stored
triacylglycerols

• Hydrolysis
of
TAGs
is
catalyzed
by
lipases

-‐
can
produce
MAGs,
DAGs,
FFA
and
glycerol

• Some
lipases
are
regulated
by
hormones

glucagon
and
epinephrine

Recall:

• 
Epinephrine
means:
“We
need
energy
now”

• 
Glucagon
means:
“We
are
out
of
glucose”

Hormones
trigger
mobiliza=on

of
stored
triacylglycerols

•  Perilipins
–
proteins
that

coat
lipid
droplets
and

restrict
access
to
lipids
to

prevent
premature

mobiliza<on

•  ê[glc]blood
è
glucagon

èè
PKA
è

phosphoryla<on
of

hormone-‐sensi<ve
lipase
&

perilipin
è
dissocia<on
of

CGI
and
ac<va<on
of

adipose
triacylglycerol

lipase

monoacylglycerol
lipase

hydrolyzes
MAGs
Serum albumin binds up to 10 f.a. noncovalently

Glycerol
from
fats
enters
glycolysis

• Only
5%
of
biologically-‐ac<ve

energy
of
TAG
is
in
glycerol

• Glycerol
kinase
ac<vates

glycerol
at
the
expense
of

ATP

• Subsequent
reac<ons

recover
more
than
enough

ATP
to
cover
this
cost

• Allows
limited
anaerobic

catabolism
of
fats

Fa'y
Acid
Transport
into
Mitochondria

• Fats
are
degraded
into
fa=y
acids
and
glycerol
in
the

cytoplasm
of
adipocytes

• Fa=y
acids
are
transported
to
other
<ssues
for
fuel

• β-‐oxida<on
of
fa=y
acids
occurs
in
mitochondria

• Small
(<
12
carbons)
fa=y
acids
diﬀuse
freely
across

mitochondrial
membranes

• Larger
fa=y
acids
(most
free
fa=y
acids)
are

transported
via
acyl-‐carni<ne/carni<ne
transporter

(carni=ne
shu'le)

• Three
steps:

Conversion
of
a
fa'y
acid
to
a
fa'y
acyl–CoA

(1)

Nucleophilic attack by f.a. anion
Phosphoester linkage between f.a.
carboxyl and α phosphate of ATP
Thioester linkage between f.a.
carboxyl and thiol group of
CoA-SH
Hydrolysis of PPi to 2Pi is
highly exergonic and pulls the
first reaction forward

Acyl-‐Carni=ne/Carni=ne
Transport

(2)

(3)

Transesterification
to carnitine
Transesterification
to CoA
2 separate pools of CoA:
Matrix CoA à used mostly in oxidative degradation (pyr, f.a., a.a.)
Cytosolic CoA à used in biosynthesis of f.a.
Carnitine-mediated entry is the rate limiting step for oxidation of f.a. in mito

Stages
of
Fa'y
Acid
Oxida=on

•  Stage
1
consists
of
oxida<ve
conversion
of
two-‐carbon

units
into
acetyl-‐CoA
via
β-‐oxida<on
with
concomitant

genera<on
of
NADH
and
FADH2

–  involves
oxida<on
of
β
carbon
to
thioester
of
fa=y
acyl-‐CoA

•  Stage
2
involves
oxida<on
of
acetyl-‐CoA
into
CO2
via
citric

acid
cycle
with
concomitant
genera<on
NADH
and
FADH2

•  Stage
3
generates
ATP
from
NADH
and
FADH2
via
the

respiratory
chain

Stages
of
Fa'y
Acid
Oxida=on

The
β-‐Oxida=on
Pathway

Each
pass
removes
one
acetyl
moiety
in
the
form
of
acetyl-‐CoA.

Palmitate (C16)
undergoes seven passes
through the oxidative
sequence
Formation of each
acetyl-CoA
requires removal
of 4 H atoms {2 e–
pairs and 4 H+})

Step
1:

Dehydrogena=on
of
Alkane
to
Alkene

•  Catalyzed
by
isoforms
of
acyl-‐

CoA
dehydrogenase
(AD)
on

the
mitochondrial
inner

membrane

–  Very-‐long-‐chain
AD

(VLCAD,
12–18
carbons)

–  Medium-‐chain
AD
(MCAD,

4–14
carbons)

–  Short-‐chain
AD
(SCAD,
4–8
carbons)

•  Results
in
trans
double
bond,
diﬀerent
from
naturally
occurring

unsaturated
fa=y
acids,
between
α
and
β
C

•  Analogous
to
succinate
dehydrogenase
reac<on
in
the
CAC

–  Electrons
from
bound
FAD
transferred
directly
to
the
electron-‐
transport

chain
via
electron-‐transferring
ﬂavoprotein
(ETF)

Step
2:

Hydra=on
of
Alkene

•  Catalyzed
by
two
isoforms
of
enoyl-‐CoA
hydratase:

–  Soluble
short-‐chain
hydratase
(crotonase)

–  Membrane-‐bound
long-‐chain
hydratase,
part
of

trifunc<onal
complex

•  Water
adds
across
the

double
bond
yielding

alcohol

•  Analogous
to
fumarase

reac<on
in
the
CAC

–  Same
stereospeciﬁcity

Step
3:

Dehydrogena=on
of
Alcohol

•  Catalyzed
by
β-‐hydroxyacyl-‐CoA
dehydrogenase

•  The
enzyme
uses
NAD
cofactor
as
the
hydride
acceptor

•  Only
L-‐isomers
of
hydroxyacyl
CoA
act
as
substrates

•  Analogous
to
malate
dehydrogenase
reac<on
in
the
CAC

•  The
ﬁrst
three
steps

create
a
much
less

stable
C-‐C
bond,

where
the
α
C
is
bound

to
2
carbonyl
groups

Step
4:

Transfer
of
Fa'y
Acid
Chain

•  Catalyzed
by
acyl-‐CoA
acetyltransferase
(thiolase)
via

covalent
mechanism

–  The
carbonyl
carbon
in
β-‐ketoacyl-‐CoA
is
electrophilic

–  Ac<ve
site
thiolate
acts
as
nucleophile
and
releases
acetyl-‐
CoA

–  Terminal
sulfur
in
CoA-‐SH

acts
as
nucleophile
and

picks
up
the
fa=y
acid
chain

from
the
enzyme

•  The
net
reac<on
is

thiolysis
of

a
carbon-‐carbon
bond

Trifunc=onal
Protein
(TFP)

•  Hetero-‐octamer

–  Four
α
subunits

•  enoyl-‐CoA
hydratase
ac<vity

•  β-‐hydroxyacyl-‐CoA
dehydrogenase
ac<vity

•  Responsible
for
binding
to
membrane

–  Four
β
subunits

•  long-‐chain
thiolase
ac<vity

•  May
allow
substrate
channeling

•  Associated
with
mitochondrial
inner
membrane

•  Processes
fa=y
acid
chains
with
12
or
more
carbons

•  Shorter
chains
are
processed
by
soluble
individual

enzymes
in
the
matrix

Similar
mechanisms
introduce
carbonyls

in
other
metabolic
pathways

Fa'y
Acid
Catabolism
for
Energy

•  For
palmi<c
acid
(C16)

–  Repea<ng
the
above
four-‐step
process
six
more
<mes
(7
total)

results
in
eight
molecules
of
acetyl-‐CoA

•  FADH2
is
formed
in
each
cycle
(7
total)

•  NADH
is
formed
in
each
cycle
(7
total)

•  Acetyl-‐CoA
enters
citric
acid
cycle
and
further
oxidizes
into
CO2

–  This
makes
more
GTP,
NADH,
and
FADH2

•  Electrons
from
all
FADH2
and
NADH
enter
ETC

•  Transfer
of
e–s
from
FADH2
and
NADH
to
O2
yields
1
H2O
per
pair

(camels
and
hiberna<ng
animals!)

