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Lecture 36
Lipid Metabolism 1
Fatty Acid Oxidation
Ketone Bodies
Key Concepts
• Overview of lipid metabolism
• Reactions of fatty acid oxidation
• Energy yield from fatty acid oxidation
• Formation of ketone bodies
Overview of Lipid Metabolism
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Carbohydrate metabolism is but one component of energy
production and storage. In fact, a much larger percentage
of the total energy reserves in animals is lipids in the form
of fat deposits.
Palmitate (16:0) is a C16 saturated fatty acid that can be
carried through the body as a protein-fatty acid complex:
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).
Fatty acids are stored as triacylglycerols (triglycerides) which are
uncharged esters of glycerol:
Why does this look familiar?
Acetyl CoA is the central intermediate in fat metabolism.
Fatty acids consist of a long
hydrocarbon chain (mostly
saturated carbons) with a
terminal carboxylate group.
Fatty acid degradation, also
called fatty acid oxidation, is
similar in many ways to fatty
acid synthesis - both the
catabolic and anabolic
pathways utilize acetyl CoA
as the activated carrier of the
two carbon product or donor.
Triglycerides obtained from food are cleaved by enzymes called lipases
in the intestine to generated freey fatty acids and monoacylglycerols. After
passing through the membrane of mucosal cells that line the intestine,
enzymes catalyze the formation of triglycerides which are then packaged
into large lipid rich particles called chylomicrons. We will look at this in
more detail in lecture 37.
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 acylCoA 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 malonyl-CoA,
the product of the acetyl-CoA carboxylase reaction, which signals that glucose
levels are high and fatty acid synthesis is favored.
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.
In other words, “non-fat” does not mean that you won’t gain fat!
Olestra is a fat substitute composed of a
sucrose molecule with several fatty acids
attached.
Why is this considered a zero calorie fat
molecule?
Why does Olestra prevent the absorption
of fat soluble vitamins?
Orlistat is an anti-obesity drug that has
been released onto the market. It binds to
and inhibits pancreatic lipase activity in the
intestine.
Why would this function as an anti-obesity
drug?
What do you think the treatment regimen
consists of, and what might be some of the
side effects?
Fatty acids stored in adopose cells are released into the blood in
response to hormone signaling by activation of cellular lipases which
cleave stored triacylglycerides. The fatty acids provide a rich source of
energy for tissues throughout the body when glycogen stores have been
depleted, especially during endurance exercise and dieting.
What two hormones signal through G protein coupled receptors and stimulate
energy mobilization, i.e., glycogen breakdown and fatty acid oxidation (lipolysis)?
What peptide hormone signals through a receptor tyrosine kinase and functions to
inhibit glycogen breakdown and lipolysis?
What is the name of this protein kinase?
Fatty acid carbon atoms are numbered starting at the carboxy terminus.
The methyl carbon atom at the distal end is called the omega (ω) carbon.
The position of double bonds can be
denoted from the carboxy end using a
delta (∆) sign followed by a superscript
number, e.g. cis- ∆9-hexadecenoate, or
from the CH3 terminal end using the
omega carbon, e.g., omega-3 (ω-3)
fatty acid.
Polyunsaturated omega-3 fatty acids are found in fish and can be
obtained from nutritional supplements.
They may play an important role in cardiovascular function and are
recommended by the American Heart Association.
There is also some recent evidence that they may improve brain
function in some patients with bipolar disorder.
Read more about this work by Dr. Andrew Stoll at McLean Hospital in
Boston. http://www.psychiatrictimes.com/p991211.html
You need to be a little careful eating too
much large fish (e.g., tuna, swordfish,
shark) because of mercury levels.
Farm trout may be a good alternative
because they are harvested young and
cultured under controlled conditions.
But there is still concern about the
growth conditions and levels of other
environmental pollutants from the water.
Fatty acids have four major physiologic roles:
1. Building blocks of membrane phospholipids.
2. Covalently attached to proteins as targeting signals.
3. Major fuel molecules in most animals.
4. FA derivatives are hormones and intracellular signals.
Fatty acids vary in chain length and degree of unsaturation
(C=C bonds).
A fully saturated fatty acid has no C=C bonds.
Triacylglycerols are very anhydrous and can store more
energy with less mass than carbohydrates (glycogen).
One gram of anhydrous fat stores SIX TIMES more energy than a gram
of hydrated glycogen.
A man weighing 155 pounds with a normal amount of stored fat, would
have to weigh 240 pounds if that same amount of energy were stored
as glycogen.
Where is the "extra" 85 pounds coming from in this example?
Why might your body weight fluctuate if you suddenly change the
relative proportion of carbohydrate, protein and fat in your diet?
