INTEGRATED METABOLISM IN
TISSUES
OVERVIEW:
 Catabolism of TAGs
 Oxidation and Synthesis of Fatty Acids
 Transfer of Acyl-CoA
 Ketone Bodies
 Catabolism of Cholesterol
 Essential Fatty Acids
CATABOLISM OF TAGS
AND FATTY ACIDS
The complete hydrolysis of Triacylglycerols gives us:
a glycerol and three fatty acids.
HOW DOES THIS HAPPEN?
Hydrolysis occurs through:
Lipoprotein lipase: non-hepatic tissue
Intracellular lipase: in liver and adipose tissue
Activated by epinephrine, norepinephrine, glucagon and
ACTH via cAMP
Activated lipase hydrolyzes one fatty acid at a time
GLYCEROL
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Glycerol is used by the liver for energy
Glycerokinase converts glycerol to glycerol phosphate
Glycerol phosphate can enter the glycolytic pathway
 Energy oxidation
or  Gluconeogenesis
(Adipose tissue cannot metabolize glycerol)
*Fatty acids are a rich source of energy
Process:
Fatty acids enter cell
Activated by Coenzyme A  Acetyl-CoA (using 2 ATP’s)
Catalyzed by Acyl-CoA synthetase
Pyrophosphate produced quickly hydrolyzed = irreversible reaction
MITOCHONDRIAL TRANSFER OF
ACYL-COA
• Fatty acid oxidation occurs in mitochondrial
matrix
• Energy produced through Oxidative
Phosphorylation*
• S-C Fatty acids pass directly into mitochondrial
matrixAcyl-CoA derivatives
• *L-C Fatty acids and the CoA derivatives cannot
-Carnitine, CAT 1, CAT 2
MITOCHONDRIAL TRANSFER OF
ACYL-COA
BETA-OXIDATION OF FATTY
ACIDS
*Breakdown of fatty acids into acetyl-CoA
• Mitochondrion
• *Cyclic Degradative Pathway
• *Dehydrogenases
Long fatty acids
Short fatty acids
BETA OXIDATION
1. Dehydrogenation forms a double bond between alpha and
beta carbons
2. Hydrogenation to unsaturated acyl-coa
3. B-hydroxy group oxidized to ketone by NAD+
4. B-ketoacyl-CoA cleaved resulting in the insertion of CoA and
cleavage of B-carbon
•
• Products are acetyl-CoA that enters Krebs cycle
And saturated coA-activated fatty acid with 2 fewer
carbons that continues the b-oxidation cycle
• *Beta-Oxidation not regulated except by TAG lipase
• Even number carbons due to 2 carbon loss at a time
• 16 carbons= 8 Acetyl-CoA molecules produced
• If fatty acid has an uneven # carbons, B12 and Biotin required
to oxidize
• Unsaturated fatty acid oxidation
ENERGY PRODUCED
• Each cleavage of saturated carbon-carbon bond  4
ATPs produced
• For each Acetyl-CoA oxidized 10 ATP produced
• The complete B-oxidation of one palmitic acid,
including the oxidation of the FADH2 and NADH
produced during this cycle yields about 106
molecules of ATP. *
FORMATION OF KETONE
BODIES*
• Another way for Acetyl-CoA to catabolize in liver
• Ketogenesis- ketone bodies formed
• Ketone bodies are three chemicals that are produced as
by-products when fatty acids are broken down for
energy.
• Only in Mitochondria
• Ketone body formation normally very low in blood.
• Situations of accelerated fatty acid oxidation with lowcarb intake => very high levels (Starvation, Low-carb
diet, or diabetes)*:
• As carb intake diminishes, oxidation of fatty acids
accelerates to provide energy through production of
TCA substrates (acetyl-CoA)
• *Shift to fat catabolism  accumulation of AcetylCoA
• Ketosis
CATABOLISM OF CHOLESTEROL
CHOLESTEROL
• Cholesterol is not an energy producing nutrient
• Its four ring structure remains intact through catabolism,
eliminated through billary system.
• The biliary system creates, transports, stores, and releases
bile into the duodenum to help in digestion.
The biliary system includes the gallbladder, bile ducts
and certain cells inside the liver, and bile ducts outside the
liver.
*Deliver y
Excretion
Delivered to the Liver
2 ways:
1. Hydrolyzed by esterases
to free form
-secreted directly into bile
canaliculi
In the form of Chylomicron
Remnants
& LDL-C and HDL-C
(low density lipoprotein
cholesterol, high density
lipoprotein cholesterol)
2. Converted into bile acids
before entering the bile
METABOLIC CHANGES
CHOLESTEROL TO BILE ACID
Key Metabolic Changes:
• Hydrocarbon Side Chain reduction at C17
• Carboxylic Acid addition on shortened chain
• Hydroxyl group addition to ring system of molecule
• Effect of these is to enhance water solubility of sterol
facilitating its excretion in the bile
• Enterohepatic circulation can return absorbed bile
salts to the liver
• *Hypercholesterolemia treated with removal of bile
salts
FATTY ACID SYNTHESIS
Non Essential Fatty Acids can be synthesized from simple
precursors
• Assembly of starter molecule
•
Acetyl-CoA and Malonyl-CoA
Acetyl-CoA + CO2 = Malonyl-CoA
Occurs in Cytosol
Catalyst- Acetyl-CoA carboxylase has biotin as prosthetic
group= “carboxylation”: Incorporates carboxyl group into
a compound using ATP
ACETYL-COA PRODUCTION &
MOVEMENT TO CYTOSOL
Production mostly occurs in mitochondria from pyruvate
oxidation, oxidation of fatty acids and degradation of some
amino acids
Some formed in cytosol through amino acid catabolism.