Palmitoyl-‐CoA
+
7CoA
+
7O2
+
28Pi+
28ADP
à
8
acetyl-‐CoA
+
28ATP
+
7H2O
(β

oxida<on)

Palmitoyl-‐CoA
+
23O2
+
108Pi+
108ADP
à
CoA
+
108ATP
+
16CO2
+
23H2O
(full

oxida<on)

NADH
and
FADH2
serve
as
sources
of
ATP

Oxida=on
of
Unsaturated
Fa'y
Acids

•  Naturally
occurring
Unsaturated
Fa=y
acids
contain

cis
double
bonds

–  Are
NOT
a
substrate
for
enoyl-‐CoA
hydratase

•  Two
addi<onal
enzymes
are
required

–  Isomerase:
converts
cis
double
bonds

star<ng
at
carbon
3

to
trans
double
bonds

–  Reductase:
reduces
cis
double
bonds
not
at
carbon
3

•  Monounsaturated
fa=y
acids
require
the
isomerase

•  Polyunsaturated
fa=y
acids
require
both
enzymes

Oxida=on
of
Monounsaturated
Fa'y
Acids

Oleate (18:1 Δ9)
converted to oleoyl-CoA
and imported into mito
via carnitine shuttle

Oxida=on
of

Polyunsaturated

Fa'y
Acids

Linoleate (Δ9,Δ12)

First
double
bond
requires
isomeriza=on

Second
requires
reduc=on/isomeriza=on

Oxida=on
of
odd-‐numbered
fa'y
acids

•  Most
dietary
fa=y
acids
are
even-‐numbered

•  Many
plants
and
some
marine
organisms
also
synthesize

odd-‐numbered
fa=y
acids

•  Propionyl-‐CoA
forms
from
β-‐oxida<on
of
odd-‐numbered

fa=y
acids

•  Bacterial
metabolism
in
the
rumen
of
ruminants
also

produces
propionyl-‐CoA

•  Oxida<on
is
iden<cal
to
even-‐numbered
long-‐chain
fa=y

acids,
but
the
last
pass
through
β-‐oxida<on
is
a
fa=y

acyl-‐CoA
with
a
5-‐C
fa=y
acid
that
is
cleaved
to
give

acetyl-‐CoA
and
propionyl-‐CoA

Carboxyla=on
of
Propionyl-‐CoA

Isomeriza=on
to
Succinyl-‐CoA
à
CAC

Isomeriza=on
in
propionate
oxida=on

requires
coenzyme
B12

Complex
Cobalt-‐Containing
Compound:

Coenzyme
B12

•  Very unstable bond
•  Breaks to yield –CH2
. and Co3+
•  Used to transfer the hydrogen
atom to a different C in the
molecule (isomerization)
•  No mixing of the transferred H
atom with the hydrogen of the
solvent (H2O)
•  The formation of this complex
cofactor occurs in one of two
known reactions that cleaves a
triphosphate from ATP

Regula=on
of
Fa'y
Acid

Synthesis
and
Breakdown

Cytosol
•  Occurs
only
when
need
for
energy
requires
it

•  2
pathways
for
f.a.CoA
in
liver:
TAG
synthesis
in
cytosol
or
f.a.
oxida<on
in
mito

•  Transfer
into
mito
is
rate
limi<ng,
once
f.a.
are
in
mito
they
WILL
undergo
oxida<on

Concn
increases
when
CHO
is
well-‐supplied

Inhibi<on
of
shu=le
ensures
oxida<on
of
f.a.
is

inhibited
when
ñenergy

ñ[NADH]/[NAD+] ý
ñAcetyl-CoA ý

Gene=c
defects
in
fa'y
acyl-‐CoA

dehydrogenases

•  Inability
to
oxidize
fats
for
energy
has
serious
eﬀects

on
health

•  More
than
20
human
gene<c
defects
in
f.a.

transport
and
metabolism
occur

•  MCAD
(medium
chain
acyl-‐CoA
dehydrogenase)

deﬁciency
is
the
most
common
syndrome
in

European
popula<ons

-‐
Unable
to
oxidize
f.a.
of
6
–
12
Cs

-‐
If
diagnosed
aoer
birth,
the
infant
can
be

treated
with
low
fat,
high
carbohydrate
diet

β-‐Oxida=on
in

Mitochondria
vs.

Peroxisomes

•  Differ
in
the
first
step:

-‐
passes
e–s
directly
to
O2
forming

H2O2
which
is
quickly
removed
by
the

ac<on
of
catalase

-‐
energy
is
lost
as
heat
instead
of

producing
ATP

•  Differ
in
f.a.
specificity:

-‐
more
ac<ve
on
very
long
f.a.
and

branched
f.a.
(α
oxida=on)

-‐
process
long
chain
f.a.
into
shorter

ones
which
are
exported
to
mito
to

complete
oxida<on

•  Zellweger
syndrome
–
inability
to
m
make
peroxisomes

ω
oxida=on

•  In
the
ER
of
liver
and
kidney

•  For
f.a.
with
10
–
12
Cs

•  Addi<on
of
OH
by
a
mixed
func=on

oxidase
(cytochrome
P450)

•  Alcohol
dehydrogenase
oxidizes
OH

to
aldehyde

•  Aldehyde
dehydrogenase
oxidizes

aldehyde
to
acid

•  CoA
can
a=ach
to
either
end
and
β

oxida<on
resumes

Forma=on
of
Ketone
Bodies

•  Entry
of
acetyl-‐CoA
into
citric
acid
cycle
requires

oxaloacetate

•  When
oxaloacetate
is
depleted,
acetyl-‐CoA
is
converted

into
ketone
bodies
(acetone,
acetoacetate
and
D-‐β-‐
hydroxybutyrate)

–  Frees
Coenzyme
A
for
con<nued
β-‐oxida<on

–  Acetone
is
exhalled

–  Acetoacetate
and
β-‐HB
are
transported
in
the
blood

•  Under
starva<on
condi<ons,
the
brain
can
use
ketone

bodies
for
energy

•  The
ﬁrst
step
is
reverse
of
the
last
step
in
the
β-‐
oxida<on:
thiolase
reac<on
joins
two
acetate
units

Release
of
Free
Coenzyme
A

Another condensation
with acetyl-CoA
yields HMG-CoA

Forma=on
of
Ketone
Bodies

Cleaved into
acetoacetate and
acetyl-CoA
Specific for the D-
isomer; don’t confuse
it with L-β-
hydroxyacyl-CoA DH
of β oxidation
Untreated
diabetes à
[acetoacetate]
is high à more
acetone
produced à
exhaled (odor)

Ketone
Bodies
as
fuel

In extrahepatic tissues:
Ketone bodies can be
used as fuels in all
tissues except the liver
The liver is a producer,
not a consumer, of
ketone bodies
ß CACFound in all
tissues except
the liver

Liver
is
the
source
of
ketone
bodies

•  Produc<on
of
ketone

bodies
increases
during

starva<on
(and
diabetes)

•  Ketone
bodies
are

released
by
liver
to

bloodstream

•  Organs
other
than
liver

can
use
ketone
bodies
as

fuels

•  High
levels
of

acetoacetate
and
β-‐
hydroxybutyrate
lower

blood
pH
dangerously

(acidosis)

•  Acidosis
due
to
ketone

bodies
-‐
ketoacidosis

Lipids
fulﬁll
a
variety
of

biological
func=ons

•  Energy
storage

•  Cons<tuents
of

membranes

•  Anchors
for
membrane
proteins

•  Cofactors
for
enzymes

•  Signaling
molecules

•  Pigments

•  Detergents

•  Transporters

•  An<oxidants

Catabolism
and
anabolism
of
fa'y
acids

proceed
via
diﬀerent
pathways

•  Catabolism
of
fa=y
acids
(excergonic
and
oxida=ve)

–  produces
acetyl-‐CoA

–  produces
reducing
power
(NADH
and
FADH2)

–  ac<va<on
of
fa=y
acids
by
CoA

–  takes
place
in
the
mitochondria

•  Anabolism
of
fa=y
acids
(endergonic
and
reduc=ve)

–  requires
acetyl-‐CoA
and
malonyl-‐CoA

–  requires
reducing
power
from
NADPH

–  ac<va<on
of
fa=y
acids
by
2
diﬀerent
–SH
groups
on
protein

–  takes
place
in
cytosol
in
animals,
chloroplast
in
plants

Subcellular
localiza=on
of
lipid
metabolism

Overview
of
Fa'y
Acid
Synthesis

•  Fa=y
acids
are
built
in
several
passes,
processing

one
acetate
unit
at
a
<me.

