5.1 Structure, Metabolism and Transport of Lipids

What is the structural backbone of triglycerides and glycerophospholipids?

Glycerol is the backbone. In triglycerides, all three hydroxyl groups are sites of esterification for fatty acids. In glycerophospholipids, one site is occupied by a phosphate group.

Describe the difference between saturated and unsaturated fatty acids.

Saturated fatty acids: Have no double bonds in their hydrocarbon side chain.

Unsaturated fatty acids: Contain one or more double bonds. The presence of double bonds greatly lowers the melting point of the lipid.

What are the key structural features of a cholesterol molecule?

Cholesterol contains a four-ring steroid nucleus, a hydrocarbon side chain (hydrophobic), and a single hydroxyl (-OH) group (hydrophilic polar head).

List the five classes of lipoprotein complexes categorized by density.

1. Chylomicrons (lowest density, largest size) 2. VLDL (Very Low-Density Lipoproteins) 3. IDL (Intermediate-Density Lipoproteins/Remnants) 4. LDL (Low-Density Lipoproteins) 5. HDL (High-Density Lipoproteins)

Match the lipoprotein to its primary transport role:

• Chylomicrons

• VLDL

• LDL

• HDL

Chylomicrons: Transport dietary (exogenous) triglycerides from the intestine to tissues.

VLDL: Transport endogenous triglycerides synthesized in the liver to adipose and muscle tissue.

LDL: Carry cholesterol to non-hepatic (peripheral) tissues.

HDL: Collect cholesterol from peripheral tissues and return it to the liver.

What is the rate-limiting step and enzyme in cholesterol biosynthesis?

The rate-limiting step is the conversion of acetyl-CoA to cholesterol, catalyzed by the enzyme HMG CoA Reductase. This enzyme is the target of statin drugs.

How does the liver dispose of excess plasma cholesterol?

The liver takes up plasma cholesterol and either converts it into bile salts (via the enzyme cholesterol-7-α-hydroxylase) for secretion into the duodenum or packages it into VLDL/LDL.

What is "Reverse Cholesterol Transport"?

It is the process where HDL collects excess cholesterol from peripheral blood vessel walls and returns it to the liver for disposal through bile secretion, protecting against atherosclerosis.

What role do ABCA1 and ABCG1 play in cholesterol recovery?

These are transporters that help move (efflux) cholesterol out of peripheral cells and onto nascent HDL particles.

What enzyme catalyzes the maturation of HDL by esterifying free cholesterol?

LCAT (Lecithin:cholesterol acyltransferase). It is activated by Apo A-1 and converts free cholesterol on the HDL surface into cholesteryl esters, which move into the core to form mature spherical HDL.

What is the function of Cholesteryl Ester Transfer Protein (CETP)?

CETP promotes the transfer of cholesteryl esters from HDL to apoB-containing lipoproteins (VLDL, IDL, and LDL). A deficiency in CETP is considered antiatherogenic because it increases HDL and decreases LDL levels.