TAG Metabolism
A) Overview
1) Fatty Acids (FA) are the principle source of energy
for vertebrates.
a) glycogen supplies glucose for 1 day
b) protein can metabolized to glucose for 1 week.
c) Tri acyl glycerol (TAG) can provide FA for energy
for 3 months.
2) Adipose is the primary site of TAG storage
3) In response to glucagon or adrenaline, TAG is broken
into glycerol and FA.
a) FA provide energy for gluconeogenesis
b) FA can be metabolized to ketone bodies in place of
glucose
c) glycerol is used as a substrate for the synthesis
of glucose.
B) TAG metabolism
1) Glucagon or adrenalin increases and binds to adipocytes
and PK is activated
a) Hormone Sensitive Lipase (HSL) is phosphorylated
a1) phosphorylation activates HSL
a2) HSL activation is the rate limiting step in TAG breakdown
a3) phosphorylated HSL moves from cytosol to lipid droplet
b) perilipins (membrane proteins enclosing TAG in lipid
bodies) are phosphorylated
b1) phosphorylation of perilipins disrupts the perilipin
shell
b2) TAG redistributes and becomes accesible to HSL
2) Activated HSL breaks down TAG
TAG --> FA + Glycerol
a) glycerol is released to liver because it is not used
by adipocytes directly (see 3 below)
3) FA are retained or released to blood stream based on
cellular glucose status
a) adipocytes require glucose for FA synthesis
a1) Glycerol 3 phosphate (GlOH 3P) is a key intermediate
for FA synthesis
a2) adipocytes cannot directly phosphorylate glycerol
to GlOH 3P
a3) adipocytes can convert dihydroxy acetone (DHAP) to
GlOH 3P
a4) DHAP is formed in glycolysis so glucose level inside
adipocytes is indirectly sensed by the level of GlOH 3P
c) if GlOH 3P is high
c1) glucose is high
c2) FA are reesterified to TAG
d) if GlOH 3P is low
d1) glucose is low
d2) FA arereleased to blood stream
Fatty Acid Oxidation
A) Overview
1) beta oxidation in occurs in mitochondrial matrix and
the peroxisome
2) FA oxidation generates energy
a) FADH2
b) NADH
c) acetyl CoA
c1) citric acid cycle
c2) ketone body synthesis
3) the Carnitine Cycle transports FA to the mitochondrial
matrix and attatches CoA to the chain
a) Long chain FA and higher (12-20 C) must be transported
to mitochondrial matrix using carnitine as a carrier
b) FA oxidation requires that the FA be linked to CoA
before than can be metabolized.
c) Carnitine is not required for short (1-3C) or medium
(4-12C) chain acyl-CoA intermediates entry to matrix
2) Fatty acid oxidation can occur in several ways
a) Saturated Even Chain beta oxidation
a1) 2 carbons removed each cycle as acetyl CoA
b) Saturated Odd chain beta oxidation
b1) 2 carbons removed each cycle as acetyl CoA
b2) terminal 3C chain converted to methylmalonyl CoA
which can be converted to succinyl CoA for use in gluconeogenesis
c) Unsaturated FA beta oxidation
c1) B-oxidation occurs until unsaturated bond interferes
c1) double bond is reduced and isomerized
c2) b oxidation continues
d) very long chain b-oxidation
d1) occurs in peroxisomes
d2) oxidation similar to saturated even chain beta oxidation
except O2 is reduced to H2O2 so no FADH2 produced
d3) Oxidation continues until octanoyl (8C)
d4) octanoyl enters mitochondria for normal beta oxidation
e) Branched Chain beta oxidation
e1) alpha oxidation removes terminal COOH in peroxisomes
e2) normal beta oxidation resumes
f) omega oxidation
f1) occurs in ER
f2) NADPH and O2 used to form dicarboxylic acid
B) Carnitine Cycle
1) Acyl CoA sythetase (FA thiokinase) attaches CoA to
the FA that will be oxidized
a) occurs on the outer mitochondrial membrane on the
inter mitochondrial space side
b) two step reaction
b1) FA is reacted with ATP to form an acyl adenylate
intermediate
FA + ATP --> acyl-AM + Ppi
b2) acyl adenylate intermediate reacts with high energy
SH of CoA to form acyl CoA with a high energy thioester bond
Acyl-AMP + CoA --> Acyl-CoA + AMP
b3) High energy bond required for reaction with carnitine
or any FA oxidation
c) short (1-3C) or medium (4-12C) chain acyl-CoA intermediates
do not require Carnitine carrier for entry to matrix
2) Carnitine Acyl Transferase I (CAT I)- attatches a carnitine
to long chain acyl CoA intemediates
AcylCoA + Carnitine --> Acyl-Carnitine + CoA
a) Oxidation occurs in matrix
b) long chain FA are transported to matrix by a carnitine
carrier
c) CAT I is inhibited by the key intermediate in FA synthesis:
malonyl CoA. If synthesis is on, oxidation needs to be off and visa versa.