•  The
acetate
is
coming
from
ac<vated
malonate
in

the
form
of
malonyl-‐CoA.

•  Each
pass
involves
reduc<on
of
a
carbonyl
carbon

to
a
methylene
carbon.

Malonyl-‐CoA
is
formed
from
acetyl-‐CoA
and

bicarbonate

•  The
reac<on
carboxylates
acetyl
CoA

•  Catalyzed
by
acetyl-‐CoA
carboxylase
(ACC)

–  Enz
has
three
subunits:

• One
unit
has
Bio<n
covalently
linked
to
Lys

• Bio<n
carries
CO2

• In
animals,
all
three
subunits
are
on
one

polypep<de
chain

–  HCO3
−
(bicarbonate)
is
the
source
of
CO2

The
Acetyl-‐CoA
Carboxylase
(ACC)
Reac=on

•  Two-‐step
rxn
similar
to
carboxyla<ons
catalyzed

by
pyruvate
carboxylase
(gluconeogenesis)
and

propionyl-‐CoA
carboxylase
(odd
f.a.
metabolism)

•  CO2
binds
to
bio<n

-  CO2
is
ac<vated
by
a=achment
to
N
in
ring
of
bio<n

Synthesis
of
fa'y
acids
is
catalyzed
by
fa'y

acid
synthase
(FAS)

•  FAS
system:

–  Catalyzes
a
repea<ng
four-‐step
sequence
that
elongates
the

fa=y
acyl
chain
by
two
carbons
at
each
step

–  Uses
NADPH
as
as
the
electron
donor

–  Uses
two
enzyme-‐bound
-‐SH
groups
as
ac<va<ng
groups

•  FAS
I
in
vertebrates
and
fungi

•  FAS
II
in
plants
and
bacteria

FAS
I
vs.
FAS
II

FAS
I

•  Single
polypep<de
chain
in

vertebrates

•  Leads
to
single
product:

palmitate
16:0

•  C-‐15
and
C-‐16
are
from
the

acetyl
CoA
used
to
prime

the
rxn

FAS
II

•  Made
of
separate,
diﬀusible

enzymes

•  Makes
many
products

(saturated,
unsaturated,

branched,
many
lengths,

etc.)

•  Mostly
in
plants
and

bacteria

Fa'y
Acid
Synthesis

•  Overall
goal:

a=ach
two-‐C
acetate
unit
from
malonyl-‐CoA
to
a

growing
chain
and
then
reduce
it

•  Reac<on
involves
cycles
of
four
enzyme-‐catalyzed
steps

–  Condensa<on
of
the
growing
chain
with
ac<vated
acetate

–  Reduc<on
of
carbonyl
to
hydroxyl

–  Dehydra<on
of
alcohol
to
trans-‐alkene

–  Reduc<on
of
alkene
to
alkane

•  The
growing
chain
is
ini<ally
a=ached
to
the
enzyme
via
a

thioester
linkage

•  During
condensa<on,
the
growing
chain
is
transferred
to
the

acyl
carrier
protein
(ACP)

•  Aoer
the
second
reduc<on
step,
the
elongated
chain
is

transferred
back
to
fa=y
acid
synthase

The
General
Four-‐Step
Fa'y
Acid
Synthase
I

Reac=on
in
Mammals
(1)

Prep:

Malonyl
CoA
and
acetyl
CoA
(or
longer
fa=y

acyl
chain)
are
bound
to
FAS
I

-‐
bind
via
thioester
terminus
of
a
Cys
of
the
FAS

-‐
ac<vates
the
acyl
group

Step
1:

Condensa<on
rxn
a=aches
two
C
from

malonyl
CoA
to
the
a=ached
acetyl-‐CoA
(or
longer

fa=y
acyl
chain)

-‐
also
releases
CO2
from
malonyl-‐CoA

-‐
the
decarboxyla<on
facilitates
the
rxn

-‐
creates
β-‐keto
intermediate

Step
1
of
FAS
I:

Elonga=on

Step
2:

1st
Reduc<on:

NADPH
reduces
the
β-‐
keto
intermediate
to
an
alcohol

Step
3:

Dehydra<on:

OH
group
from
C-‐2
and

H
from
neighboring
CH2
are
eliminated,

crea<ng
double
bond
(trans-‐alkene)

Step
4:

2nd
Reduc<on:

NADPH
reduces

double
bond
to
yield
saturated
alkane

Step
5:
Transloca<on:
The
growing
chain
is

moved
from
ACP
to
–SH
on
FAS

The
General
Four-‐Step
Fa'y
Acid
Synthase
I

Reac=on
in
Mammals

Steps
2-‐4
of

the
FAS
I
rxn

Overall
Palmitate
Synthesis

Acyl
Carrier
Protein
(ACP)
serves
as
a
shu'le

in
fa'y
acid
synthesis

•  Contains
a
covalently
a=ached
prosthe<c

group
4’-‐phosphopantetheine

–  Flexible
arm
to
tether
acyl
chain
while

carrying
intermediates
from
one

enzyme
subunit
to
the
next

•  Delivers
malonate
to
the
fa=y
acid

synthase

•  Shu=les
the
growing
chain
from
one

ac<ve
site
to
another
during
the
four-‐step

reac<on

Charging
ACP
and
FAS
I
with
acyl

groups
ac=vates
them

•  Two
thiols
must
be
charged
with
the
correct
acyl

groups
before
condensa<on
rxn
can
begin

–  Thiol
from
4’-‐phosphopantethine
in
ACP

–  Thiol
from
Cys
in
fa=y
acid
synthase

1) Acetyl
group
of
acetyl-‐CoA
is
transferred
to
ACP

–  Catalyzed
by
malonyl/acetyl-‐CoA
transferase
(MAT)

–  ACP
passes
this
acetate
to
the
Cys
of
the
β-‐ketoacyl-‐ACP
synthase
(KS)

domain
of
FAS
I

2)  ACP
–SH
group
is
re-‐charged
with
malonyl
from

malonyl-‐CoA

Charging,
Ac=va=on
with
ACP,
and
the

Four-‐Step
Sequence
of
Mammalian
Fa'y

Acid
Synthesis

•  Ac<vated
acetyl
and

malonyl
groups
form

acetoacetyl-‐ACP
and

CO2

–  Claisen
condensa<on
rxn

•  Catalyzed
by
β-‐ketoacyl-‐
ACP
synthase
(KS)

•  Coupling
condensa<on

to
decarboxyla<on
of

malonyl-‐CoA
makes
the

rxn
energe<cally

favorable

•  Carbonyl
at
C-‐3
is
reduced
to
form
D-‐β-‐
hydroxybutyryl-‐ACP

–  NADPH
is
e−
donor

•  Catalyzed
by
β-‐ketoacyl-‐ACP
reductase
(KR)

•  OH
and
H
removed
from
C-‐2
and
C-‐3
of
β-‐hydroxybutyryl-‐
ACP
to
form
trans-‐Δ2-‐butenoyl-‐ACP

