Metabolism of Fats

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Metabolism of Fats
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Learning Objectives
Describe saturated and unsaturated fatty acids
note the naturally occuring cis isomer and the
absence of conjugated double bonds in unsaturated
fatty acids. Describe with structures the effect
of cis double bonds on the three-dimensional
configuration of fatty acids, and comment on the
effect this has in membranes.
Describe the physical properties of fatty
acids. Draw the structure of a triacylglycerol.
Describe the digestion, mobilization, transport,
and storage of fats.
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Learning Objectives
Describe the enzymatic action of
lipases. Describe the carnitine-mediated transfer
of fatty acids into the mitochondrial
matrix. Identify carnitine acyl transferase I as
the enzyme inhibited by malonyl-CoA. Identify the
rate limiting step in the utilization of fatty
acids as fuels. Identify the products of the
b-oxidation of fatty acids. Identify the function
of enoyl CoA isomerase.
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Fatty acids are carboxylic acids with hydrocarbon
chains ranging from 4 to 36 carbons long (C6 to
C36). In some fatty acids, this chain is
saturated (no double bonds). In others, the chain
contains one or more double bonds (unsaturated).
Oleic acid 181 D9
Stearic acid 180
Saturated fatty acids adopt an extended
conformation. The cis double bond in oleic acid
does not permit rotation and introduces a rigid
bend in the hydrocarbon tail. All other bonds
are free to rotate.
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The packing of fatty acids into stable aggregates
Fully saturated fatty acids in the extended form
pack into nearly crystalline arrays, stabilized
by many hydrophobic interactions. The presence
of one or more cis double bonds interferes with
this tight packing and results in less stable
aggregates.
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The double bonds of polyunsaturated fatty acids
are almost never conjugated (alternating single
and double bonds), but are separated by a
methylene group (red)
CH CH CH2 CH CH
In nearly all naturally occurring fatty acids,
the double bonds are in the cis configuration
C C
H
H
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Physical properties of fatty acids
The physical properties of free fatty acids
(unesterified fatty acids having a free carboxyl
group) are largely determined by the length and
degree of unsaturation of the hydrocarbon chain.
Fatty acids are poorly soluble in water. Any
free fatty acids in the blood are non-covalently
bound to serum albumin. Most fatty acids in the
blood are not free, but esterified to glycerol or
glycerol derivatives. Lacking the charged
carboxylate group, esterified fatty acids are
even less soluble in aqueous solutions.
Glycerol
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Triacylglycerols are fatty acid esters of glycerol
When glycerol has two different fatty acids at
C-1 and C-3, C-2 is a chiral center.
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In most eukaryotic cells, triacylglycerols form a
separate phase of microscopic, oily droplets in
the aqueous cytosol, serving as depots of
metabolic fuel.
In vertebrates, specialized cells called
adipocytes (fat cells) store large amounts of
triacylglycerols as fat droplets that nearly fill
the cell.
Cross section of four guinea pig adipocytes
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Food fats
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The major component of bees wax
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Phospholipids
The backbone of phospholipids L-glycerol-3-phosph
ate (or D-glycerol-1-phosphate)
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Glycerolphospholipids
Membrane lipids in which two fatty acids are
attached in ester-linkage to the first and second
carbon of glycerol the third carbon of glycerol
has a highly polar or charged group attached
through a phosphodiester linkage.
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The distribution of fatty acids in
glycerolphospholipids is specific for different
organisms, and specific for different tissues of
the same organism. The biological significance
of the variation in fatty acids and head groups
is not yet understood.
In general, glycerolphospholipids contain a C10
or C18 saturated fatty acid at C-1, and a C18 to
C20 unsaturated fatty acid at C-2.
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Phospholipids with ether-linked fatty acids
ether-linked alkene
plasmalogen
About half of the phospholipids in vertebrate
heart tissue are plasmalogens.
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ether-linked alkane
platelet-activating factor
This molecular signal is released from leukocytes
(basophils), and stimulates platelet aggregation
and the release of serotonin (a vasoconstrictor)
from platelets. It also plays an important role
in inflammation and the allergic response.
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Sphingolipids
These lipids have a polar head group and two
non-polar tails but contain no glycerol.
Sphingolipids are composed of one molecule of the
long-chain amino alcohol sphingosine, one
molecule of a long-chain fatty acid, and a polar
head group that is joined by a glycosidic or
phosphodiester linkage.
