Lipid Digestion Flashcards
What are the physiological roles of fatty acids?
- They are the building blocks of phospholipids and glycolipids
- They serve as hormones and intracellular signaling molecules
- They are fuel molecules, stored as triglycerides (triacylglycerol)
Why is storing energy as fate more efficient than storing it as glycogen?
- Oxidation of fatty acid is ~9kcal/g (vs. carbs/protein which is ~4kcal/g
- Stored dat is nearly anhydrous (containing no water) while glycogen binds ~2 g of water
What are challenges on lipid digestion?
- Lipids are not water-soluble
- Triglycerides are too large to be absorbed
What is the digestive solution involved in lipid digestion?
Triglycerides mix with bile and pancreatic secretions
What are pancreatic secretions?
- HCO3-
- Lipase
- Colipase
What are micelles?
Where can they be found?
lipids with bile acids and lipase/colipase
Intestinal lumen
How do lipid-soluble components enter the cell?
Micelles interact with enterocyte (cells of
the intestinal lining)
Lipid-soluble components diffuse across the enterocyte membrane into the cell
What is bile?
Where is it made?
Where is it stored?
- Responsible for fat emulsification (detergent action)
- Made in the liver
- Stored in gallbladder
How are lipids digested?
- Bile salts emulsify lipids
- Pancreatic lipase acts on triglycerides
(triglycerides –> sn-2 monoglyceride + 2 fatty acids)
How does pancreatic colipase helps in digestion of lipids?
- It is activated by trypsin
- It interacts with triglyceride and pancreatic lipase (protein cofactor of lipase)
- It displaces bile to allow recycling of bile salts
- It improves activity of pancreatic lipase
What are the products of the reaction between pancreatic lipase and triglycerides?
2-monoacylglycerol
two fatty acids
How is dietary lipids absorbed?
- lipids are packaged in micelles (fatty acids, 2-monoacylglycerol, cholesterol and fat-soluble vitamins) surrounded by bile salts and diffuse through the unstirred layer
- fatty acids have a charged (hydrophilic) end
- as they are absorbed from the solution, more lipids partition out of the micelles
How is dietary lipids absorbed through the entire GI tract?
Stomach: emulsification droplet (fat droplet) + lingual lipase –> smaller fat droplets
Duodenum: smaller fat droplets + pancreatic lipase (& colipase) –> fatty acids and 2-monoacylglycerol (which diffuse to micelles
Duodenum, jejunum, ileum: micelles diffuse to brush border followed by absorption of fatty acids and monoglycerides
What happens to bile salts during the absorption process?
- More than 95% of all bile salts are reabsorbed primarily in the terminal ileum
- Bile salts are sent into portal circulation and are cleared by the liver
Which enzyme breaks down cholesterol?
What are the products?
- Cholesterol esterase
- Fatty acid + cholesterol
Which enzyme breaks down phosphatidylcholine?
What are the products?
- Phospholipase A2
_ Fatty acid + lysophosphatidylcholine
How are dietary fatty acids absorbed?
- medium and short chain fatty acids (<10 C) diffuse through gut epithelium, into hepatic portal blood supply
- Long chain fatty acids (more than 12C) are transported to ER of gut epithelial cells for re-synthesis into triglycerides (class of lipids)
- Fatty acids incorporated into triglycerides must first be activated into their CoA derivative
How are fatty acids activated?
FA + ATP –> FA-AMP + CoASH –> FA-CoA
AMP leaves
How is triacylglycerol synthesized?
2-Monoacylglycerl + FA1CoA –> Diacylglycerol (CoASH leaves)
Diacylglycerol + FA3CoA –> Triacylglycerol (CoASH leaves)
How are Nascent chylomicrons synthesized?
Triacylglycerol + other lipids + apoprotein B48 –> nascent chylomicrons
What is the purpose of chylomicrons?
- Long chain FA enter the ER
- They are activated and incorporated into TG
- TG and cholesterol are packaged into nascent chylomicrons and released into the lymphatic system
How do nascent chylomicrons become mature chylomicrons?
- Once in circulation, nascent chylomicrons interact with another lipoprotein, HDL
- It acquires 2 apoproteins from HDL:
apoprotein CII and apoprotein E - This converts nascent chylomicron to a mature chylomicron
What are chylomicrons composed off?
Triacylglycerol (major component)
Cholesterol
Cholesterol ester
Phospholipid
What is the fate of chylomicrons?
- ApoCII on the mature chylomicron activates the enzyme lipoprotein lipase (LPL) on the inner surface of the capillary endothelial cells of muscle and adipose tissue
- LPL digests the TG in chylomicron, producing free FA and glycerol
- FA enter adjacent organs either for energy production (muscle) or fat storage (adipocyte)
- Glycerol is metabolized in the liver
- As chylomicron loses triglyceride, its density increases and it becomes a chylomicron remnant. It’s taken up by liver receptors that recognize apoE
- In the liver, chylomicron remnant is degraded into its component parts
How can fat absorption (obesity) be prevented (via olestra)?
- Olestra is a commercial lipid produced from the esterification of fatty acid with sucrose
- 6 -8 fatty acids are added
- Fatty acid-sucrose molecules take like natural lipid
- They cannot be hydrolyzed to absorbable constituents and are excreted
How can fat absorption (obesity) be prevented (via orlistat)?
- Orlistat is a non-hydrolysable analog of triglycerides
- It inhibits pancreatic lipase
What is lipid malabsorption?
How does it happen?
- Loss of lipid in feces, known as steatorrhea
- Lipid loss includes:
fat-soluble vitamins: A, D, E, K
essential fatty acids
The most abundant component of chylomicrons is which of the following?
A. apoB-48 B. Triglyceride C. Phospholipid D. Cholesterol E. Cholesterol ester
B
The conversion of nascent chylomicrons to mature chylomicrons requires which of the following?
A .Bile salts B. 2-Monoacylglycerol C. Lipoprotein lipase D. High-density lipoprotein E. Lymphatic system
D