•  Catalyzed
by
β-‐hydroxyacyl-‐ACP
dehydratase
(DH)

•  NADPH
is
the
electron

donor
to
reduce
double

bond
of
trans-‐Δ2-‐butenoyl-‐
ACP
to
form
butyryl-‐ACP

•  Catalyzed
by
enoyl-‐ACP

reductase
(ER)

Enzymes
in
Fa'y
Acid
Synthase

•  Condensa<on
with
acetate

–  β-‐ketoacyl-‐ACP
synthase
(KS)

•  Reduc<on
of
carbonyl
to
hydroxyl

–  β-‐ketoacyl-‐ACP
reductase
(KR)

•  Dehydra<on
of
alcohol
to
alkene

–  β-‐hydroxyacyl-‐ACP
dehydratase
(DH)

•  Reduc<on
of
alkene
to
alkane

–  enoyl-‐ACP
reductase
(ER)

•  Chain
transfer/charging

–  Malonyl/acetyl-‐CoA
ACP
transferase

The
Transferase
and
FAS
rxns
are

repeated
in
new
rounds

•  Product
of
ﬁrst
round

is
butyryl-‐ACP

–  (bound
to
phosphopantetheine-‐SH
group
of
ACP)

•  Butyrul
gp
is
transferred
to
the
Cys
of
β-‐
ketoacyl-‐ACP
synthase

–  In
the
ﬁrst
round,
acetyl-‐CoA
was
bound
here

•  New
malonyl-‐CoA
binds
to
ACP

•  Aoer
new
round
of
four
steps,
six-‐C
product
is

made
(bound
to
ACP)

Beginning
of
the

Second
Round
of

Fa'y
Acid
Synthesis

Stoichiometry
of
Synthesis
of

Palmitate
(16:0)

1)  7
acetyl-‐CoAs
are
carboxylated
to
make
7

malonyl-‐CoAs…
using
ATP

7
AcCoA
+
7
CO2
+
7
ATP
à
7
malCoA
+
7
ADP
+
7
Pi

2)  Seven
cycles
of
condensa<on,
reduc<on,

dehydra<on
and
reduc<on…using
NADPH
to

reduce
the
β-‐keto
group
and
trans-‐double
bond

AcCoA
+
7
malCoA
+
14
NADPH
+
14
H+
àPalmitate
+
7
CO2
+

8
CoA
+
14
NADP+
+
6
H2O

Note:

Eukaryotes
have
one
addi<onal
energy
cost.

(Next
slide)

Acetyl-‐CoA
is
transported
into
the

cytosol
for
fa'y
acid
synthesis

•  In
nonphotosynthe<c
eukaryotes…

is
made
in
the
mitochondria

•  But
fa=y
acids
are
made
in
the
cytosol

•  So
Acetyl-‐CoA
is
transported
into
the

cytosol
with
a
cost
of
2
ATPs

•  Therefore,
cost
of
FA
synthesis
is
3
ATPs

per
2-‐C
unit

Fa'y
acid
synthesis
occurs
in
cell
compartments

where
NADPH
levels
are
high

•  Cytosol
for
animals,
yeast

•  Chloroplast
for
plants

•  Sources
of
NADPH:

–  In
adipocytes:

pentose
phosphate
pathway
and
malic

enzyme

–  NADPH
is
made
as
malate
converts
to
pyruvate
+
CO2

–  In
hepatocytes
and
mammary
gland:

pentose

phosphate
pathway

• 
NADPH
is
made
as
glucose-‐6-‐phosphate
converts
to
ribulose
6-‐
phosphate

–  In
plants:

photosynthesis

Pathways
for
NADPH
Produc=on

Acetyl-‐CoA,
generated
in
the
mitochondria,
is

shu'led
to
the
cytosol
as
citrate

•  In
most
eukaryotes,
the
acetyl-‐CoA
for
lipid
synthesis

is
made
in
the
mitochondria

–  But
lipid
synthesis
occurs
in
the
cytosol

• And
there
is
no
way
for
acetyl-‐CoA
to
cross

mitochondrial
inner
membrane
to
the
cytosol

•  So
acetyl-‐CoA
is
converted
to
citrate

–  Acetyl-‐CoA
+
oxaloacetate
à
citrate

• Same
rxn
as
occurs
in
CAC

• Catalyzed
by
citrate
synthase

• Citrate
passes
through
citrate
transporter

Citrate
is
cleaved
to
regenerate

acetyl-‐CoA

•  Citrate
(now
in
cytosol)
is
cleaved
by
citrate

lyase

–  Regenerates
acetyl-‐CoA
and
oxaloacetate

–  Rxn
requires
ATP

–  Acetyl-‐CoA
can
now
be
used
for
lipid
synthesis

•  What
happens
to
the
oxaloacetate
because

there
is
no
oxaloacetate
transporter
either?

Oxaloacetatecyt
is
converted
to
malate

•  Malate
dehydrogenase
in
cytosol
reduces

oxaloacetate
to
malate

•  Two
poten<al
fates
for
malate:

–  Can
be
converted
to
NADPHcyt
and
pyruvatecyt
via
the

malic
enzyme

•  NADPH
used
for
lipid
synthesis

•  Pyruvatecyt
sent
back
to
mito
via
pyruvate
transporter

•  Converted
back
to
oxaloacetatemito
by
pyruvate
carboxylase,

requires
ATP

–  Can
be
transported
back
to
mito
via
malate
-‐α-‐
ketoglutarate
transporter

•  Malatemito
is
reoxidized
to
oxaloacetatemito

Shu'le
for
Transfer
of
Acetyl
Groups
from

Mitochondria
to
Cytosol

Fa'y
acid
synthesis
is
=ghtly
regulated

via
ACC

•  Acetyl
CoA
carboxylase
(ACC)
catalyzes
the
rate-‐
limi<ng
step

–  ACC
is
feedback-‐inhibited
by
palmitoyl-‐CoA

–  ACC
is
acEvated
by
citrate

•  Remember
citrate
is
made
from
acetyl-‐CoAmito

•  Citrate
signals
excess
energy
to
be
converted
to

fat

–  When
[acetyl-‐CoA]mito
↑,
converted
to
citrate…citrate

exported
to
cytosol

Importance
of
Citrate
to
Regula=on
of

Fa'y
Acid
Synthesis

•  In
animals,
citrate
s<mulates
fa=y
acid

synthesis!

–  Precursor
for
acetyl-‐CoA

• Sent
to
cytosol
and
cleaved
to
become
AcCoA
when

AcCoA
and
ATP
↑
(energy
excess)

–  Allosteric
ac<vator
of
ACC

–  Inhibitor
of
PFK-‐1

• Reduces
glycolysis

ACC
is
also
regulated
by
covalent

modiﬁca=on

•  Inhibited
when
energy
is
needed

•  Glucagon
and
epinephrine:

–  reduce
sensi<vity
of
citrate
ac<va<on

–  lead
to
phosphoryla<on
and
inac<va<on
of

ACC
via
PKA

• ACC
is
ac<ve
as
dephosphorylated
monomers

• When
phosphorylated,
ACC
polymerizes
into
long

inac<ve
ﬁlaments

• Dephosphoryla<on
reverses
the
polymeriza<on

Regula=on
of
Fa'y
Acid
Synthesis
in

Vertebrates

Addi=onal
Modes
of
Regula=on
in
Fa'y

Acid
Synthesis

•  Changes
in
gene
expression

–  Example:

Fa=y
acids
(and
eicosanoids)
bind
to

transcrip<on
factors
called
Peroxisome

Proliferator-‐Ac=vated
Receptors
(PPARs)
à

inducing
gene
expression
of
some
genes

•  Reciprocal
regula<on

–  Malonyl-‐CoA
inhibits
fa=y
acid
import
into
mito

• One
of
many
ways
to
ensure
that
fat
synthesis
and

oxida<on
don’t
occur
simultaneously

Palmitate
can
be
lengthened
to

longer-‐chain
fa'y
acids

•  Elonga<on
systems
in
the
endoplasmic

re<culum
and
mitochondria
create
longer

fa=y
acids

•  As
in
palmitate
synthesis,
each
step
adds

units
of
2
C

•  Stearate
(18:0)
is
the
most
common

product

Palmitate
and
stearate
can
be

desaturated

•  Palmitate(16:0)àpalmitoleate(16:1;
Δ9)

•  Stearate
(18:0)àoleate
(18:1;
Δ9)

–  Catalyzed
by
fa=y
acyl-‐CoA
desaturase
in

animals

• Also
known
as
the
fa=y
acid
desaturases

• Requires
NADPH;
enzyme
uses
cytochrome
b5
and

cytochrome
b5
reductase

Note
that
this
is
a
Δ9-‐desaturase!

