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Lipid Metabolism 1:
Overview of lipid transport in animals, fatty acid
oxidation, ketogenesis in liver mitochondria
Bioc 460 Spring 2008 - Lecture 35 (Miesfeld)
Prime rib contains large
amounts of saturated fats in
the form of triacylglycerols
Adipose tissue is the primary
triacylglycerol storage depot in
animals, fats are an excellent
form of redox energy
Stored fat comes from the
conversion of carbohydrates
into fatty acids in the liver
Key Concepts in Lipid Metabolism
•
Stored lipids is the primary source of energy in most organisms. Lipids, such as
triacylglycerols, are much more reduced than carbohydrates and are
hydrophobic, which makes them ideal storage forms of high energy compounds.
•
The three sources of triacylglycerols in animals are dietary lipids, stored
triacylglycerols in adipose tissue, and the conversion of carbon from either
carbohydrate or protein into fatty acids in the liver.
 -oxidation is the mitochondrial process by which fatty acids are oxidized to yield
NADH, FADH2, and acetyl-CoA. These metabolites are oxidized by the citrate
cycle and electron transport system to yield large amounts of ATP.
•
Ketogenesis takes place in liver mitochondria when acetyl-CoA levels are high
and oxaloacetate levels are low. Acetoacetate and D--hydroxybutyrate are
exported are exported and converted back into acetyl-CoA by peripheral tissues.
Overview of Lipid Transport in Animals
There are three basic sources of
fatty acids in animals that can be
used for energy conversion
processes:
1) fatty acids present in
triacylglycerols obtained from
the diet,
2) fatty acids stored as
triacylglycerols in adipose tissue
that are released by hydrolysis
following hormone stimulation
(glucagon or epinephrine
signaling)
3) fatty acids synthesized in the
liver from excess carbohydrates
and exported as triacylglycerols.
Fat is stored in fat cells
(adipocytes). Obesity,
especially childhood obesity,
can be due to both more fat
storage per cell, and to a
larger number of adipocytes.
In contrast, in normal
healthy adults, the onset of
old age and reduced
metabolic rates leads to
weight gain resulting
primarily from storing more
fat per cell (although adults
can also add more fat cells if
they become obese).
Review of lipid structures:
Fatty acids are stored as triacylglycerols
Glycerol
Fatty acid #2
Fatty acid #1
Fatty acid #3
Glycerol esterification of fatty acids
protects cell membranes from the
amphipathic nature of fatty acids. Soap
is made out of fatty acids and works well
to remove oils from hands and clothes by
forming micelles that trap the lipids in a
water soluble particle.
Lipid metabolism is critical to
animals who depend on
lipids as a major energy
source. Plants only use
seeds as a major lipid
storage depot. Conversion
of carbohydrates to fatty
acids is thought to be a
major contributing factor to
obesity and diabetes in
developed countries over the
last 30 years. The primary
source of these
carbohydrates are soft
drinks, and processed foods
(snack foods) that have
been prepared with refined
sugar and flour. Another
dietary demon contributing
to obesity has been
transesterified fats.
Pathway Questions
1. What purpose does fatty acid metabolism serve in animals?
– Fatty acid oxidation in mitochondria is responsible for providing
energy to cells when glucose levels are low. Triacylglycerols
stored in adipose tissue of most humans can supply energy to
the body for ~3 months during starvation.
– Fatty acid synthesis reactions in the cytosol of liver and adipose
cells convert excess acetyl CoA that builds up in the
mitochondrial matrix when glucose levels are high into fatty
acids that can be stored or exported as triacylglycerols.
Pathway Questions
2. What are the net reactions of fatty acid degradation and
synthesis for the C16 fatty acid palmitate?