Fatty acid synthesis localized in cytosol, but acetyl-CoA
produced in matrix is unable to exit through mitochondrial
membrane.
Acetyl-CoA gets to cytosol by reacting with oxoloacetate to
form citrate, which can pass through inner membrane.
Citrate lyase converts the citrate back to oxaloacetate and
acetyl-CoA.
MITOCHONDRIAL MATRIX
TRANSFER
http://www.dnatube.com/video/641/Fatty-Acid-Biosynthesis
FATTY ACID SYNTHASE SYSTEM
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• Enzymes involved in fatty acid synthesis arrangement.
In cytosol
*Enzymes: ACP (Acyl Carrier Protein) & CE (Condensing
Enzyme)
Both have free SH group that Acetyl-CoA and MalonylCoA attach to before synthesis can begin
Acetyl-CoA transferred to ACP, losing its CoA  Acetyl-ACP
Acetyl group then transferred again to SH of CE leaving ACPSH
Malonyl group attaches to this molecule, losing it’s CoA
Now the fatty acid chain can be extended
STARTER MOLECULE
STEPS OF CHAIN ELONGATION
1. Carbonyl carbon of acetyl group to C2 of Malonyl-Acp, lose
CO2 with malonyl carboxyl group
2. B-Ketone reduce using NADPH (from PPS)
3. Alchohol dehydrated  double bond
4. Double bond reduced to butyryl-ACP from NADPH
5. Butyryl transferred to CE exposing ACP SH site to a 2 nd
malonyl-coa molecule
6. The second malonyl-coA condenses with ACP
7. Second condensation rxn takes place, with coupling of
butyryl group on the CE to C2 of malonyl-ACP. 6C chain
reduced and transferred to CE in a repetition of steps 2-5.
8. The cycle repeats to form a c16 fatty acid (palmitic)
*ESSENTIAL FATTY ACIDS
•
Humans cannot introduce double bonds beyond D-9 site
•
•
Linoleic and alpha linoleic- Plant products
Prostaglandins, Thromboxanes and Leukotriene's can be
formed from LA (n-6) (favored in the western diet) &
ALA (n-3)
EFA’S METABOLISM AND
ROLE
• EFA’s enter Smooth ER for metabolism
• LA  y-linoleic acid  dihomo-Y-linoleic acid
arachidonic acid
• ALA  Eicosapentaenoic acid (EPA)
• N-6 and n-3 fatty acids compete for enzymes and
take the same path, which can affect the conversion
of one or the other
• Eicosanoids transferred to membranes in the form
of TAGs or phospholipids. Go through further
elongation and desaturations in smooth ER,
transferred to the peroxisome and undergo Boxidation to DHA.
•
AA, ALA, EPA and DHA containing phospholipids or
TAG are incorporated into any of the cell’s membranes
or the neutral lipid. AA is predominant in membranes.
•
The higher fluidity from unsaturation = better expression
of hormone receptors
•
Eicosanoids- Important for hormone-receptor binding
sites*
AA (N-6) VS. EPA AND DHA
(N-3)
AA
EPA AND DHA
Pro-inflammatory
Pro-arrythmic
Activate platelets
Vasoconstrictors
Anti-inflammatory
Anti-Arrythmic
Inhibits platelets
Vasodilators
DHA: nervous system, vision,
neuroprotection, successful
aging, and memory.*
Deep-water fish: Herring,
Salmon, Tuna
SYTNHESIS OF
TRIACLYGLYCEROLS
• Precursors: CoA-activated
fatty acids and G-3-P
• De novo,(a Latin expression
meaning "from the
beginning,”), major route
• Salvage pathway increases
when a deficiency of essential
amino acid methionine exists.
SYNTHESIS OF CHOLESTEROL
 Nearly all tissues in body capable of synthesizing
cholesterol from acetyl-CoA
 Liver = 20% of endogenous synthesis
 80% from extrahepatic tissues, intestine most active
 1 g/day endogenously synthesized
 Average daily cholesterol intake 300 mg/day, only half is
absorbed
 Endogenous synthesis 2/3 total cholesterol
26 STEPS, 3 STAGES
1. Cytoplasmic sequence by which
3-hydroxy-3-methylutaryl-CoA
(HMG-CoA) formed from 3 mol
acetyl-CoA
2. Conversion of HMG-CoA to squalene,
including rate limtiing step of
cholesterol synthesis, in which
HMG-CoA reduced to mevalonic
Acid by HMG-CoA reductase
3. Formation of cholesterol from squalene
CHOLESTEROL SYNTHESIS
http://www.dnatube.com/video/253/Cholesterol--biosynthesis
CHOLESTEROL INHIBITORS
• As total body cholesterol increases, the rate of
synthesis decreases. ( negative feedback regulation of
HMG-CoA reductase reaction.)
• Suppression of cholesterol synthesis by dietary
cholesterol is unique to liver.
• Statins: HMG-CoA inhibitors, block endogenous
cholesterol synthesis
SUMMARY
• The complete hydrolysis of TAGs  Glycerol and 3 fatty Acids
• Fatty Acids are a rich source of energy
• Long Chain fatty acids cannot cross inner membrane, require
carnitine.
• The breakdown of fatty acids into acetyl-CoA “B-Oxidation”
• The synthesis of fatty acids is essentially the reverse of B-Oxidation
• Ketone bodies are produced when fatty acids are broken down for
energy
• Ketosis is a result which disrupts the body’s acid/base balance,
Diabetes
• Cholesterol is secreted into bile canliculi or converted to bile acids.
• N-6 EFA’s vs. N-3 EFA’s