It
reduces
the
bond
between
C-‐9
and
C-‐10.

Vertebrate
fa'y
acyl
desaturase
is
a
non-‐
heme,
iron-‐containing,
mixed
func=on

oxidase

•  O2
accepts
four
electrons
from
two
substrates

•  Two
electrons
come
from
saturated
fa=y
acid

•  Two
electrons
come
from
ferrous
state
of
cytochrome

b5

Desatura=on
of
a
Fa'y
Acid
by

Fa'y
Acyl-‐CoA
Desaturase

Plants
can
desaturate
posi=ons

beyond
C-‐9

•  Humans
have
Δ4,
Δ5,
Δ6,
and
Δ9
desaturases
but
cannot

desaturate
beyond
Δ9

•  Plants
can
produce:

–  linoleate
18:2(Δ9,12)

–  α-‐linolenate
18:3
(Δ9,12,15)

•  These
fa=y
acids
are
“essen=al”
to
humans

–  Polyunsaturated
fa=y
acids
(PUFAs)
help
control
membrane

ﬂuidity

–  PUFAs
are
precursors
to
eicosanoids

•  Implica<ons
of
stearoyl-‐ACP
desaturase
(SCD)
on
obesity

–  SCD1-‐mutant
mice
are
resistant
to
diet-‐induced
obesity!

Oxidases,
Monooxygenases,

and

Dioxygenases

Many
enzymes
use
oxygen
as
an
e−
acceptor,
but
not
all

of
them
incorporate
oxygen
into
the
product.

• 
Oxidases
do
not
incorporate
oxygen
into
the
product

–  Oxygen
atoms
usually
end
up
in
H2O2

• 
Oxygenases
do
incorporate
oxygen
into
the
product

–  Monooxygenases
incorporate
one
of
the
oxygen
atoms

into
the
product

–  Dioxygenases
incorporate
both
oxygen
atoms
into
the

product

Monooxygenases
incorporate
one

oxygen
into
the
product

AH
+
BH2
+
O-‐O
àA-‐OH
+
B
+
H2O

• 
Product
is
ooen
hydroxylated,
so
also
called

hydroxylases
or
mixed-‐func=on
oxygenases

–  Example:

Phenylanine
hydroxylase
hydroxylates

phenylalanine
to
form
tyrosine

–  Deﬁciency
causes
phenylketonuria
(PKU)

Cytochrome
P450s
are

monooxygenases

•  Important
in
drug
metabolism

•  Hydroxylate
nonpolar
molecules

–  usually
inac<va<ng
them
and
making
them

more
H2O-‐soluble
for
excre<on

•  If
two
drugs
(or
alcohol
and
a
drug)
use
the

same
P450,
they
will
compete,
and
levels
of

the
drug
or
alcohol
will
not
be
cleared
as

quickly

–  Can
be
deadly

Dioxygenases
incorporate
two
oxygens

in
the
product

•  Usually
metalloproteins

–  Ac<ve
sites
have
Fe
or
Mn

ions

•  Rxns
ooen
involve
opening

an
aroma<c
ring

•  Example:
Tryptophan
2,3-‐
dioxygenase

Eicosanoids
are
potent
short-‐range

hormones
made
from
arachidonate

•  Eicosanoids
are
paracrine
signaling
molecules

•  They
include
prostaglandins,
leukotrienes,

thromboxanes

•  Created
from
arachidonic
acid,
20:4
(Δ5,8,11,14)

•  Arachidonate
is
incorporated
into
the

phospholipids
of
membranes

•  In
response
to
s<muli
(hormone,
etc.),

phospholipase
A2
is
ac<vated
and
a=acks
the

C-‐2
fa=y
acid,
releasing
arachidonate

Prostaglandins
are
made
by
prostaglandin
H2

synthase
(cyclooxygenase,
COX)

•  COX
(aka
PGH2
synthase)
is
a
bifunc<onal

ER
enzyme:

•  Step
1:

cyclooxygenase
ac<vity
of
PGH2

synthase
adds
2
O2
to
form
PGG2

•  Step
2:

peroxidase
ac<vity
converts

peroxide
to
alcohol,
creates
PGH2

•  PGH2
is
precursor
to
other
eicosanoids

Conversion
of

Arachidonate
to

Prostaglandins
and

Other
Eicosanoids

•  Thromboxane
synthase

present
in
thrombocytes

converts
PGH2
to

thromboxane
A2

•  Induce
the
constric<on
of

blood
vessels
and
blood

clovng

•  Low
doses
of
aspirin

reduce
the
risk
of
heart

a=acks
and
strokes
by

reducing
thromboxane

produc<on

PGH2
synthase
has
two
isoforms

•  COX-‐1
catalyzes
synthesis
of
prostaglandins

that
regulate
gastric
mucin
secreEon

•  COX-‐2
catalyzes
synthesis
of
prostaglandins

that
mediate
pain,
inﬂammaEon,
and
fever

NSAIDs
inhibit
cyclooxygenase
ac=vity

•  Aspirin
(Acetylsalicylate)
is
an
irreversible

inhibitor

–  Acetylates
a
Ser
in
the
ac<ve
site

–  Blocks
ac<ve
site
in
both
COX
isozymes

•  Ibuprofen
and
naproxen
are
compe<<ve

inhibitors

–  Resemble
substrate,
also
block
the
ac<ve
site

in
both
isozymes

–  Undesired
side
eﬀects
such
as
stomach

irritaEon,
why?

