Physiological Chemistry
Lipid Metabolism
Chapter 25
Digestion
• 98% of total dietary lipids are triacyglycerols (fats
and oils)
• Stomach:
– Mostly physical changes take place (i.e.,
formation of lipid droplets) – high fat meals
have a longer transit time through the stomach
since it takes longer to “disperse” lipids in this
manner
– Gastric lipases carry out ~10% hydrolysis
Digestion
• Small Intestines
– As chyme enters the duodenum, intestinal cells
secrete cholecystokinin (CCK), which stimulates
contraction of the gall bladder and release of bile into
the small intestine
– bile salts serve to emulsify triacylglcerols, which
allows pancreatic lipase to hydrolyze them to two
fatty acids and monoacylglycerols, which eventually
form “micelles”, which are readily absorbed through
intestinal cell membranes
Digestion (hydrolysis) of a triacylglycerol
A micelle
Products of Lipid Digestion
• Smaller components
– Free fatty acids (short, medium and long chain)
– Monoacylglycerols
– Phospholipids and lysophospholipids
– Free cholesterol
– Fat soluble vitamins
– Bile acids
Absorption
• Micelles are absorbed by passive diffusion
through the cell membrane of small intestinal
cells
• Nearly all dietary fat and 50% of dietary
cholesterol are absorbed
• About 80% of bile acids are reabsorbed
Metabolism of Absorbed Lipid
• Free fatty acids (FA) absorbed in small intestinal
cell:
– Short-chained FA go directly into blood and bind to
albumin
• Transported through portal circulation to liver for “chain
elongation” or catabolism
– Other FA are re-esterified to triacylglycerols,
phospholipids, and cholesterol esters
• Newly formed triacylglycerols, cholesterol esters,
free cholesterol, and phospholipids are assembled
with proteins to form chylomicrons
Chylomicrons
• Lipoprotein made by small intestinal cells
• Move from small intestines to lymphatic system and
then to blood stream
• Removed from blood by lipoprotein lipases in
tissues (mostly adipose and muscle), which break
them down to free fatty acids and glycerol before
absorbing them to use either as sources of fuel or to
store them as triacylglycerols (TAGs)
• Appear in blood within 2 hours, peak at about 5-6
hours and are completely cleared in about 10 hours
post meal (dependent upon fat content of meal)
– May take up to 16 hours
Chylomicrons
Digestion and Absorption
Figure 25.3: The events that must occur before triacylglycerols can
reach the bloodstream through the digestive process.
CCK
Triacylglycerol (TAG) Storage
•
•
•
•
TAG stored in adipocytes (adipose tissue) mostly
Provides insulation and protection
TAG droplet occupies most of cell
As TAG accumulates, cell enlarges and then splits
into 2 cells
• TAG can be stored virtually limitlessly!
• Obesity can result from either an increased
number of fat cells or an enlargement of existing
fat cells
Adipocytes
Figure 25.4: Structural
characteristics of the
adipose cell.
Triacylglycerol Mobilization
• Release of free fatty acids and glycerol from fat cells by
hydrolysis of TAGs, stimulated by epinephrine and
norepinephrine (fasting or physical activity)
• Hormone binds to external receptor and activates adenyl
cyclase, causing production of cAMP in the inside of the
adipocyte
• cAMP activates hormone sensitive lipase (HSL) in
adipocyte, which hydrolyzes TAGs to produce free fatty
acids (FFAs) and glycerol that are released into blood
and go to the muscle tissue (FFAs are transported bound
to plasma albumin)
• Relatively slow process (~10% replaced daily)
Epinephrine Action
Figure 25.5
Triacylglycerol Mobilization
• TAGs represent the largest energy reserve in the body,
and are the SECOND type of compound to be utilized as
energy after carbohydrates
Glycerol Metabolism
• Occurs in the cytosol
• Involves phosphorylation and oxidation
• Consumes one ATP and produces one NADH
• DHAP enters glycolysis to produce
glyceraldehyde-3-P
Fatty Acid -Oxidation
• Occurs in mitochondria
• Makes acetyl-CoA, ATP, and NADH/FADH2
• Involves three steps
