403: Dietary Lipids and Lipid Transport

Triacylglycerol Synthesis Location

LIVER and adipose

Primary storage of TAG

Adipose tissue

Structure of TAGs

Glycerol + 3 fatty acids (R1,2,3) with ester bonds

Primary site of fat digestion

Small intestine lumen

Bile salts

Produced in liver and stored in gall bladder

Needed for digestion of fats in small intestine

What enzyme is involved in digestion in small intestine?

Pancreatic lipase - ONLY lipase to know

Pancreatic lipase

• Digests dietary fats in small intestine (secreted by pancreas)

  • Hydrolysis at 1 and 3 ester linkages

• REQUIRES COLIPASE

  • And calmodulin

Colipase role

Stabilizes interaction between fat and pancreatic lipase

Pancreatic lipase products:

TAG → 2-monoacylglycerol + 2 fatty acids (water-soluble)

Ways digested fat gets into blood from small intestine:

1. Diffusion (short chain FA)

2. Transport (monoacylglycerol and long-chain FA)

What organelle do absorbed lipids go?

Endoplasmic reticulum

Bile salts are released to lumen - can’t diffuse through

What molecule transports lipids through blood?

Chylomicrons

Where are chylomicrons synthesized?

Endoplasmic reticulum of small intestine

Chylomicron Components

• MAINLY TAGs

• Phospholipids

• Cholesterols

• Proteins (apolipo)

Lipoproteins

Soluble aggregates involved in lipid metabolism

Include chylomicrons, VLDL, LDL, IDL, and HDL

Where are lipoproteins produced?

Liver and small intestinal cells

Inside of lipoproteins

Hydrophobic components: TAGs and cholesteryl esters

Outside of lipoproteins:

Amphipathic components: apolipoprotein, cholesterol, glycerophospholipids

Do lipoproteins have a bilayer?

NO: they have phospholipids but NOT bilayer

Are lipoproteins or water more dense?

Lipoproteins because lipids are less dense

5 Types of Lipoproteins:

• Chylomicrons (LEAST DENSE)

• VLDL

• LDL

• IDL

• HDL (MOST DENSE)

What is the most dense lipoprotein?

HDL

Important lipoproteins:

• ApoC-II - fat catabolism (digestion)

• ApoE - cholesterol transport

What enzyme gets fat out of lipoprotein?

Lipoprotein lipase

Lipoprotein lipase products:

Monoacylglycerol + 2 FA (same as pancreatic lipase)

Location of lipoprotein lipase

Cell surface in cardiac muscle, skeletal muscle, and adipose

Apo-CII

Apolipoprotein that activates lipoprotein lipase to digest TAGs

Chylomicron remnant

Product of lipoprotein lipase

Transport of chylomicron remnant

To the liver to be degraded in lysosome and recycle cholesterol

VLDL

Transports TAGs made in liver to cells

Degraded by lipoprotein lipase → IDL

Where is VLDL made?

The liver (to transport TAGs → cells)

VLDL Remnant

IDL, which is further degraded to LDL

Cholesterol Transport

BY LDL in the liver

Cholesterol Transport Pathway

1. LDL receptors in ER of liver

2. LDL receptors to surface

3. ApoE on LDL binds to LDL receptors

4. LDL internalized

5. Endosome → LDL dissociates

6. Endosome fuses with lysosome

7. LDL degraded by lysosome, freeing cholesterol

HDL Synthesized by…

The LIVER! So it can take up excess lipids and recycle in liver

HDL is also called…

A trash can - because it takes up excess lipids

Fat digestion summary:

1. Digested by pancreatic lipase in small intestine

2. Chylomicron synthesized in small intestine

3. Fat transported in chylomicron through blood

4. Lipoprotein lipase degrades chylomicron in liver

5. VLDL transports lipids from liver

6. IDL → LDL

7. LDL delivers cholesterol to cells via ApoE and lysosomes