A
Few
NSAIDs
that
Inhibit
PGH2

Arachidonate
(substrate)
Advil, motrin Aleve

COX-‐2-‐speciﬁc
inhibitors
have
a

checkered
history

•  Developed
to
inhibit
prostaglandin
forma<on

without
harming
stomach

•  Includes
Vioxx,
Bextra,
and
Celebrex

•  Vioxx
and
Bextra
removed
from
market
due
to

increased
rates
of
stroke
and
heart
a=ack

–  May
disrupt
balance
between
blood-‐thinning

prostacyclin
and
blood-‐clovng
thromboxanes

Leukotriene
synthesis
also
begins

with
arachidonate

•  O2
is
added
to
arachidonate
via
lipoxygenases

•  Creates
species
that
diﬀer
in
the
posi<on
of
the
OOH
group

•  Not
inhibited
by
NSAIDs

Biosynthesis
of
Triacylglycerols

•  Synthesized
or
ingested
fa=y
acids
are
either

stored
for
energy
or
used
in
membranes

depending
on
the
needs
of
the
organism

•  Animals
and
plants
store
fat
for
fuel

–  Plants:
in
seeds,
nuts

–  Typical
70-‐kg
human
has
~15
kg
fat

• Enough
to
last
12
wks

• Compare
with
12
hrs’
worth
glycogen
in
liver
and
muscle

•  Animals
and
plants
and
bacteria
make

phospholipids
for
cell
membranes

The
precursor
for
the
backbone
of
fat
and

phospholipids
is
glycerol
3-‐phosphate

•  Both
pathways
start
by
the
forma<on
of
fa=y
acyl

esters
of
glycerol

•  The
substrates
are
fa=y
acyl-‐CoAs
and
L-‐glycerol
3-‐
phosphate

•  Most
glycerol
3-‐phosphate
comes
from

dihydroxyacetone
phosphate
(DHAP)
from
glycolysis

–  via
glycerol
3-‐phosphate
dehydrogenase

•  Some
glycerol
3-‐phosphate
made
from
glycerol

–  via
glycerol
kinase

–  Minor
pathway
in
liver
and
kidney
only

Acyl
transferases
a'ach
two
fa'y

acids
to
glycerol
3-‐phosphate

•  Phospha<dic
acid
is
the

precursor
to
TAGs
and

phospholipids

–  Made
of
glycerol
3-‐phosphate

+
2
fa=y
acids

–  Fa=y
acids
are
a=ached
by

acyl
transferases

–  Release
of
CoA

To
make
TAG,
phospha=dic
acid
is

dephosphorylated
and
acylated

•  Phospha<dic
acid
phosphatase
(lipin)

removes
the
3-‐phosphate
from
the

phospha<dic
acid

–  Yields
1,2-‐diacylglycerol

•  Third
carbon
is
then
acylated
with
a
third

fa=y
acid

–  Yields
triacylglycerol

Conversion
of

Phospha=dic
Acid
into

Triacylglycerol

Regula=on
of
Triacylglycerol

Synthesis
by
Insulin

•  Insulin
results
in
s<mula<on

of
triacylglycerol
synthesis

•  Lack
of
insulin
results
in:

–  Increased
lipolysis

–  Increased
fa=y
acid
oxida<on

• Some<mes
to
ketones,
if
citric

acid
cycle
intermediates

(oxaloacetate)
that
react
with

acetyl
CoA
are
depleted

–  Failure
to
synthesize
fa=y

acids

Regula=on
of
Fat
Metabolism
by

Glucagon
and
Epinephrine

•  Glucagon
and
epinephrine
result
in

s<mula<on
of
triacylglycerol
breakdown

(mobiliza<on
of
fa=y
acids)

–  Also
decrease
glycolysis

–  Also
increase
gluconeogenesis

Triacylglycerol
breakdown
and

re-‐synthesis
create
a
fu=le
cycle

•  Seventy-‐five
percent
of
free
fa=y
acids
(FFA)

released
by
lipolysis
are
reesterified
to
form
TAGs

rather
than
be
used
for
fuel

–  Some
recycling
occurs
in
adipose
<ssue

–  Some
FFA
from
adipose
cells
are
transported
to
liver,

remade
into
TAG,
and
re-‐deposited
in
adipose
cells

•  Although
the
distribu<on
between
these
two
paths

may
vary
(the
flux
of
FFA
into
and
out
of
the

adipose),
overall,
the
percentage
of
FFA
being

esterified
remains
at
~75%.

The
Triacylglycerol
Cycle

*
In
mammals,

TAG
molecules

are
broken

down
and

resynthesized

in
a
TAG
cycle

even
during

starva<on.

Benefits
of
this
fu=le
cycle?

•  Recycling
con<nues
even
in
starvaEon

•  Specula<on:

–  energy
reserve
for
“fight
or
flight”
crises
that

might
occur
during
fas<ng

•  The
total
#
of
FFA
in
flux
may
change
but
the

%
recycled
remains

–  unless
a
pharamacological
interven<on
happens

(i.e.,
thiazolidinedione
drugs,
type
2
DM)

What
is
the
source
of
the
glycerol
3-‐
phosphate
needed
for
fa'y
acid

reesteriﬁca=on?

•  During
lipolysis
(s<mulated
by
glucagon
or

epinephrine),
glycolysis
is
inhibited

–  So
DHAP
is
not
readily
available
to
make
glycerol
3-‐
phosphate

•  And
adipose
cells
don’t
have
glycerol
kinase
to

make
glycerol
3-‐phosphate
on-‐site

•  So
cells
make
DHAP
via
glyceroneogenesis

Glyceroneogenesis
makes
DHAP
for

glycerol
3-‐phosphate
genera=on

•  Glyceroneogenesis
contains
some
of
the

same
steps
of
gluconeogenesis

–  Converts
pyruvate
à
DHAP

–  Basically,
a
shortened
version
of

gluconeogenesis
in
the
liver
and
adipose
<ssue

•  Explains
why
adipose
cells
express
pyruvate

carboxylase
and
PEPCK
even
though
fat
cells
don’t

make
glucose

Regula=on
of
PEPCK
expression
is

=ssue-‐dependent

•  Cor<sol
and
glucagon
both
increase
PEPCK

expression
in
liver.

–  Results
in
more
TAG
synthesis,
so
more
released
to
the

blood

•  Cor<sol
and
other
glucocor<coids
decrease
PEPCK

expression
in
adipose
<ssue

–  ↓
glyceroneogenesis
in
adipose
means
less
recycling;

more
FFA
are
released
into
the
blood

–  Most
glycerol
freed
from
TAG
in
adipose
is
sent
to
liver

and
converted
to
glucose

Regula=on
of
Glyceroneogenesis
via
Glucocor=coid

Hormones

Cor=sol
and
glucagon
can
elevate
blood

sugar

1)  ↑
PEPCK
expression
in
liver
à↑
gluconeogenesis

(so
↑
[glucose])

2)  ↓
PEPCK
expression
in
adipose
<ssue
à
glycerol

freed,
sent
to
liver,
converted
to
glucose

3)  Plus,
the
FFA
associated
with
increased
ﬂux

through
TAG
cycle
à
interfere
with
glucose

uptake
in
muscle,
keep
[glucose]blood
high
à
may

lead
to
insulin
resistance
(type
2
DM)

Thiazolidinedione
drugs
target
insulin

resistance
by
increasing
glyceroneogenesis

•  Elevated
FFA
levels
seem
to
promote
insulin

resistance

•  Thiazolidinediones
upregulate
PEPCK
in

adipose
<ssue
via
PPARγ,
lead
to
↑

glyceroneogenesis,↑
resynthesis
of
TAG
in

adipose
<ssue
and
↓
release
of
FFA

•  Thus
the
drugs
promote
sensi<vity
to
insulin

Thiazolidinediones/Glitazones

Have
this
group

in
common

Avandia
(Rosiglitazone)
–
removed
from

market
due
to
associa<on
with
heart
a=ack

Pioglitazone
(Actos)

Regula=on
of
Glyceroneogenesis
via

Thiazolidinediones

Biosynthesis
of
Membrane

Phospholipds

•  Begin
with
phospha<dic

acid
or
diacylglycerol

•  A=ach
head
group
to
C-‐3

OH
group

–  C-‐3
has
OH,
head
group
has

OH

–  New
phospho-‐head
group

created
when
phosphoric

acid
condenses
with
these

two
alcohols

–  Eliminates
two
H2O

Further
Details
on
A'aching
the

Head
Group

•  Either
one
of
the

alcohols
is
ac<vated

by
a=aching
to
CDP

(cy<dine
diphosphate)

•  The
free
(not
bound
to

CDP)
alcohol
then

does
nucleophilic

a=ack
on
the
CDP-‐
ac<vated
phosphate

•  Releases
CMP
and
a

glycerophospholipid
E. coli: CDP-DAG
Eukaryotes: both

Synthesis
of
Phospha=dylethanolamine
and

Phospha=dylcholine
in
Yeast

•  Phospha<dylserine
is
decarboxylated
to

phospha7dylethanolamine

–  phospha<dylserine
decarboxylase

•  Phospha<dylethanolamine
acted
on
by

S-‐adenosylmethionine
(methyl
group

donor),
adds
three
methyl
groups
to

amino
group
à
phopsha7dylcholine

(lecithin)