3) Carnitine/Acyl-Carnitine Translocase: transfers Acyl-Carnitine
to matrix
a) Acyl-Carnitine enters matrix and simultaneously
b) Free carnitine is transported out of matrix
4) Carnitine acyl transferase II (CAT II): frees FA from
carnitine and adds CoA back to FA
a) on matrix side of inner mitochondrial membrane
Acyl-Carnitine + CoA --> acyl-CoA + carnitine
b) oxidation requires that CoA is complexed to FA chain
c) free carnitine is recycled back to inner mitochondrial
space for use on CAT I by Carnitine/Acyl-Carnitine Translocase (3 above)
C) Saturated Even Chain Beta Oxidation
1) 2 carbon units are removed from each acyl-CoA chain
as acetyl CoA with each cycle
a) 1 FADH2 and 1 NADH2 is produced by each cycle
b) acetyl CoA can enter TCA for metabolism (1FADH2, 3NADH,
1GTP)
c) Net ATP per Cycle:
c1) 2xFADH2 4 ATP
c2) 4xNADH 12ATP
c3) 1xGTP 1ATP
c4) Total 17 ATP
d) for palmityl (C16), 7 cycles (last 4C broken into
2xacetyl CoA so 8 cycles do not occur) occur to produce 8 acetyl CoA
C16 --> 8 acetyl CoA + 7 FADH2 + 7 NADH
d1) 130 net ATP per C16
2) There are four general reactions per cycle
a) Oxidation producing FADH2
b) Hydration
c) Oxidation producing NADH
d) Thiolyis releasing acetyl CoA
3) Enzymes catalyzing each of the general reactions are
specific for chain length and are named with the appropriate prefix.
a) long (C12-C18)
b) medium (C4-C12)
c) short (C4-C6)
4) FA oxidation is not a significant fuel for all tissues
a) Since RBCs do not have mitochondria they do not use
FA for energy
b) Brain and nervous tissue do not use FA as significant
fuel source
5) Specific Reactions
a) acyl CoA dehydrogenase (CAD)- oxidation
acyl-CoA + FAD --> enoyl-CoA + FADH2
a1) generation of FADH2
a2) double bond created between alpha and beta carbon
a3) double bond must be trans to proceed
a4) defficiency in medium chain length CAD decreases
FA oxidation, causes severe hypoglycemia and is the cause of up to 10% of SIDS
b) enoyl CoA hydratase: hydration
enoyl-CoA + H2O --> beta hydroxy-acyl CoA
b1) water added across double bond to create alcohol
on beta
c) beta hydroxy acyl CoA dehydrognase: oxidation
beta hydroxy-acyl-CoA + NAD --> beta keto-acyl-CoA + NADH
c1) NADH generated
c2) alcohol on beta carbon is converted to a ketone
d) Acyl CoA acyltransferase: thiolysis
beta keto-acyl-CoA + CoA --> Acyl-CoA (-2C) + acetyl CoA
d1) acetyl CoA released and new acyl CoA intermediate
formed
d2) final cycle (#7 for C16, 4C step), generates 2 acetyl
CoA
D) Saturated Odd Chain beta oxidation
1) identical to sturated even chain beta oxidation until
last cycle
a) last cycle (#7 for C15) generates a 3 carbon intermediate:
propionyl CoA (ProCoA)
b) ProCoA is converted to Methyl Malonyl CoA (MM CoA)
c) MM CoA forms Succinyl CoA
2) the reactions
a) propionyl CoA Carboxylase: MM CoA formed
ProCoA + HCO3 + ATP --> MM CoA + ADP
a1) ATP used to add CO2
b) methyl malononyl CoA mutase
MM CoA --> Succinyl CoA
b1) requires B12
b2) because succinyl CoA is part of the citric acid cycle,
it can be used for gluconeogenesis
b3) Net Metabolism in TCA
1 GTP +1ATP
1FADH2 2ATP
1NADH +3ATP
-1ATP (Formation) -1ATP
total 5ATP
b4) Net Metabolism of 2 acetyl CoA (even chain)
2GTP 2ATP
6NADH 18ATP
2FADH2 4ATP
24ATP
b5) 1 odd chain C less cost 19 ATP
E) Unsaturated FA beta oxidation