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Digestion, mobilization, and transport of fats
Vertebrates (1) obtain fats in the diet (2)
mobilize fats stored in adipose tissue
(adipocytes or fat cells) (3) convert excess
dietary carbohydrates to fats in the liver for
export via the blood to other tissues (lipid
biosynthesis)
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Fats ingested in diet
Fatty acids are oxidized as fuel or re-esterified
for storage
Fatty acids enter cells
Lipoprotein lipase releases fatty acids and
glycerol
Bile salts emulsify fats in the small intestine
Chylomicrons move through lymphatic system and
bloodstream
Intestinal lipases degrade triacylglycerols to
free fatty acids
Fatty acids are taken up by intestinal mucosa and
Triacylglycerols, cholesterol, and
apolipoproteins are incorporated into chylomicrons
converted to triacylglycerols
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Before ingested triacylglycerols can be absorbed
through the intestinal wall, they must be
converted from insoluble macroscopic fat
particles to finely dispersed microscopic
micelles. Bile salts (such as tauocholic acid)
are synthesized in the liver from cholesterol,
stored in the gallbladder, and released into the
small intestine after a fatty meal. Bile salts
act as biological detergents, converting dietary
fats into mixed micelles of bile salts and
triacylglycerols. This allows intestinal,
water-soluble lipases to access and degrade the
triacylglycerols into free fatty acids and
glycerol.
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CH2 CH CH2
CH2 CH CH2
OH
OH
OH
glycerol
O
O
O
lipase
free fatty acids
triacylglycerol
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The free fatty acids and glycerol diffuse into
the epithelial cells lining the intestinal
surface (the intestinal mucosa) where they are
reconverted into triacylglycerols and packaged
with dietary cholesterol and lipoproteins into
lipoprotein aggregates called chylomicrons.
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The surface of the chylomicron is a layer of
phospholipids, with head groups facing the
aqueous phase. Triacylglycerols in the interior
make up more than 80 of the mass.
Apolipoproteins that protrude from the surface
act as signals in the uptake and metabolism of
chylomicron contents.
Molecular structure of a chylomicron
Apolipoproteins
Apolipoproteins are lipid-binding proteins in the
blood responsible for the transport of
triacylglycerols, phospholipids, cholesterol, and
cholesterol esters between organs. Apo
designates the protein in its lipid-free form.
Phospholipids
Cholesterol
Triacylglycerol and cholesterol esters
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Adipocyte
Hormone bound to cell membrane receptor
(adipose tissue)
cAMP
Adenylyl cyclase
ATP
triacylglycerol
fatty acids
lipase
Mobilization of triacylglycerols stored in
adipose tissue
glycerol
Serum albumin
Myocyte
Blood stream
(muscle tissue)
Production of ATP via b-oxidation of fatty acids,
the TCA cycle, and the electron transport chain
fatty acid transporter
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When blood glucose levels drop, the hormones
epinephrine and glucagon are secreted into the
blood stream. They bind to receptors on the
plasma membrane of adipose cells and activate the
membrane-bound enzyme adenylyl cyclase. Adenylyl
cyclase converts ATP to the second messenger
molecule cyclic AMP (cAMP). cAMP binds to a
cAMP-dependent protein kinase (this enzyme is
inactive unless cAMP is bound). The kinase
phosphorylates a hormone-sensitive
triacylglycerol lipase thereby activating it. The
now active lipase catalyzes the hydrolysis of
ester linkages in triacylglycerols, releasing
free fatty acids which pass into the blood and
bind to serum albumin for transport to skeletal
muscle, heart, and renal cortex. Here they are
utilized for energy via b-oxidation and
subsequent processing through the TCA cycle and
the electron transport chain.
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Fatty acids are activated and transported into
mitochondria
The enzymes of fatty acid oxidation are located
in the mitochondrial matrix. The free fatty
acids that enter the cytosol from the blood
cannot pass directly through the mitochondrial
membranes, but must first undergo a series of
three enzymatic reactions.
(1) acyl-CoA synthetase forms a thioester
linkage from CoA-SH to the carboxyl group of a
free fatty acid
fatty acid CoA ATP fatty
acyl-CoA AMP PPi
R is the hydrocarbon chain of the fatty acid
different acyl-CoA synthetase isozymes have
specificities for short, intermediate, and long
carbon chains.
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Outer Membrane
Biochemical anatomy of a mitochondria
Freely permeable to small molecules and ions
Matrix
Inner Membrane
Contains
Impermeable to most small molecules and ions,
including H
Pyruvate dehydrogenase complex
Contains
Respiratory electron carriers (Complexes I-IV)
(oxidative phosphorylation)
TCA cycle enzymes
Amino acid oxidation enzymes
ATP synthase (FoF1)
Fatty acid b-oxidation enzymes
ADP-ATP translocases
Other membrane transporters
DNA, ribosomes
Mg2, Ca2, K
Porin channels
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Fatty acyl-CoAs are formed in the outer
mitrochondrial membrane, and they do not cross
the inner mitochondrial membrane intact.