–  Catalyzed
by
methyltransferase

Phospholipid
Synthesis
in
Mammals

•  Phospha7dylserine
isn’t
synthesized
from
CDP-‐
diacylglycerol
as
it
is 
in
yeast
and
 
bacteria

•  Made
“backwards”  
from
PE
or
PC
via

head
group
exchange

rxns

–  Catalyzed
by
speciﬁc
synthases

–  Pathway
“salvages”
the
choline

Sphingolipids
are
made
in
four
steps

1)  Synthesis
of
sphinganine
from
palmitoyl-‐CoA

and
serine

2)  A=achment
of
fa'y
acid
via
amide
linkage

3)  Desatura=on
of
N-‐acylsphinganine

(dihydroceramide)

• Yields
N-‐acylsphingosine
(ceramide)

4)

A=achment
of
head
group

• Can
yield
a
cerebroside
or
ganglioside

ER
Golgi

Phospholipids
must
be
transported

from
the
ER
to
membranes

•  Phospholipids
are:

–  synthesized
in
the
smooth
ER

–  transported
to
Golgi
complex
for
addi<onal
synthesis

•  Must
be
inserted
into
specific
membranes
in
specific

propor7ons
but
can’t
diffuse
because
they
are

nonpolar

•  So
transported
in
membrane
vesicles
that
fuse
with

target
membrane

•  Details
of
the
process
are
not
well-‐understood

Four
Steps
of
Cholesterol
Synthesis

1)  Three
acetates
condense
to

form
5-‐C
mevalonate

2)  Mevalonate
converts
to

phosphorylated
5-‐C
isoprene

3)  Six
isoprenes
polymerize
to

form
the
30-‐C
linear
squalene

4)  Squalene
cyclizes
to
form
the

four
rings
that
are
modiﬁed
to

produce
cholesterol

Step
1:

Forma=on
of

Mevalonate
from
Acetyl-‐CoA

•  2
Acetyl-‐CoAs
àAcetoacetyl-‐CoA

–  Catalyzed
by
acetyl-‐CoA
acyl
transferase

(thiolase)

+
Acetoacetyl-‐CoA
à
β-‐
hydroxyl-‐β-‐methylglutaryl-‐CoA

(HMG-‐CoA)

–  Catalyzed
by
HMG-‐CoA
synthase

•  NOT
the
mitochondrial
HMG-‐CoA

synthase
used
in
ketone
body
forma<on

•  HMG-‐CoA
+
2
NADPH
àmevalonate

–  Catalyzed
by
HMG-‐CoA
reductase

–  Rate-‐limi7ng
step
and
point
of

regula7on!

–  HMG-‐CoA
reductase
is
a
target
for
some

cardiovascular
drugs

Sta=n
drugs
inhibit
HMG-‐CoA

reductase
to
lower
cholesterol

•  Sta<ns
resemble
HMG-‐CoA
and
mevalonate
à

compe<<ve
inhibitors
of
HMG-‐CoA
reductase

•  First
sta<n,
lovasta<n,
was
found
in
fungi

•  Lowers
serum
cholesterol
by
~20
–
40%

•  Also
reported
to
improve

circula<on,
stabilize

plaques
by
removing
chol

from
them,
reduce
vascular

inﬂamma<on

•  Most
circulaEng
chol
comes

from
internal
manufacture
rather
than
the
diet

Step
2:

Conversion
of
Mevalonate

to
Two
Ac=vated
Isoprenes

•  3
PO4
3−
transferred
stepwise
from

ATP
to
mevalonate

•  Decarboxyla<on
and
hydrolysis

creates
a
diphosphorylated
5-‐C

product
(isoprene)
with
a
double

bond

•  Isomeriza<on
to
a
second

isoprene

•  The
two
“ac<vated”
isoprene

units
are
Δ3-‐isopentyl

pyrophosphate
and
dimethylallyl

pyrophosphate

Step
3:

Six
Ac=vated
Isoprene
Units

Condense
to
Form
Squalene

•  The
two
isoprenes
join
head

-‐to-‐tail,
displacing
one
set
of

diphosphates

à
forms10-‐C
geranyl

pyrophopshate

•  Geranyl
pyrophosphate
joins
to

another
isopentenyl

pyrophosphate

à
forms
15-‐C
farnesyl

pyrophosphate

•  Two
farnesyl
pyrophosphates

join
head-‐to-‐head
to
form

phosphate-‐free
squalene

Step
4:

Conversion
of
Squalene
to

Four-‐Ring
Steroid
Nucleus

•  Squalene
monooxygenase
adds
one
oxygen
to

the
end
of
the
squalene
chain

à
forms
squalene
2,3-‐epoxide

•  Here
pathways
diverse
in
animal
cells
vs.
plant

cells

•  The
cycliza<on
product
in
animals
is
lanosterol,

which
converts
to
cholesterol

•  In
plants,
the
epoxide
cyclizes
to
other
sterols

such
as
s<gmasterol

Conversion
of

Squalene
to

Cholesterol

Fates
of
Cholesterol
Aner
Synthesis

•  In
vertebrates,
most
cholesterol

synthesized
in
the
liver,
then
exported:

-  As
bile
acids,
biliary
cholesterol
or
cholesteryl

esters

•  Other
<ssues
convert
cholesterol
into

steroid
hormones,
etc.

Bile
Acids
Assist
in
Emulsiﬁca=on
of
Fats

•  Bile
is
stored
in
the
gall
bladder,
secreted

into
small
intes<ne
aoer
fa=y
meal

•  Bile
acids
such
as
taurocholic
acid
emulsify

fats

–  Surround
droplets
of
fat,

increase
surface
area
for

a=ack
by
lipases

Cholsteryl
esters
are
more
nonpolar

than
cholesterol

•  Contain
a
fa=y
acid
esteriﬁed
to
the
oxygen

–  Comes
from
a
fa=y
acyl-‐CoA

–  Makes
the
cholesterol
more
hydrophobic,

unable
to
enter
membranes

•  Transported
in
lipoproteins
to
other
<ssues

or
stored
in
liver

Cholesterol
and
other
lipids
are

carried
on
lipoprotein
par=cles

•  Lipids
are
carried

through
plasma

on
spherical

par<cles

–  Surface
is
made

of
apolipoprotein

and
phospholipid

monolayer

–  Interior
contains
cholesterol,
TAGs,
cholesteryl
esters

Four
Major
Classes
of
Lipoprotein

Par=cles

•  Named
based
on
posi<on
of
sedimenta<on
(density)

in
centrifuge

•  Large
enough
to
see
in
electron
microscope

•  Includes:

–  Chylomicrons
(largest
and
least
dense)

–  Very
low-‐density
lipoproteins
(VLDL)

–  Low-‐density
lipoproteins
(LDL)

–  High-‐density
lipoproteins
(HDL)
–
smallest,
most
dense

Electron
Microscope
Pictures
of
Lipoproteins

Apolipoproteins
in
Lipoproteins

•  “Apo”
for
“without”…

–  So
“apolipoprotein”
refers
to
the
protein
part

of
a
lipoprotein
par<cle

•  Provide
sites
for
the
par<cle
to
bind
to
cell

surface
receptors,
ac<vate
enzymes,
etc.

•  At
least
ten
have
been
characterized
in

humans

Chylomicrons
carry
fa'y
acids
to

=ssues

•  Have
more
TAG
and
less
protein
à
hence,

least
dense.

