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22
Dietary Triacylglycerols
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Hydrolysis occurs when chylomicrons encounter lipoprotein lipase anchored
in capillary walls
When Energy is in good supply: Converted back to Triacylglycerols for
storage in adipose tissue
When energy is in low supply: Fatty acid carbon atoms are activated then
oxidized as acetyl-CoA
Acetyl-CoA generates energy via the citric acid cycle and oxidative
phosphorylation
...

Stored Triacylglycerols are hydrolyzed to fatty acids
Free fatty acids and glycerols are released in bloodstream
These fatty acids travel in association with Albumins where they are
converted to Acetyl-CoA for energy generated

Glycerol from Triacylglycerols
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Glycerol produced from TAG
Hydrolysis is carried to
liver/kidneys
There it is converted to
Glyceraldehyde-3-Phosphate
and Dihydroxyacetone (DHAP)
DHAP enters
Glycolysis/gluconeogenesis
pathways
 Linking lipid and carbohydrate
paths

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Fate of Dietary Triacylglycerols
TAGs undergo hydrolysis → Fatty Acids and Glycerol
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Fatty Acids Undergo:
– Resynthesis of triacylglycerols for storage
– Conversion to acetyl-CoA

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Glycerol→Glyceraldehyde & DHAP which participate in:
– Glycolysis
– Gluconeogenesis
– Triacylglycerol Synthesis (Energy storage)

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Acetyl-CoA participates in:
– Triacylglycerol synthesis
– Ketogenesis
– Synthesis of sterols and other lipids
– Krebs Cycle
– Electron Transport Chain

Storage and Metabolism of Triacylglycerols
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After a meal, blood glucose levels increase, insulin levels rise and glucagon
levels drop
Glucose enters cells, rate of glycolysis increases
Insulin activates the synthesis of triacylglycerols for storage
When digestion is finished, blood glucose levels return to normal
 Insulin levels drop, Glucagon leves rise

Lower insulin and higher glucagon activate Tag Lipase (Enzyme that controls
hydrolysis of stored triacyglycerols)
When glycerol 2-phosphate is in short supply, fatty acids and glycerol
produced by hydrolysis of stored TAGs are released to bloodstream for
transport to energy generating cells
Otherwise, fatty acids and glycerol are cycled back into new Tags for storage
...

Fatty Acid synthesis and catabolism are similar in that they both proceed 2
Carbo atoms at a time and in that they are both recursive, spiral pathways

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1
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Pancreatic lipases partially hydrolyze the triacylglycerols in the micelles
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Insoluble hydrolysis products are carried to the lining in micelles, where they
are absorbed and reassembled into triacylglycerols
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2
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3
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Triacylglycerols in storage are similarly hydrolyzed within adipocytes
...
Acetyl-CoA can participate in
resynthesis of fatty acids (lipogenesis), formation of ketone bodies
(ketogenesis), steroid synthesis, or energy generation via the citric acid cycle
and oxidative phosphorylation
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4
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The synthesis requires dihydroxyacetone phosphate (from glycolysis or
glycerol) for conversion to glycerol 3-phosphate, to which fatty acyl groups are
added one at a time to yield triacylglycerols
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5
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The fatty acyl-CoA molecules are transported into the mitochondrial matrix
and are then oxidized two carbon atoms at a time to acetyl-CoA by repeated
trips through the b-oxidation spiral
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What is the function of ketogenesis?
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The ketone bodies are 3-hydroxybutyrate, acetoacetate, and acetone
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Their production is increased when energy generation from the citric acid
cycle cannot keep pace with the quantity of acetyl-CoA available
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The ketone bodies are water-soluble and can travel unassisted in the
bloodstream to tissues where acetyl-CoA is produced from acetoacetate and
3-hydroxybutyrate
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