1) identical to beta oxidation of saturated FA until a
double bond at C4-C5 occurs
a) double bond must be eliminated to proceed
b) double bond is reduced and then isomerized
2) the reactions
a) acyl CoA dehydrogenase (CAD)- oxidation
acyl-CoA + FAD --> 2,4 dienoyl-CoA + FADH2
a1) This is identical to step 1 of saturated FA beta
oxidation above except a double bond is also between C4-C5 because the FA was
unsaturated
a2) generation of FADH2
a3) double bond created between alpha and beta carbon
b) 2,4 dienoyl reductase: reduction
2,4 dienoyl-CoA + NADPH --> cis-delta3-enoyl CoA + NADP
b1) NADPH is consumed
b3) C2-C3 double bond and C4-C5 double bond reduced to
form a cis C3 C4 double bond
c) cis delta3 enoyl isomerase: isomerize
cis delta3-enoylCoA --> trans delta2-enoyl CoA
c1) cis C3-C4 double bond is isomerized to trans C2-C3
double bond
c2) this is the identical intermediate to that generated
in step 1 (CAD) of sturated b oxidation.
d) Hydratase, Dehydrogenase, and Thiolysis cycle resumes
identical to that in saturated FA beta oxidation (above)
F) Very Long Chain FA beta oxidation
1) very long (C14-C20)
2) occurs in peroxisomes
3) similar to saturated beta oxidation except:
a) no FADH2 is produced in first step because e- are
transfered to molecular O2 to generate H2O2
b) peroxisomal beta oxidation continues until octanoyl
(8C), c) after oxidation to octanoyl, the 8C molecule is transported
to mitochondria to complete oxidation there
4) the reactions
a) acyl CoA oxidase: oxidation
acyl-CoA + O2 --> enoyl-CoA + H2O2
a1) No FADH2 produced so ATP yield is reduced by 2 for
every cycle until 8C compared to mitochondrial beta oxidation. In addition, reducing
equivalents (NADH)must be transported into the mitochondria by the glycerol 3
phosphate or malate/aspartate shuttle.
a2) e- transfered directly to molecular oxygen to form
H2O2
a3) C2-C3 double bond created
b) enoyl CoA Hydratase: hydration
b1) similar to mitochondrial beta oxidation
c) hydroxy enoyl dehydrogenase: oxidation
c1) similar to mitochondrial beta oxidation
c2) NADH produced
d) ketoacyl CoA thiolase: thiol cleavage
d1) similar to mitochondrial beta oxidation
e) cycle continues until 8C (octanoyl reached) and 8C
is transported to mitochondria to complete beta oxidation
G) Branched chain beta oxidation
1) Branched chain FA (ie phytanic acid from Chloryphyll)
can be metabolized by a slight alteration of very long chain beta oxidation.
a) "branched" means methyl groups come off of main chain
FA, and these methyl groups block beta oxidation
b) also occurs in peroxisome
c) Acyl CoA is attatched to long chain by ligase
d) alpha oxidation removes C terminal COOH
e) peroxisomal beta oxidation then occurs, so first step
is acyl CoA oxidase and no FADH2 is produced.
f) last step of each cycle releases propionyl CoA not
Acetyl CoA
2) the reactions
a) long chain FA acyl CoA ligase: attatches CoA to phytanic
acid
phytanic acid + CoA + ATP --> phytanoyl CoA + AMP + Ppi
b) SCP2: alpha oxidation remoces C terminal COOH group
b1) step 1
phytanoyl CoA + 1/2 O2 --> pristanal + formyl CoA
b2) step 2
pristanal --> pristanic acid
b3) Refsums disease: alpha oxidation step does not occur,
phytanic acid accumulates causing neurological complications.
c) pristanic acid undergoes peroxisomal beta oxidation.