•  Have
ApoB-‐48,
ApoE,
and
ApoC-‐II

•  ApoC-‐II
ac<vates
lipoprotein
lipase
to
allow

FFA
release
for
fuel
in
adipose
<ssue,
heart,

and
skeletal
muscle

Chylomicron
remnants
deposit

their
cholesterol
in
the
liver

•  When
chylomicrons
are
depleted
of
their

TAG,
“remnants”
go
to
liver

•  ApoE
receptors
in
liver
bind
the
remnants,

take
them
up
by
endocytosis

•  Remnants
release
their
cholesterol
in
the

liver

VLDLs
transport
endogenous
lipids

•  Cholesteryl
esters
and
TAGs
from
excess
FA

and
cholesterol
are
packed
into
very
low-‐
density
lipoproteins
(VLDL)

•  Excess
carbohydrate
in
the
diet
can
also
be

made
into
TAG
in
the
liver
and
packed
into

VLDL

•  Contain
apoB-‐100,
apoC-‐I,
apoC-‐II,
apoC-‐III,

and
apoE

VLDLs
take
TAGs
to
adipose
=ssue

and
muscle

•  Again,
ApoC-‐II
ac<vates
lipoprotein
lipase

to
release
free
fa=y
acids

•  Adipocytes
take
up
the
FFA,
reconvert

them
to
TAGs,
and
store
them
in
lipid

droplets

•  Muscle
uses
the
TAG
for
energy

VLDL
remnants
become
LDL

•  Removal
of
TAG
from
VLDL
produces
LDL

•  Because
TAG
removed,
LDL
is
enriched
in

cholesterol/chloesteryl
esters

•  ApoB-‐100
is
the
major
apolipoprotein

LDLs
carry
cholesterol
from
liver
to

muscle
and
adipose
=ssue

•  Muscle
and
adipose
<ssue
have
LDL

receptors,
recognize
apoB-‐100

à
Enable
myocytes
and
adipocytes
to
take

up
cholesterol
via
receptor-‐mediated

endocytosis

Cholesterol
Uptake
by
Receptor-‐
Mediated
Endocytosis

Familial
hypercholesterolemia
is
associated

with
LDL
receptor
muta=ons

•  Muta<ons
in
LDL
receptor
prevent
normal

uptake
of
LDL
by
liver
and
other
<ssues

•  LDL
accumulates
in
blood

•  Heterozygous
individuals
have
risk
of
heart

a=ack
greater
than
normal

•  Homozygous
individuals
have
much

increased
risk
of
heart
a=ack

HDL
carries
out
reverse
cholesterol

transport

•  HDLs
contain
a
lot
of
protein

–  Including
ApoA-‐I
and

lecithin-‐cholesterol
acyl

transferase
(LCAT)

•  Catalyzes
the
forma<on
of

cholesteryl
esters
from

lecithin
and
cholesterol

•  Enzyme
converts
chol
of

chylomicron
and

VLDL
remnants
to

cholesteryl
esters

•  HDL
picks
up
cholesterol
from
cells
and
returns

them
to
the
liver

Five
Modes
of
Regula=on
of
Cholesterol

Synthesis
and
Transport

1)  Covalent
modiﬁca<on
of
HMG-‐CoA
reductase

2)  Transcrip<onal
regula<on
of
HMG-‐CoA
gene

3)  Proteoly<c
degrada<on
of
HMG-‐CoA
reductase

4)  Ac<va<on
of
ACAT,
which
increases
esteriﬁca<on

for
storage

5)  Transcrip<onal
regula<on
of
the
LDL
receptor

Regula=on
of

Cholesterol

Metabolism

HMG-‐CoA
reductase
is
most
ac=ve
when

dephosphorylated

1)  AMP-‐dependent
protein
kinase

-‐
when
AMP
rises,
kinase
phosphorylates
the

enzyme
à
ac<vity
↓,
cholesterol
synthesis
↓

2) 
Glucagon,
epinephrine

-‐
cascades
lead
to
phosphoryla<on,
↓
ac<vity

3)  Insulin

-‐
cascades
lead
to
dephosphoryla<on,↑
ac<vity

Covalent

modiﬁca=on
provides
short-‐term
regula=on.

LOW
Energy
Level

Longer-‐term
Regula=on
of
HMG-‐CoA

Reductase
through
Transcrip=onal
Control

•  Sterol
regulatory
element-‐binding
proteins

(SREBPs)

–  When
sterol
levels
are
high,
SREBP
is
in
ER

membrane
with
other
proteins

–  When
sterol
levels
decline,
complex
is
cleaved,

moves
to
the
nucleus

–  SREBP
ac<vates
transcrip<on
of
HMG-‐CoA

reductase
and
LDL
receptor
as
well
as
other

genes
à
more
cholesterol
produced
and

imported

Regula=on
of
Cholesterol
Synthesis
by
SREBP

Regula=on
of
HMG-‐CoA
Reductase
by

Proteoly=c
Degrada=on

•  Insig
(insulin-‐induced
gene
protein)
senses

cholesterol
levels.

–  Binds
to
HMG-‐Co-‐A
reductase,

–  Triggers
ubiquina<on
of
HMG-‐CoA
reductase

–  Targets
the
enzyme
for
degrada<on
by

proteasomes

–  Also
prevents
the
synthesis
of
HMG-‐CoA

reductase
(complexing
and
inhibi<ng
SREBP)

Cardiovascular
disease
(CVD)
is

mul=-‐factorial

•  Very
high
LDL-‐cholesterol
levels
tend
to

correlate
with
atherosclerosis

–  Although
many
heart
a=ack
vic<ms
have

normal
cholesterol,
and
many
people
with
high

cholesterol
do
not
have
heart
a=acks

•  Low
HDL-‐cholesterol
levels
are
nega<vely

associated
with
heart
disease

How
Plaques
Form

•  LDL
with
partly
oxidized
fa=y
acyl
groups

s<cks
to
the
lining
of
arteries

•  A=racts
macrophage
cells
of
the
immune

system

•  These
cells
don’t
regulate
their
uptake
of

sterols,
so
they
accumulate
cholesterol
and

cholesteryl
esters

•  The
macrophages
become
foam
cells

(named
for
appearance)

How
Plaques
Form
(cont.)

•  Foam
cells
undergo
apoptosis

•  Remnants
accumulate,
along
with
scar

<ssue,
etc.

•  Can
occlude
a
blood
vessel
or
break
oﬀ
and

travel
to
another
artery

•  Occlusion
of
blood
vessels
in
the
heart

cause
heart
a=ack;
occlusion
in
the
brain

causes
stroke

Familial
Hypercholesterolemia

•  Due
to
gene<c
muta<on
in
LDL
receptor

•  Impairs
receptor-‐mediated
uptake
of
cholesterol

from
LDL

•  Cholesterol
accumulates
in
the
blood
and
in
foam

cells

•  Regula<on
mechanisms
based
on
cholesterol

sensing
inside
the
cell
don’t
work

•  Homozygous
individuals
can
experience
severe

CVD
as
youths

Reverse
cholesterol
transport
by
HDL
explains

why
HDL
is
cardioprotec=ve

•  HDL
picks
up
cholesterol
from
non-‐liver

<ssues,
including
foam
cells
at
growing

plaques

•  ABC
(ATP-‐Binding
Casse=e)
transporters

bring
cholesterol
from
inside
the
cell
to
the

plasma
membrane

•  HDL
carries
cholesterol
back
to
liver

Reverse
Cholesterol
Transport

Ques=on
7
(Take
home
exam)

Due:
NEXT
WEEK
(js=ban@birzeit.edu)

•  Please
solve
ques=ons:

1.  6
(uncouplers)

2.  17
(ATP
turnover)

3.  22
(alanine)

4.  24
(diabetes)

For
wri[en
answers,
I
prefer
to
have
them
typed
in
Word.
I

can
accept
the
assignment
in
one
ﬁle
sent
to
my
email.
For

answers
that
require
solving
mathemaEcally,
you
can
either

type
them
or
write
them
down
and
scan
them.

Fatty Acid Catabolism: A Major Energy Source

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