– “activation” of the fatty acid
– transport into mitochondrial matrix
– repeated -Oxidation: sequence of FOUR reactions
Activation and Transport of FA
A. Conversion to acyl-CoA on the outer
mitochondrial space
Process consumes TWO high energy phosphate bonds
B. A transporter then facilitates translocation of
acyl-CoA into mitochondrial intermembrane
space
Activation and Transport of FA
C. Acyl-CoA is converted to acyl-carnitine by
carnitine palmitoyltransferase, which then
translocates acyl-carnitine into the matrix, and
regenerates acyl-CoA from free CoA from within
the matrix as carnitine is recycled back into the
intermembrane space
Activation and Transport of FA
-Oxidation
• Sequential removal of 2 C (acetyl) units from the
carboxyl end of FA in a repetitive FOUR-step cycle
1. Oxidation (dehydrogenation)
2. Hydration
-Oxidation
3. Oxidation (dehydrogenation)
4. Cleavage
Unsaturated Fatty Acids
• Two additional enzymes are needed
– Epimerase: converts D--hydroxyacyl-CoA that
results from hydration of a cis double bond (the most
abundant) to the L isomer
Unsaturated Fatty Acids
• Two additional enzymes are needed
– Cis-trans isomerase: converts cis double bonds at oddnumbered positions to trans double bonds at evennumbered positions, which is required for the
hydratase enzyme
Complete Oxidation and ATP Production
• For C18 (Stearate): 8 cycles occur
– 32 ATPs from 8 NADH (× 2.5) and 8 FADH2 (× 1.5)
– 90 ATPs (via Citric Acid Cycle) from 9 Acetyl-CoAs
(9×3×2.5 + 9×1×1.5 + 9×1)
– 2 ATPs are used to form acyl CoA (initially)
– Total Net = 120 ATP from C18 Fatty Acid!!
Energy Comparison
Comparing equal masses:
• 1 g stearic acid (18:0) x 1 mole x 120 ATP = 0.423 moles of ATP
284 g
1 mole
• 1 g glucose x 1 mole x 30 ATP = 0.17 moles ATP
180 g 1 mole
• You get 2.5 times more energy (0.423/0.17) from 1 g fat than from
1 g carbohydrate; thus:
• 1 g carb = 0.17 × 24.5 ~ 4 kcal
• 1 g fat = 0.43 × 24.5 ~ 9 kcal
Energy Comparison
• Both carbs and fats are useful “body fuels”
• Skeletal muscle: prefers glucose when active and
fatty acids when resting
• Cardiac muscle: prefers fatty acids then ketone
bodies, glucose, lactate
• Liver: prefers fatty acids
• Brain: prefers glucose (fatty acids can’t cross
BBB) then ketone bodies
• RBC: needs glucose (no mitochondria)
Energy in Exercise
The initial stages of
exercise are fueled
primarily by glucose
(glycogen); within
about 20 – 30 minutes,
triacylglycerols become
the primary fuel
Ketone Bodies
• Produced via alternative pathway of acetyl-CoA
metabolism when it accumulates
– acetoacetate → acetone
– β-hydroxybutyrate
• Caused by high rate of fat catabolism relative to glucose
catabolism in liver mitochondria
• When glucose catabolism decreases and/or fat
catabolism rises, there is not enough pyruvate to
synthesize oxaloacetate (by carboxylation), so Krebs
cycle cannot consume all the acetyl-CoA
Ketogenesis Pathway
• Step 1: Self-condensation of acetyl-CoA
• Step 2: Hydrolysis of acetyl-CoA and condensation of
acetate with acetoacetyl-CoA
Ketogenesis Pathway
• Step 3: Fragmentation of HMG CoA to acetoacetate
(ketone body) + acetyl CoA
• Step 4a: Reduction of acetoacetate to β-hydroxybutyrate
Ketogenesis Pathway
• Step 4b: Spontaneous decarboxylation of acetoacetate to
acetone (ketone body) + CO2 (acetone is then expired
from the lungs and excreted via skin pores and urine)
Figure 25.8
How do ketone bodies provide energy?
• β-hydroxybutyrate can be reversibly converted to
acetoacetate, which produces NADH that can be used in
the electron transport chain to produce ATP
• Acetoacetate can be “activated” by transfer of a CoA
group from succinyl-CoA (from Krebs cycle), and the
resulting acetoacetyl-CoA can be cleaved to two acetylCoA molecules that can enter Krebs cycle
• Thus, β-hydroxybutyrate and acetoacetate represent
water-soluble transportable forms of acetyl-CoA that
can be used to generate energy as needed
CoA‒SH
succinate
succinyl-CoA
How do ketone bodies provide energy?
