(4.5) Apolipoprotein

Apo A

Apolipoproteins: They are specific proteins that comprise the protein portion of a lipoprotein.

• _____:

  • Major protein component of HDL (High-Density Lipoprotein).

  • Major components are ____-I (75%) and ____-II (20%).

  • Synthesized by the liver and intestines.

  • Responsible for the activation of LCAT (Lecithin-Cholesterol Acyltransferase), which catalyzes the esterification of cholesterol.

Apo B

Apolipoproteins: They are specific proteins that comprise the protein portion of a lipoprotein.

• _____:

  • Major protein constituent of LDL, VLDL (40%), and chylomicrons.

  • ____-100 is the most common form; it is found in VLDL and LDL and is synthesized in the liver.

  • ____-48 is the other major form; it is found exclusively in chylomicrons and is synthesized in the intestines.

Apo C

Apolipoproteins: They are specific proteins that comprise the protein portion of a lipoprotein.

• _____:

  • Major protein constituent of VLDL.

  • Minor protein constituent of HDL and LDL.

  • Primarily synthesized in the liver.

Apo D

Apolipoproteins: They are specific proteins that comprise the protein portion of a lipoprotein.

• _____:

  • Minor component of HDL (about 5%).

  • Structurally belongs to the lipocalin family (unlike other apolipoproteins).

  • Acts as a multi-ligand binding protein, likely involved in the transport of small hydrophobic molecules like bilirubin and steroids.

Apo E

Apolipoproteins: They are specific proteins that comprise the protein portion of a lipoprotein.

• _____:

  • Found in VLDL, HDL, and Chylomicrons.

  • Highly associated with Alzheimer’s disease (specifically, the ____ε4 allele is a major genetic risk factor).

  • Serves as a critical ligand for the LDL receptor and the Remnant receptor, facilitating the hepatic uptake of chylomicron remnants and VLDL remnants (IDL).

Apo A-I

Lipoprotein Carrier: HDL, chylomicrons

Function: Cofactor for LCAT

Apo A-II

Lipoprotein Carrier: HDL

Function: Unknown

Apo-IV

Lipoprotein Carrier: HDL, chylomicrons

Function: Activates LCAT

Apo B-48

Lipoprotein carrier: Chylomicrons

Function: Secretion of TG from intestines

Apo B-100

Lipoprotein Carrier: VLDL, LDL

Function: LDL receptor binding

Apo C-I

Lipoprotein Carrier: Chylomicrons, VLDL, LDL

Function: Activates LCAT

Apo C-II

Lipoprotein Carrier: Chylomicrons VLDL, LDL

Function: Activates LpL

Apo C-III

Lipoprotein Carrier: Chylomicrons VLDL, LDL

Function: Inhibits activation of LpL by C-II

Apo E

Lipoprotein Carrier: Chylomicrons VLDL, LDL

Function: Facilitate uptake of remnant particles (chylomicrons and lDL)