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Biochem 1 > Lipid Digestion > Flashcards

Lipid Digestion Flashcards

1
Q

What are the physiological roles of fatty acids?

A
  • 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)
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2
Q

Why is storing energy as fate more efficient than storing it as glycogen?

A
  • 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
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3
Q

What are challenges on lipid digestion?

A
  • Lipids are not water-soluble

- Triglycerides are too large to be absorbed

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4
Q

What is the digestive solution involved in lipid digestion?

A

Triglycerides mix with bile and pancreatic secretions

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5
Q

What are pancreatic secretions?

A
  • HCO3-
  • Lipase
  • Colipase
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6
Q

What are micelles?

Where can they be found?

A

lipids with bile acids and lipase/colipase

Intestinal lumen

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7
Q

How do lipid-soluble components enter the cell?

A

Micelles interact with enterocyte (cells of
the intestinal lining)
Lipid-soluble components diffuse across the enterocyte membrane into the cell

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8
Q

What is bile?
Where is it made?
Where is it stored?

A
  • Responsible for fat emulsification (detergent action)
  • Made in the liver
  • Stored in gallbladder
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9
Q

How are lipids digested?

A
  • Bile salts emulsify lipids
  • Pancreatic lipase acts on triglycerides
    (triglycerides –> sn-2 monoglyceride + 2 fatty acids)
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10
Q

How does pancreatic colipase helps in digestion of lipids?

A
  • 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
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11
Q

What are the products of the reaction between pancreatic lipase and triglycerides?

A

2-monoacylglycerol

two fatty acids

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12
Q

How is dietary lipids absorbed?

A
  • 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
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13
Q

How is dietary lipids absorbed through the entire GI tract?

A

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

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14
Q

What happens to bile salts during the absorption process?

A
  • 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
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15
Q

Which enzyme breaks down cholesterol?

What are the products?

A
  • Cholesterol esterase

- Fatty acid + cholesterol

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16
Q

Which enzyme breaks down phosphatidylcholine?

What are the products?

A
  • Phospholipase A2

_ Fatty acid + lysophosphatidylcholine

17
Q

How are dietary fatty acids absorbed?

A
  • 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
18
Q

How are fatty acids activated?

A

FA + ATP –> FA-AMP + CoASH –> FA-CoA

AMP leaves

19
Q

How is triacylglycerol synthesized?

A

2-Monoacylglycerl + FA1CoA –> Diacylglycerol (CoASH leaves)

Diacylglycerol + FA3CoA –> Triacylglycerol (CoASH leaves)

20
Q

How are Nascent chylomicrons synthesized?

A

Triacylglycerol + other lipids + apoprotein B48 –> nascent chylomicrons

21
Q

What is the purpose of chylomicrons?

A
  • 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
22
Q

How do nascent chylomicrons become mature chylomicrons?

A
  • 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
23
Q

What are chylomicrons composed off?

A

Triacylglycerol (major component)
Cholesterol
Cholesterol ester
Phospholipid

24
Q

What is the fate of chylomicrons?

A
  1. 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
  2. LPL digests the TG in chylomicron, producing free FA and glycerol
  3. FA enter adjacent organs either for energy production (muscle) or fat storage (adipocyte)
  4. Glycerol is metabolized in the liver
  5. As chylomicron loses triglyceride, its density increases and it becomes a chylomicron remnant. It’s taken up by liver receptors that recognize apoE
  6. In the liver, chylomicron remnant is degraded into its component parts
25
Q

How can fat absorption (obesity) be prevented (via olestra)?

A
  • 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
26
Q

How can fat absorption (obesity) be prevented (via orlistat)?

A
  • Orlistat is a non-hydrolysable analog of triglycerides

- It inhibits pancreatic lipase

27
Q

What is lipid malabsorption?

How does it happen?

A
  • Loss of lipid in feces, known as steatorrhea
  • Lipid loss includes:
    fat-soluble vitamins: A, D, E, K
    essential fatty acids
28
Q

The most abundant component of chylomicrons is which of the following?

A. apoB-48
B. Triglyceride
C. Phospholipid
D. Cholesterol
E. Cholesterol ester
A

B

29
Q

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
A

D

Biochem 1 (22 decks)

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