• Heart muscle and the renal cortex use
acetoacetate in preference to glucose
• The brain adapts to utilization of acetoacetate
with starvation or diabetes (75% of brain fuel
needs are obtained from acetoacetate during
prolonged starvation)
Ketosis vs. Ketoacidosis
• The presence of significant levels of acetoacetate, βhydroxybutyrate, and acetone is called ketosis
• Acetone gives a “sweet smell” to breath and urine
• Acetoacetate and β-hydroxybutyrate can decrease plasma
pH (by decreasing plasma bicarbonate), which if left
uncontrolled can lead to metabolic acidosis, also called
ketoacidosis
• Increased solutes in plasma can cause a fluid shift from
ICF to ECF if left uncontrolled
Ketosis vs. Ketoacidosis
• This can initially lead to extracellular edema, and
ultimately to intracellular dehydration
– Loss of solutes in the urine causes loss of water as
well (by osmotic diuresis), eventually leading to total
body dehydration
• Occurs when glucose metabolism is not occurring at a
reasonable rate compared to fat metabolism for a
prolonged period of time
• Common in: uncontrolled type I diabetes, and extended
low carbohydrate diets, fasting, or starvation
Lipogenesis
• Synthesis of fatty acids from acetyl CoA
– Occurs in the cytosol, mostly in liver, adipose tissue,
and mammary glands
– NOT the exact reverse of fatty acid degradation
– Carried out by the “Fatty Acid Synthase Complex”
– Fatty acid is bonded to acyl carrier protein (ACP)
– Requires NADPH as a reducing agent
– Acetyl-CoA initially forms malonyl CoA (3 carbons)
– Converts “excess caloric intake” from carbohydrates,
protein, and alcohol (i.e., excess acetyl CoA) to fats
Lipogenesis
• Humans can convert both glucose and fats to
acetyl CoA, but acetyl-CoA can only be
converted back to fatty acids (via lipogenesis),
NOT to glucose (acetyl-CoA cannot be
converted to pyruvate or oxaloacetate)
• Thus to lose extra fat, humans must burn fat as
an energy source (being physically active before
and/or after meals)
Lipogenesis
• The major steps or lipogenesis are:
– Translocation of acetyl-CoA from the mitochondrial
matrix to the cytosol (via citrate-malate shuttle)
– Formation of malonyl CoA
– Formation of acyl-ACP
– Chain elongation
Citrate-Malate Shuttle
• Acetyl CoA is produced in the mitochondria and
lipogenesis occurs in the cytosol, so acetyl CoA
needs to be transported out of the mitochondria
• Acetyl CoA + oxaloacetate → citrate (via Krebs)
• Citrate is then transported out of the matrix
through a protein called the citrate transporter
Citrate-Malate Shuttle
• Once in the cytosol
– citrate → acetyl CoA + oxaloacetate
– consumes one ATP
• acetyl CoA goes into lipogenesis
• oxaloacetate + NAD+ → malate + NADH
• malate transported into the matrix by a protein
called the malate transporter
• malate + NADH → oxaloacetate + NAD+
• Oxaloacetate is used to react with more acetyl
CoA to form citrate (cycle repeats)
Formation of Acetyl & Malonyl ACP
1. Transfer of acetyl group from CoA to ACP
2. Conversion of acetyl-CoA to malonyl-ACP
(Only occurs where ATP levels are high; requires Mn2+ and biotin)
Chain Elongation
1. Attachment of acetyl- and malonyl-ACP to the
Fatty Acid Synthase Complex
2. Much like β-oxidation, a sequence of FOUR
steps then take place, which are the reverse of βoxidation
A. condensation
B. hydrogenation
C. dehydration
D. hydrogenation
Chain Elongation
1. Condensation (with decarboxylation)
2. Hydrogenation (requires NADPH, from pentose
phosphate pathway)
Chain Elongation
3. Dehydration
4. Hydrogenation (requires NADPH, from pentose
phosphate pathway)
Further elongation requires other enzyme, but still result in fatty
acids with an even carbon number
Synthesis of Unsaturated Fatty Acids
• Requires molecular oxygen and NADPH
• In humans, points of unsaturation can only occur at
carbon atoms 4 through 9 in fatty acid
– Thus fatty acids with points of unsaturation initially
beyond carbon 10 (e.g., linoleic acid and alpha
linolenic acid) cannot be synthesized
Cholesterol Synthesis
• Essential structural component of all cell membranes
• Precursor to steroid hormones (progestins, estrogens,