Fatty acid oxidation:
Palmitate + 7 NAD+ + 7 FAD + 8 CoA + 7 H2O + ATP →
8 acetyl CoA + 7 NADH + 7 FADH2 + AMP + 2 Pi + 7 H+
Fatty acid synthesis:
8 Acetyl CoA + 7 ATP + 14 NADPH + 14 H+ →
Palmitate + 8 CoA + 7 ADP + 7 Pi + 14 NADP+ + 6 H2O
Pathway Questions
3. What are the key enzymes in fatty acid metabolism?
Fatty acyl CoA synthetase – enzyme catalyzing the "priming" reaction in
fatty acid metabolism which converts free fatty acids in the cytosol into
fatty acyl-CoA using the energy available from ATP and PPi hydrolysis.
Carnitine acyltransferase I - catalyzes the commitment step in fatty acid
oxidation which links fatty acyl-CoA molecules to the hydroxyl group of
carnitine. The activity of carnitine acyltransferase I is inhibited by malonylCoA, the product of the acetyl-CoA carboxylase reaction, which signals
that glucose levels are high and fatty acid synthesis is favored.
Pathway Questions
3. What are the key enzymes in fatty acid metabolism?
Acetyl CoA carboxylase - catalyzes the commitment step in fatty acid
synthesis using a biotin-mediated reaction mechanism that carboxylates
acetyl-CoA to form the C3 compound malonyl-CoA. The activity of acetyl
CoA carboxylase is regulated by both reversible phosphorylation (the
active conformation is dephosphorylated) and allosteric mechanisms
(citrate binding stimulates activity, palmitoyl-CoA inhibits activity).
Fatty acid synthase - this large multi-functional enzyme is responsible for
catalyzing a series of reactions that sequentially adds C2 units to a
growing fatty acid chain covalently attached to the enzyme complex. The
mechanism involves the linking malonyl-CoA to an acyl carrier protein,
followed by a decarboxylation and condensation reaction that extends the
hydrocarbon chain.
Pathway Questions
4. What are examples of fatty acid metabolism in real life?
A variety of foods are prominently advertised as "non-fat," even though they
can contain a high calorie count coming from carbohydrates. Eating too
much of these high calorie non-fat foods (e.g., non-fat bagels) activates the
fatty acid synthesis pathway resulting in the conversion of acetyl-CoA to
fatty acids, which are stored as triacylglycerols.
Transport and storage of fatty acids and triacylglycerols
Much of the triacylglycerol stored in adipose tissue originates from
dietary lipids. Fats that enter the small intestine from the stomach
are insoluble and must be emulsified by bile acids such as
glycocholate which are secreted by the bile duct and function as
detergents to promote the formation of micelles.
Lipases are water soluble enzymes in the small intestine that
hydrolyze the acyl ester bonds in triacylglycerols to liberate free fatty
acids which then pass through the membrane on the lumenal side of
intestinal epithelial cells. Pancreatic lipase cleaves the ester bond at
the C-1 and C-3 carbons to release two free fatty acids and
monoacylglyclerol.
Transport and storage of fatty acids and triacylglycerols
Transport and storage of fatty acids and triacylglycerols
Chylomicrons transport the triacylglycerols to adipose tissue for
storage, and to muscle cells for energy conversion processes.
Apolipoprotein C-II on the surface of chylomicrons binds to and
activates lipoprotein lipase on endothelial cells which leads to the
release of fatty acids and glycerol.
Fatty acids diffuse into the endothelial cells and then enter nearby
adipose and muscle cells where they are stored or used for energy
conversion pathways. The glycerol produced by lipoprotein lipase
returns to the liver where it is converted to dihydroxyacetone
phosphate.
Transport and storage of fatty acids and triacylglycerols
Fatty acids are synthesized in the liver from carbohydrates
Dietary lipids are not the only
source of triacylglycerols
stored in adipocytes. The
liver synthesizes
triacylglycerols from fatty
acids when glucose levels
are high and the amount of
acetyl CoA produced
exceeds the energy
requirements of the cell.
Glucose provides the
necessary substrates for
triacylglycerol synthesis
(acetyl CoA for fatty acid
synthesis and glycerol) using
reactions in the glycolytic
pathway and the citrate cycle.