Enzymes carry out similar reactions, their names are different to reflect the
substrate they work on.
c1) pristanoyl CoA oxidase: oxidation no FADH2
c2) pristenoyl CoA hydratase: hydration
c3) 3-hydroxy pristanoyl CoA dehydrogenase: oxidation
and NADH produced
c4) 3-ketopristanoyl CoA thiolase: thiol cleavage -Last
step eleases propionyl CoA not Acetyl CoA
-propionyl CoA can be converted to Succinyl CoA as shown
for odd chain FA beta oxidation.
H) Omega Oxidation
1) Occurs in ER
2) cytochrome p450 catalyzes the formation of COOH group
addition to the omega carbon using molecular oxygen and NADPH.
--See NADPH usage in hexose monophosphate shunt under
glycolysis
3) dicarboxylic acid formed
4) omega oxidation of keratins prevents H2O leakage from
skin.
Ketone Body metabolism
A) Overview
1) there are 3 ketone bodies we are concerned with
a) acetoacetate (AcAc)
b) beta hydroxy butyrate (B-HB)
c) acetone
2) Ketone bodies are made in liver mitochondria
a) Ketone are normal fuels of respiration
a1) heart and renal Cortex use AcAc in preference to
glucose
a2) brain can adapt to use AcAc during starvation: 75%
of fuel during starvation
b) liver makes, but cannot utilize ketone bodies because
it lacks ketoacid transferase
3) Ketone body usage
a) heart and renal cortex prefer AcAc: gluconeogenesis
usage in liver decreased, protein utilization spared
b) brain utilizes AcAc during starvation: gluconeogenesis
usage in liver decreased, protein utilization spared
B) Synthesis of Ketone Bodies
1) When fasting or eating a high fat low carbohydrate
diet, liver is synthesizing glucose and metabolizing FA
a) increased FA in blood stream, increases beta oxidation,
which increases acetyl CoA, NADH and ATP
b) gluconeogenesis converts lactate or alanine to pyruvate
and then converts pyruvate to oxaloacetate (OA)
c) OA can only leave mitochondria as malate or aspartate
to enter cytosol.
d) excess NADH, due to FA metabolism, shunts large levels
of OA into malate
OA + NADH --> malate + NAD
e) OA is depleted in matrix
f) TCA stops because citrate synthetase can't form citrate
without OA and because NADH is still accumulating
g) Slowed TCA while FA oxidation still making acetyl
CoA causes acetyl CoA and NADH to accumulate further
h) high concentration of acetylCoA and NADH triggers
ketone bony synthesis
2) thiolase: combines 2 acetyl CoA to form acetoacetyl
CoA
acetyl CoA + acetyl CoA --> acetoacetyl CoA + CoA
3) HMG CoA Synthetase generates HMG CoA
Acetyl CoA + acetoacetyl CoA --> HMG-CoA + CoA
a) HMG = 3 hydroxy 3 methyl Glutaryl
b) fasting induces the synthesis of HMG CoA synthetase
c) HMG CoA can lead to steroid metabolism through HMG
CoA reductase
4) HMG CoA Lyase: converts HMG CoA to acetoacetate (AcAc)
HMG CoA --> acetyl CoA + AcAc
5) AcAc has three different metabolic fates
a) AcAc enters blood stream
b) AcAc is reduced to beta hydroxybutyrate (B-HB) by
3 hydroxybutyrate dehdrogenase and enters blood
AcAc + NADH --> B-HB + NAD
b1) Overall level of NADH/NAD determines the relative
amounts of AcAc and B-HB produced (ie high NADH, low NAD, regenerate NAD by forming
B-HB)
b2) humans tend to produce more B-HB than AcAc
c) AcAc spontaneously decomposes to acetone which enters
blood stream
AcAc ---> Acetone + CO2
C) Ketone Body Oxidation
1) Occurs in cell mitochondria except for liver
2) KetoAcid transferase (succinyl CoA:AcAc CoA transferase):
forms acetoacetyl CoA from succinyl CoA
AcAc + succinyl CoA --> acetoacetyl CoA + succinate
a) ketoacid transferase is missing in liver so ketone
body oxidation does not occur in the liver
3) Acetoacetyl CoA thiolase: reforms acetyl CoA for use
in the TCA
acetoacetyl CoA + CoA --> 2 Acetyl CoA
4) beta hydroxy butyrate dehydrogenase converts B-HB to
AcAc
B-HB + NAD --> AcAc + NADH
a) AcAc is metabolized through KetoAcid transferase (#2
above)
b) the production of NADH makes the metabolism of B-HB
yield more energy than the metabolism of AcAc.