androgens, glucocorticoids, and mineralcorticoids),
vitamin D, and bile acids/salts
• Occurs primarily in the liver and small intestines from 18
acetyl CoA molecules, which ultimately yield a structure
with 27 carbons
• Approximately 1.5 – 2.0 g/day is synthesized and nearly
0.30 g/day is consumed in the diet
Cholesterol Synthesis
• Five major stages
Key enzyme: HMG-CoA reductase [target of anti-cholesterol
drugs called “statins”, e.g., atorvastatin (Lipitor), lovastatin
(Mevacor), simvastatin (Zocor)]
Cholesterol Synthesis
×6
Cholesterol Synthesis
Several steps
Statin Structure
Steroid Synthesis
Relationship between Lipids and Carbohydrates
• Acetyl CoA is primary link
• Both fat and carbohydrates produce acetyl-CoA
for citric acid cycle for energy
• Excess acetyl-CoAs from carbohydrate
metabolism can produce fatty acids and eventually
lead to fat storage (they cannot be used to regenerate glucose because they cannot be converted
to pyruvate or oxaloacetate)
Relationship between Lipids and Carbohydrates
• Ketosis occurs when there is an increase in fatty
acid metabolism relative to glucose metabolism
• Ketoacidosis occurs when there is excessive fat
metabolism that is not well controlled
• Fatty acid and cholesterol synthesis occurs when
the body is in an acetyl CoA-rich state
Lipoproteins (Chapter 20)
• Chylomicrons – carry exogenous (dietary) fat
and other lipids from the small intestine into the
blood and ultimately to tissue sites (mostly
adipose tissue and muscle)
• VLDL – carry mostly endogenous (synthesized)
fat, from the liver through the blood to adipose
tissue
Lipoproteins
• LDL – carry fats and larger amounts of cholesterol
from the liver through the blood to the peripheral
tissues (made from VLDL) (< 100 mg/dL is
optimal)
• HDL – carry fats and larger amounts of cholesterol
from the peripheral tissues through the blood to the
liver for metabolism to bile acids followed by
excretion in bile via the gall bladder (> 60 mg/dL is
optimal)
– Ratio of total cholesterol to HDL cholesterol is
better of < 3.5 is optimal
©2001 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license.
Size and Compositions of the Lipoproteins
Lipoprotein Electrophoresis
• Sample (serum) is applied to a support medium
• Placed in a chamber with a buffer and electrodes
• Negatively charged particles move to the anode
• After the development period, the strip is removed and
stained (Oil Red O)
• This sticks to the lipids and they can be quantified in
several ways
Lipoprotein Electrophoresis
• A normal (fasting for > 8 hours) lipoprotein
electrophoresis pattern should contain prominent HDL
(alpha) and LDL (beta) bands, and a weak VLDL (prebeta) band
• Chylomicrons are cleared from the blood within 8-12
hours
anode
cathode
Hyperlipidemia
• Type I Hyperlipidemia (Frederickson Classification) is
the inability to clear chylomicrons from the plasma in a
normal time
– Results from a deficiency in lipoprotein lipase (breaks
down chylomicrons) or low HDL levels
– People with this condition display significantly
elevated TAG levels in the blood (> 600 mg/dL;
normal is < 150 mg/dL)
– Must severely restrict fats from diet, which may result
in essential fatty acid and fat-soluble vitamin
deficiency
Hyperlipidemia
• Restricted diet may be supplemented with a
“medium-chain TAG” mixture (recall that
smaller fatty acids escape the re-esterification
into TAG and incorporation into chylomicrons –
they are carried as FFAs by albumin)
• Instead, these (and shorter chain fatty acids) are
transported to the liver for “chain elongation”
Familial Hypercholesterolemia
• Genetic disorder characterized by high cholesterol levels,
specifically very high levels of low-density lipoprotein
(LDL, “bad cholesterol”), in the blood and early
cardiovascular disease
• Many patients have mutations in the LDLR gene that
encodes the LDL receptor protein, which normally
removes LDL from the circulation, or apolipoprotein B
(ApoB), which is the part of LDL that binds with the
receptor
Familial Hypercholesterolemia
• Patients who have one abnormal copy of the LDLR gene
(i.e., heterozygous) may have premature cardiovascular
disease at the age of 30 to 40.