The fatty acid  oxidation pathway in mitochondria
Fatty acids must first be
activated by a two step
reaction catalyzed by
medium chain fatty acyl
CoA synthetase.
In the first step, the
carboxylate ion of the
fatty acid attacks a
phosphate in ATP to form
an acyl-adenylate
intermediate and release
pyrophosphate (PPi)
which is quickly
hydrolyzed by the
enzyme inorganic
pyrophosphatase to
form 2 Pi.
In the second step of the fatty acyl CoA synthetase
reaction, the palmitoyl-adenylate intermediate is
attacked by the thiol group of CoA to form the thioester
palmitoyl-CoA product and release AMP.
Fatty acid are transported into mitochondria by carnitine
The fatty acyl-CoA products of the fatty acyl CoA synthetase reaction
have two fates. If the energy charge of the cell is low, then they will be
imported into the mitochondrial matrix by the carnitine transport cycle
and degraded by the fatty acid oxidation reactions to yield acetyl CoA,
FADH2 and NADH. However, if the energy charge is high, and fatty acid
synthesis is favored, then mitochondrial import of fatty acyl-CoA is
inhibited and the fatty acyl-CoA molecule is used instead for
triacylglycerol or membrane lipid synthesis in the cytosol.
Carnitine acyltransferase I is located in the outer mitochondrial
membrane and replaces CoA with carnitine to form fatty acyl carnitine
which is translocated across the inner mitochondrial membrane. The
carnitine translocating protein is an antiporter that exchanges a fatty
acyl carnitine molecule for a carnitine. Once inside the mitochondrial
matrix, fatty acyl carnitine is converted back to fatty acyl CoA in a
reaction catalyzed by carnitine acyltransferase II releasing the
carnitine so that it can be shuttled back across the membrane.
Fatty acid are transported into mitochondria by carnitine
-oxidation yields
large amounts of ATP
Once the electron-rich
carbons of fatty acids are
moved into the mitochondrial
matrix, their high energy
redox potential is traded in for
a substantial payout of ATP
This energy conversion
process of fatty acid --> ATP
involves oxidation of fatty
acids by sequential
degradation of C2 units
leading to the generation
FADH2, NADH, and acetyl
CoA. The subsequent
oxidation of these reaction
products by the citrate cycle
and oxidative
phosphorylation generates
large amounts of ATP.
-oxidation reactions
The -oxidation pathway occurs at the 
carbon of the fatty acid, thereby releasing
the C-1 carboxyl carbon and  carbon as
the acetate component of acetyl CoA.
In the first of four reactions, the enzyme
acyl CoA dehydrogenase catalyzes a
dehydrogenation reaction (oxidation) that
introduces a trans C=C bond between the
 and  carbons of the fatty acyl-CoA
molecule using a mechanism that
reduces an enzyme bound FAD to form
FADH2.
Mitochondria contain three isozymes of
acyl CoA dehydrogenase which differ in
their specificity for hydrocarbon chains of
different lengths, long chain (C12 to C18),
medium chain (C4 to C14) and short chain
(C4 to C8 ) acyl CoA dehydrogenases.
-oxidation reactions
The second reaction in the  oxidation
pathway is a hydration step catalyzed by
the enzyme enoyl CoA hydratase that
adds H2O across the C=C bond to convert
trans-2-enoyl-CoA to 3-L-hydroxyacyl-CoA.
The third reaction is another
dehydrogenation (oxidation) step in which
the enzyme -hydroxyacyl-CoA
dehydrogenase removes an electron pair
from the substrate and donates it to NAD+
to form NADH.
Finally, coenzyme A is used in thiolysis
reaction catalyzed by the enzyme acyl CoA
acetyltransferase (also called thiolase)
that releases a molecule of acetyl CoA and
in the process, results in the formation of an
fatty acyl CoA product that is two carbons
shorter than the starting substrate.
