Lipids metabolism 1234665585258255 .pptx

1. LIPID METABOLISM

2. Lipids
Lipids include a wide variety of chemical substances such as:
• Neutral fat (triacylglycerol or triglycerides).
• Fatty acids and their derivatives.
• Phospholipids.
• Glycolipids.
• Sterols.
• Fat soluble vitamins (A, D, E and K).

3. Components of dietary lipids
The major components of dietary lipids are:
1. Approximately 90 % of dietary lipid is in the form of triacylglycerols
(triglycerides).
2. Cholesterol and cholesterol esters.
3. Phospholipids.
4. Free fatty acids.

4. Dietary lipids
Triacylglycerols
(90 %)
Cholesterol and
cholesterol esters
Phospholipids Free fatty acids

5. Digestion of lipids
•The major site of lipid digestion is the small intestine.
•Little or no digestion occurs in the mouth or stomach.
In the stomach, lipids are mixed with hydrochloric acid
to form chyme.
•The lipids are digested in the intestine by pancreatic
enzymes.

6. Digestion in Small Intestine
• The acidic stomach contents called chyme, containing dietary
fat leaves the stomach and enters the small intestine.
• Digestion of fat occurs in the duodenum by emulsification of
fat.
• In the duodenum the dietary fat is emulsified by the action
of bile salts.

7. Emulsification of fat
• Emulsification is the breakdown of fat globules
duodenum into tiny droplets.
in the
• Emulsification provides a larger surface area on which the
enzyme pancreatic lipase can act to digest the fats into fatty
acids and glycerol.
• Emulsification is assisted by the action of the bile salts.

8. Emulsification of fat
Problem of lipids- hydrophobic and enzymes work in hydrophilic environment Bile acids produced in liver and stored in gall
bladder. Forms amphipathic micelles in small intestine with fat globules. Pancreatic lipases enter micelles and digest lipids

10. Hydrolysis of dietary triacylglycerols
• Emulsified triacylglycerols are hydrolyzed by pancreatic lipase.
Lipase hydrolyses fatty acid in the 1 and 3 positions of the
triacylglycerol, producing 2-monoacylglycerols and two molecules of
fatty acids.
• Triglycerides (TG)
TG + H2O → Diglyceride
H2O →
+ fatty acid (FA)
Monoglyceride (MG) + FA
Diglyceride +

11. Hydrolysis of dietary phospholipids
• Dietary glycerophospholipids are digested by pancreatic
phospholipase-A2. This enzyme catalyzes the hydrolysis of fatty acid
residues at the 2-position of the phospholipid, leaving
lysophospholipids and a molecule of fatty acid.

12. Hydrolysis of cholesterol ester
• Cholesterol esters are hydrolyzed by pancreatic cholesterol ester
hydrolase (cholesterol esterase), which produces cholesterol plus
free fatty acid.

13. Products of Lipid Digestion (Micelle Formation)
• Free fatty acids, free
cholesterol, 2-monoacylglycerol
and lysophospholipid are the
primary products of dietary lipid
digestion. These, together with
bile salts, form mixed micelles.
• Fat soluble vitamins A, D, E and
K are also packaged in these
micelles and are absorbed from
the micelles along with the
products of dietary lipid
digestion.

14. Absorption of Lipids by Intestinal Mucosal Cells
• 2-monoacylglycerol, fatty acids, cholesterol, and lysophospholipid
packaged in mixed micelles are absorbed from the intestinal lumen
into the intestinal mucosal cells by diffusion.
• Inside the mucosal cells the following events occur:
1. 2-monoacylglycerols are reconverted to triacylglycerols.
2.The absorbed lysophospholipids and cholesterol are also
reconverted to phospholipids and cholesterol esters.
3.The triacylglycerol resynthesized in intestinal cells combine with
cholesterol, phospholipids and proteins to form chylomicrons.

15. Transport of lipids
• Triacylglycerol, phospholipid and cholesterol esters resynthesized in
the intestinal mucosa are absorbed into the lymph in the form of
lipoprotein known as chylomicrons.
• Chylomicrons are composed of:
- Triacylglycerols (85 – 90%).
- Cholesterol and cholesterol ester (5%).
- Phospholipids (7%).
- Protein (apolipoprotein, 1–2%).

18. • The chylomicrons pass from the lymph into the blood
through the thoracic duct.
After a fatty meal, the plasma is milky in appearance
due to the presence of chylomicrons

20. Fate of absorbed lipids
The chylomicron enters the blood stream via the lymphatic system.
Lipoprotein lipase then hydrolyses the TAG to Free Fatty Acids (hydrophobic)
Lipoprotein lipase is an extracellular enzyme on the vascular endothelial surface, anchored
to capillary walls. This enzyme is predominantly in adipose tissue, muscle, and heart tissue
but not in the liver, as the liver has hepatic lipase. Lipoprotein functions to convert
triglycerides to fatty acids and glycerol.
FFA then bind to albumin for transport target organs like skeletal and cardiac muscles FFA in
the cells are either oxidised for energy, re-esterified back to TAG in adipose tissue for storage
Glycerol
Moves to the liver, phosporylated to glycerol phosphate which can enter the gluconeogensis
or the glycolysis pathways
Can be re-esterified with fatty acids to TAG