CH3
CH3 N CH2 CH CH2 COO-
CH3
Carnitine
OH
(2) The fatty acyl group is transiently attached
to the hydroxyl group of carnitine by the enzyme
carnitine acyltransferase I on the outer face of
the inner membrane.
Carnitine acyltransferase I is inhibited by
malonyl-CoA, the first intermediate in fatty acid
biosynthesis. This inhibition prevents the
simultaneous synthesis and degradation of fatty
acids.
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The fatty acyl-carnitine ester enters the matrix
by facilitated diffusion through the
acyl-carnitine/carnitine transporter of the inner
mitochondrial membrane.
(3) In the matrix, carnitine acyltransferase II
transfers the fatty acid to mitochondrial CoA-SH
and regenerates carnitine which passes through
the transporter to the intermembrane space.
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The carnitine-mediated entry process is the
rate-limiting step for oxidation of fatty acids
in the mitochondria. Once inside the
mitochondria, the fatty acyl-CoA is quickly acted
upon by a set of enzymes in the matrix.
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Stages of fatty acid oxidation in the
mitochondrial matrix
(1) b-oxidation a long chain fatty acid is
oxidized to yield acetyl residues in the form of
acetyl-CoA.
(2) TCA cycle the acetyl groups are oxidized to
CO2.
(3) Electrons generated in the first two stages
are transferred to O2 in the mitochondrial
electron transport chain providing the energy for
ATP synthesis.
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b-oxidation the repetitive four-step process by
which fatty acids are converted into acetyl-CoA.
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b-Oxidation of fatty acids
In each pass through this sequence of reactions,
one acetyl residue (2 carbons) is removed in the
form of acetyl-CoA from the carboxyl end of the
fatty acid chain.
In addition, one molecule of FADH2 and one
molecule of NADH is produced. (These carry the
electrons from the oxidation process to the
electron transfer chain).
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For palmitic acid, a 16 carbon, saturated fatty
acid, the first pass through the sequence of
reactions yields a 2-carbon acetyl residue (as
acetyl-CoA) and a 14-carbon fatty acyl-CoA
(myristic acid).
Six more passes through the sequence of reactions
yields seven more acetyl-CoAs.
A total of 8 acetyl-CoAs are produced from
palmitic acid.
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The overall equation for the complete oxidation
of palmitoyl-CoA (including TCA cycle oxidation
and oxidative phosphorylation via the electron
transport chain)
Palmitoyl-CoA 23 O2 108 Pi 108 ADP
CoA 108 ATP 16 CO2 23 H2O
notice the water.
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Fat bears carry out b-oxidation in their sleep
Hibernating grizzly bears remain in a continuous
state of dormancy for periods as long as seven
months. These animals use body fat as their sole
fuel. Fat oxidation yields sufficient energy for
maintenance of body temperature, synthesis of
amino acids and proteins, and other
energy-requiring
activities. Fat oxidation releases large
quantities of water which replenishes water lost
during breathing. The glycerol released by
degradation of triacylglycerols is converted into
blood glucose by gluconeogenesis. Urea formed
during breakdown of amino acids is reabsorbed by
the kidneys and recycled.
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In the first reaction of b-oxidation,
dehydrogenation of fatty acyl-CoA produces a
double bond between the a and b carbons. The
configuration of this double bond is trans. The
double bonds in naturally occurring, unsaturated
fatty acids are cis. The oxidation of unsaturated
fatty acids requires additional steps.
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Oxidation of the monounsaturated fatty acid,
oleic acid, requires an additional enzyme,
enoyl-CoA isomerase, to reposition the double
bond, converting the cis isomer to a trans
isomer, a normal intermediate in b-oxidation.
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Oxidation of fatty acids with an odd-number of
carbons
Succinyl-CoA feeds into the TCA cycle
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Methylmalonyl-CoA mutase
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Coenzyme B12
The cofactor for methylmalonyl-CoA mutase.
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Pernicious anemia results from a failure to
efficiently absorb vitamin B12 from the intestine
where it is synthesized by bacteria or obtained
from digestion of meat. Individuals with this
disease do not produce sufficient amounts of
intrinsic factor, a glycoprotein essential to
vitamin B12 absorption. The pathology in
pernicious anemia includes reduced production of
erythrocytes, reduced levels of hemoglobin, and
severe, progressive impairment of the central
nervous system. Administration of large doses of
vitamin B12 alleviates these symptoms in at least
some cases.