Reactions of fatty acid oxidation
Stored triacylglycerols are first hydrolyzed by lipases to release glycerol
and free fatty acids.
Conformational changes in Lipase accommodate fatty acid binding
Glycerol is further metabolized to dihydroxyacetone phosphate (DHAP)
following phosphorylation and dehydrogenation:
What is the metabolic fate of the dihydroxyacetone phosphate under
conditions of low energy charge in the cell?
Fatty acids are activated by ATP through an activation process on the
outer mitochondrial membrane requiring the enzyme acyl-CoA
synthetase to form fatty acyl-CoA. This reaction is driven by the
hydrolysis of inorganic pyrophosphate.
What enzyme cleaves PPi and why would this pull the acyl-CoA
synthetase reaction to the right?
Fatty acyl-CoA derivatives are carried across the inner mitochondrial
membrane by conjugation to carnitine.
The carrier process requires carnitine acyltransferase I (outside) and
carnitine acyltransferase II (inside).
The two carnitine acyltransferase enzymes (I and II) function
on either side of the inner mitochondrial membrane
Once inside the mitochondrial matrix, fatty acids are oxidized 2 carbons at
a time, releasing acetyl-CoA, NADH and FADH2.
There are four steps in fatty acid oxidation pathway:
1) oxidation, 2) hydration, 3) oxidation, 4) thiolysis.
1
This is why it is called
Beta Oxidation.
2
3
There are four steps in fatty acid oxidation pathway:
1) oxidation, 2) hydration, 3) oxidation, 4) thiolysis.
3
4
Taken together with acyl-CoA synthetase and carnitine
acyltransferase reactions, 6 steps are actually required to
release the first acetyl-CoA from a fatty acid starting in the
cytosol.
After this initial set of reactions, each round of FA oxidation releases 2
carbons as acetyl-CoA, and produces 1 NADH and 1 FADH2. It requires
7 rounds of FA oxidation to metabolize palmitate, a C16 fatty acid. The
electron pair from FADH2 is donated directly to ubiquinone in the
electron transport system via the ETF:Q oxidoreductase complex as
described in lecture 29.
Fatty Acid Oxidation of C16 palmitate
yields 106 ATP !
Let's do the math to calculate the ATP yield
Palmitoyl-CoA + 7 FAD + 7 NAD + 7 CoA + 7 H2O →
8 acetyl-CoA + 7 FADH2 + 7 NADH + 7H+
8 acetyl-CoA = 24 NADH + 8 FADH2 + 8 ATP[GTP]
31 NADH = 31 x 2.5 ATP = 77.5 ATP
15 FADH2 = 15 x 1.5 ATP = 22.5 ATP
Grand Total:
8 ATP + 77.5 ATP + 22.5 ATP = 108 ATP
The net energy yield from palmitate oxidation is only 106 ATP.
What reaction required 2 high energy phosphate bonds (the equivalent of 2 ATP)?
Note that ATP synthesis and the oxidation of NADH
and FADH2 yields a large amount of H2O (subtracting
the investment of H2O in beta oxidation).
ADP + PO42- ---> ATP + H20
2 NADH + 2 H+ + O2 ---> 2 H20
2 FADH2 + O2 ---> 2 H20
In fact, desert animals like the kangaroo
rat, derive much of their water from fuel
metabolism. The complete oxidation of
palmitate generates 145 moles of H2O:
~10 mls of H2O per gram of palmitate.
Formation of ketone bodies
Acetyl-CoA derived from fatty acid oxidation enters the Citrate Cycle only
if carbohydrate metabolism is properly balanced.
When fatty acid oxidation produces more acetyl-CoA 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. The ketone bodies are exported to other parts of the body, such as
the heart muscle, where they are converted back to 2 acetyl-CoA units.
Why must carbohydrate metabolism be balanced for efficient use of
acetyl-CoA?
Formation of ketone bodies
FAT is the fuel of the carbohydrate furnace - if the furnace is
working at capacity, and too much fuel is piling up, then it is
converted to ketone bodies and shipped out of the cell!
What cyclic pathway functions as the "metabolic furnace" in
this analogy?
In fasting or diabetes, oxaloacetate (OAA) is consumed to
form glucose via the gluconeogenic pathway, therefore, the
citric acid cycle is no longer able to function at full capacity.
The acetyl-CoA that is accumulating from fatty acid oxidation
is used instead to make acetoacetate, acetone, and
hydroxybutyrate.
While the brain under normal conditions
prefers glucose, the heart muscle can
metabolize acetoacetate to yield 2 moles of
acetyl-CoA (the brain can switch over
during starvation).
Can animals convert fatty acids into glucose?
Wait a minute, can't acetyl-CoA be used to make OAA, which can then be
converted to pyruvate as a substrate for gluconeogenesis?