Lipid Digestion and Metabolism Flashcards
What type of lipids are most common in the diet?
Most dietary lipid is triglyceride (TG) with small amounts
of phospholipid & cholesterol
Where does lipid digestion and uptake occur? How does it occur?
Lipid digestion and uptake occurs in intestine and is facilitated by bile salts which emulsify the dietary lipids into micelles
Pancreatic lipase is secreted into duodenum and binds to
TG-containing micelles
The lipase cleaves fatty acids from TG at C1 and C3 position to form 2 free FA & monoacylglycerol
Phospholipids are hydrolyzed to 1 free FA and lysophospholipid by pancreatic phospholipase A2.
Free FA, monoacyglycerol, lysophospholipid, and
cholesterol form mixed micelles that are freely absorbed
into small intestine cells.
What is the exogenous pathway?
Exogenous pathway: lipoproteins transport of TG and
other lipid nutrients to the body’s tissues via lipoprotein
What is the endogenous pathway?
Endogenous pathway: lipoproteins transport of lipids
produced in the liver to the body’s cells.
How are TGs digested and absorbed?
- In the intestinal lumen, the dietary fat
emulsified (solubilized) by mixing with bile
salts (an amphipathic molecule with detergent
properties). - Digested by pancreatic lipase into two fatty
acids (FAs) and monoacylglycerol - Transported into enterocytes (endocytosis)
- Short (C4-C6) and medium (C6-C12) chain
FAs transferred directly into blood, where it
binds to albumin and carried to liver - Long chain FAs delivered to SER, where they
are incorporated into TG - In enterocytes the TG combines with dietary
cholesterol, newly synthesized phospholipid
and lipoprotein-B48 to form nascent (newly
made) chylomicrons (large LDL) - The nascent chylomicrons are:
- secreted into lymph and into blood stream at
the thoracic duct
- Converted into mature chylomicrons as they
are circulation in lymph and blood when
HDLs transfer two lipoprotein molecules
- Lipoprotein lipase (LPL) converts the TGs
in chylomicrons into fatty acids and glycerol
- Fatty acids are taken up from the blood by muscle and adipose tissue cells, where as
glycerol is carried in the blood to the liver - LPL removes 90% of the TGs in chylomicrons
Remaining 10% of TGs from the chylomicron
remnants are digested the liver - Cholesterol released from chylomicron
remnants
How are TGs made?
Fatty acids + glycerol-3-phosphate (or DHAP)
In triacylglycerol synthesis, glycerol-3-phosphate or dihydroxyacetone phosphate reacts sequentially with three molecules of acyl-CoA
What happens to FAs removed from the blood?
In liver, FAs removed from blood are
used for TG that are incorporated into
VLDL
How is glycerol removed from blood?
By the liver
What can happen to fatty acids in the body?
Converted to triacylglycerols
Degraded to generate energy
Used for membrane synthesis
What is the function of the TG cycle?
TG cycle is the mechanism that regulates the level of fatty acids available to the body for energy generation
What is glyceroneogenesis? What enzymes are involved?
Glyceroneogenesis is an
abbreviated version of
gluconeogenesis
Glycerol-3-phosphate is
synthesized from
substrates other than
glucose or glycerol
Key enzymes are PC and
PEPCK-C
How are fats mobilized throughout the body?
TAG is hydrolyzed to FA & glycerol which diffuse
into blood stream.
FA are transported in blood by non-covalent binding
to serum albumin
FA taken up by heart, liver, & muscle by co-transport
with sodium.
After absorption by muscle, heart or liver cell the FA is
activated
How are fats transported into the mitochondria?
The activated FA is transported into mitochondria by
shuttle system that uses carnitine as FA carrier
What is beta-oxidation?
Most fatty acids are degraded by the sequential removal of
two carbon fragments from the carboxyl end as acetylCoA;
this is known as β-oxidation
What is carnitine?
Carnitine is used to transfer acyl groups into the mitochondrion where most β-oxidation occurs