-oxidation reactions
The complete oxidation of palmitoyl-CoA (C16) requires seven rounds of the 
oxidation pathway to convert one molecule of palmitoyl CoA into eight molecules
of acetyl CoA in a net reaction that can be written as:
Palmitoyl-CoA + 7 CoA + 7 FAD + 7 NAD+ + 7 H2O -->
8 acetyl CoA + 7 FADH2 + 7 NADH + 7 H+
After seven rounds
of  oxidation,
palmitoyl-CoA yields
8 acetyl CoA, 7
NADH and 7 FADH2.
The oxidation of
acetyl CoA by the
citrate cycle then
generates 24 NADH,
8 FADH2 and 8 GTP
(ATP).
-oxidation reactions
The combined reactions of the electron transport system and
oxidative phosphorylation converts these
31 NADH (31 x ~2.5 ATP) = ~77.5 ATP
15 FADH2 (15 x ~1.5 ATP) = ~22.5 ATP
For a grand total = 100 ATP
After subtracting the 2 ATP required for fatty acyl CoA
activation (AMP --> PPi)
And adding the 8 ATP obtained from eight turns of the citrate
cycle;
The total payout for the complete oxidation
of palmitate is 106 ATP
-oxidation is a chemical source of water for desert animals
Besides the payout of ATP that comes from fatty acid oxidation,
another benefit is the generation of H2O that occurs when O2 is
reduced by the final reaction in the electron transport system, as well
as, the formation of H2O in the ATP synthesis reaction of oxidative
phosphorylation as shown in the three reactions below:
2 NADH + 2 H+ + O2 --> 2 H2O
2 FADH2 + O2 --> 2 H2O
ADP + PO42- --> ATP + H2O
The water production that accompanies fatty oxidation benefits
animals that live in dry climates where liquid water is scarce, for
example, the desert kangaroo rat and Arabian camel. Large animals
that hibernate over the winter, like the Alaskan brown bear, also take
advantage of fatty acid oxidation in order to replace H2O that is lost
by respiration.
Ketogenesis
Acetyl-CoA derived from fatty acid oxidation enters
the Citrate Cycle only if carbohydrate metabolism
is properly balanced.
When fatty acid oxidation produces more acetylCoA than can be combined with OAA to form
citrate, then the "extra" acetyl-CoA is converted
to acetoacetyl-CoA and ketone bodies, including
acetone. Ketogenesis (synthesis of ketone bodies)
takes place primarily in the liver.
Ketogenesis
Three mitochondrial reactions are required
to convert two acetyl CoA molecules into
acetoacetate which is then reduced to
form D--hydroxybutyrate. Acyl-CoA
acetyltransferase (thiolase) is the same
enzyme that releases one molecule of
acetyl CoA in reaction 4 of the  oxidation
pathway, however in this case, the
reaction is driven toward condensation by
the high concentration of acetyl CoA in the
mitochondria under ketogenic conditions.
In the next step, the enzyme HMG-CoA
synthase fadds another acetyl CoA group
to form the intermediate -hydroxy-methylglutaryl-CoA, abbreviated as HMGCoA, and then the enzyme HMG-CoA
lyase removes one of the original acetyl
CoA groups to yield acetoacetate.
Ketones are an energy
source for tissues
Acetoacetate and D-hydroxybutyrate are
exported from the liver and
used by other tissues such
as skeletal and heart
muscle to generate acetyl
CoA for energy
conversion reactions.
Even the brain which
prefers glucose as an
energy source, can adapt to
using ketone bodies as
chemical energy during
times of extreme starvation.
Ketogenesis occurs when glycogen
stores are depleted such as during
fasting and in undiagnosed diabetics
Diabetes is a metabolic form of
carbohydrate "starvation," and
characterized by elevated
concentrations of acetoacetate and D-hydroxybutyrate in the blood and
urine. Diabetics can have high levels
of acetone in their blood which can be
detected on their breath as a fruity
odor. Acetone is a spontaneous
breakdown product of acetoacetate
(decarboxylation), or is formed by
enzymatic cleavage of acetoacetate by
the enzyme acetoacetate
decarboxylase