• Having two abnormal copies (being homozygous) may
cause severe cardiovascular disease in childhood.
• Heterozygous FH is a common genetic disorder,
occurring in 1:500 people in most countries;
homozygous FH is much rarer, occurring in 1 in a
million births.
Familial Hypercholesterolemia
• Heterozygous FH is normally treated with statins, bile
acid sequestrants, or other hypolipidemic agents that
lower cholesterol levels.
• New cases are generally offered genetic counseling
• Homozygous FH often does not respond to medical
therapy and may require other treatments, including LDL
apheresis (removal of LDL in a method similar to
dialysis) and occasionally liver transplantation
Familial Hypercholesterolemia
• High cholesterol levels normally do not cause any
symptoms early on.
• Cholesterol may be deposited in various places in the
body that are visible from the outside, such as in
yellowish patches around the eyelids (xanthelasma
palpebrarum), the outer margin of the iris (arcus senilis
corneae) and in the form of lumps in the tendons of the
hands, elbows, knees, and feet, particularly the Achilles
tendon (tendon xanthoma).
Familial Hypercholesterolemia
Xanthelasma palpebrarum, yellowish patches consisting
of cholesterol deposits above the eyelids. These are
common in people with FH.
Familial Hypercholesterolemia
Patient’s hands showing multiple xanthoma tendinosum
Familial Hypercholesterolemia
• Accelerated deposition of cholesterol in the walls of
arteries leads to atherosclerosis, the underlying cause of
cardiovascular disease.
• The most common problem in FH is the development of
coronary artery disease (atherosclerosis of the coronary
arteries that supply the heart) at a much younger age than
would be expected in the general population.
• This may lead to angina pectoris (chest tightness on
exertion) or heart attacks.
Familial Hypercholesterolemia
• Less commonly, arteries of the brain are affected; this
may lead to transient ischemic attacks (brief episodes
of weakness on one side of the body or inability to talk)
or occasionally stroke.
• Peripheral artery occlusive disease (obstruction of the
arteries of the legs) occurs mainly in people with FH
who smoke; this can cause pain in the calf muscles
during walking that resolves with rest (intermittent
claudication) and problems due to a decreased blood
supply to the feet (such as gangrene)
Familial Hypercholesterolemia
• If lipids start infiltrating the aortic valve (the heart valve
between the left ventricle and the aorta) or the aortic root
(just above the valve), thickening of these structures may
result in a narrow passage called aortic stenosis.
• Aortic stenosis is characterized by shortness of breath,
chest pain, and episodes of dizziness or collapse.
• Atherosclerosis risk is increased further with age and in
those who smoke, have diabetes, high blood pressure,
and a family history of cardiovascular disease.
Familial Hypercholesterolemia
• Cholesterol levels may be determined as part of health
screening for health insurance or occupational health, when
the external physical signs such as xanthelasma, xanthoma,
arcus are noticed, symptoms of cardiovascular disease
develop, or a family member has been found to have FH.
• Hyperlipoproteinemia type IIa on the Fredrickson
classification is typically found:
– raised level of total cholesterol
– markedly raised level of low-density lipoprotein (LDL)
– normal level of high-density lipoprotein (HDL)
– normal level of triglycerides
Summary of HyperLipoproteinemia Phenotypes
Type metabolic defect
LP abnormality
predominant
hyperlipidemia
I
deficient
fasting
hypertriglyceridemia
lipoprotein lipase chylomicrons
IIa
floating creamy
layer over clear
serum
hypercholesterolemia clear serum
increased LDL
IIb
deficient apoB
receptors
deficient apoB
receptors
and deficient HDL
III
defective apoE
IV
deficient HDL
increased IDL and
hypercholesterolemia
chylomicron remnants hypertriglyceridemia
increased VLDL
hypertriglyceridemia
V
deficient
lipoproteinlipase
and deficient HDL
increased LDL
fasting chylomicrons
and increased VLDL
appearance
hypercholesterolemia
hypertriglyceridemia
hypertriglyceridemia
clear (if trig < 500
mg/dl)
lipemic (if trig > 600
mg/dl)
lipemic
clear (if trig < 500
mg/dl)
lipemic (if trig > 600
mg/dl)
floating creamy
layer
over lipemic