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Peroxisomes also carry out b-oxidation
Although the major site for lipid degradation is
the mitochondria, peroxisomes contain a set of
enzymes capable of oxidizing fatty
acids. Peroxisomes are membrane-enclosed cellular
compartments in which fatty acid oxidation
produces H2O2 (hydrogen peroxide). This strong
and potentially damaging oxidant is immediately
cleaved to H20 and O2 by catalase.
2 H2O2 2 H20 O2
catalase
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Mitochondria
Peroxisome
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Regulation
Oxidation of fatty acids consumes a precious
fuel, and it is regulated to occur only when the
need for energy requires it. In the liver, fatty
acyl-CoA formed in the cytosol has two major
pathways open to it (1) b-oxidation by enzymes
in the mitochondria (2) conversion into
triacylglycerols and phospholipids by enzymes in
the cytosol
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The pathway taken depends upon the rate of
transfer of fatty acyl-CoAs into the
mitochondria. Once fatty acyl-CoAs have entered
the mitochondria, they are committed to oxidation
to acetyl CoA. The three-step process by which
fatty acyl-CoAs are transported into the
mitochondria is rate limiting for fatty acid
oxidation and is an important point of regulation.
Carnitine acyltransferase I is inhibited by
malonyl-CoA, the first intermediate in fatty acid
biosynthesis. This inhibition prevents the
simultaneous synthesis and degradation of fatty
acids.
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Malonyl-CoA increases in concentration whenever
the animal is well supplied with
carbohydrate. Excess glucose that is not oxidized
or stored as glycogen is converted in the cytosol
into fatty acids for storage as
triacylglycerols. The inhibition of carnitine
acyltransferase I by malonyl-CoA assures that the
oxidation of fatty acids is inhibited whenever
the liver is amply supplied with glucose as fuel
and is actively making triacylglycerols from
excess fatty acids.
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Ketone Bodies
In the liver, acetyl CoA (produced by oxidation
of fatty acids) can enter the TCA cycle or be
converted to the ketone bodiesacetone,
acetoacetate, and b-hydroxybutyrate.
acetone
acetoacetate
The term bodies is a historical artifact the
term is occasionally applied to insoluble
particles. But acetone, acetoacetate, and
b-hydroxybutyrate are quite soluble in blood and
urine.
b-hydroxybutyrate
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Ketone bodies are synthesized from acetyl CoA
2 acetyl CoA
2 HS-CoA
acetoacetate
acetone
b-hydroxybutyrate
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b-hydroxybutyrate and acetoacetate can be used as
fuels
TCA cycle
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Ketone bodies are overproduced in diabetes and
during starvation
During starvation, blood glucose levels
drop. Hormonal controls (epinephrine and
glucagon) mobilize triacylglycerols from adipose
tissue to increase the levels of blood fatty
acids. The liver shifts to gluconeogenesis to
produce glucose to maintain blood levels. This
shift in metabolism, draws off TCA cycle
intermediates (the flux through oxaloacetate is
directed toward glucose production). This diverts
acetyl CoA toward ketone body formation.
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glucose
Glycolysis
Blood
Gluconeogenesis
malonyl-CoA
fatty acids
triacylglycerols
fatty acids
b-oxidation
Storage in fat droplets
acetyl-CoA
ketone bodies
TCA Cycle
Mitochondria
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Glucose
Gluconeogenesis
Glucose-6-phosphate
Oxaloacetate is present in very low (catalytic)
concentrations.
Phosphoenolpyruvate
oxaloacetate
TCA Cycle
Pyruvate
Glucogenic amino acids
Lactate
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Untreated diabetes leads to overproduction of
ketone bodies with several associated medical
problems.
When the insulin level is insufficient,
extrahepatic tissues cannot take up glucose
efficiently from the blood (glycolysis shuts down
for lack of glucose). Under these conditions,
malonly-CoA is not formed and is not present to
inhibit carnitine acyltransferase I. Fatty acids
(mobilized from droplets within the cell)
therefore enter the mitochondria to be degraded
to acetyl-CoA (the cell needs ATP). Gluconeogenesi
s is active however since the cell needs glucose
if not for energy for other precursors. TCA
cycle intermediates have been drawn off as in
starvation.
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The accumulation of acetyl-CoA (from fatty acid
degradation) in the mitochondria accelerates the
formation of ketone bodies. These pass into the
blood where the buffering capacity is exceeded
and the pH drops (acidosis). Extreme acidosis
can lead to coma and in some cases death. A
condition called ketosis is reached when ketone
bodies in the blood and urine reach very high
levels.