1. Apolipoproteins

2. Lipoprotein complexes
Triaglycerol lipid droplets
Cholesterol esters
Phospholipids
Apolipoproteins

3. Lipoproteins Chylomicrons High density lipoproteins, HDL
Intermediate density lipoproteins, IDL
Low density lipoproteins, LDL
Very low density lipoproteins, VLDL

4. Apolipoproteins ApoA-I, II and IV ApoB-48 and 100 ApoC-I, II and III
ApoD
Cholesterol ester transfer protein
ApoE
ApoH

5. Closer look Apolipoprotein A-I Apolipoprotein A-II HDL and enzyme LCAT
Atherosclerosis ?

6. Apolipoprotein A-I ApoA-I synthesized in intestine and liver
Associated with chylomicrons and HDL

7. A look at the structure of ApoA-I
PDB ID 1AV1
Four alpha helical horseshoe shaped molecules that in the crystal form create a tightly associated elliptical ring.

8. Monomer of ApoA-I
Horseshoe shaped pseudo-continuous amphipathic alpha helix
Punctuated by kinks of regularly spaced proline residues

9. Dimer of ApoA-I
Consists of two monomers arranged antiparallel

10. Dimer of ApoA-I
One face of the dimer is hydrophilic (pink) the other face is hydrophobic (blue)

11. Tetramer of ApoA-I
In the crystal form two dimers join in an antiparallel fashion to form an elliptical tetrameric ring.
This is not the structure that binds to lipids because hydrophobic side chains are hidden

12. Proposed ApoA-I lipid binding
Binds as dimer
Elliptical shape -> circular shape
By adjustment of interhelical kinks and bends
Dimer wraps around lipid like a belt
Lipid bound structure has not been solved

13. Apolipoprotein A-II
Exchangeable apolipoprotein
Associated with HDL

14. Structure of lipid free ApoA-II
PDB ID 1L6K
Monomer
Alpha helices punctuated by proline residues, but less uniformly than in ApoA-I
Dimer
Side to side packing of two monomers joined by a salt bridge
Contains three hydrophobic patches

15. Tetramer of ApoA-II
Side to side packing of two dimers shields hydrophobic patches (cyan), when lipids not present
Top view of 3 tetramers
Side view of 3 tetramers

16. Lipid bound ApoA-II, ApoA-II-BOG
PDB ID 1L6L
ApoA-II with beta-octylglucopiranoside
Head to tail packing in the tetramers, dimers held together by hydrogen bonds and polar interactions between Gln77 and Ala2 and Lys3
Curved confirmation due to residues 31-39
Glutamine, Alanine, Lysine

17. ApoA-II-BOG
Gln77 hydrogen bond with Ala2 as well as a polar interactions with Lys3
ApoA-II-BOG, regions enabling flexibility highlighted in red. Close up of molecules involved in head to tail packing shown

18. ApoA-I and II bind to HDL
HDL, highest density lipoprotein due to it’s protein lipid ratio
Contains almost no cholesterol or cholesterol esters when synthesized
Obtains cholesterol esters from cholesterol by the HDL associated enzyme, lecithin: cholesterol acyltransferase (LCAT)

19. LCAT LCAT is synthesized in the liver
LCAT makes cholesterol esters from free cholesterol found in chylomicron remnants and VLDL remnants
LCAT transfers a fatty acid from the C-2 position of lecithin to the C-3-OH of cholesterol, generating a cholesterol ester and lysolecithin
The action of LCAT requires interaction with ApoA-I

20. ApoA-I and LCAT ApoA-I activates LCAT by binding to HDL
increasing helical content of ApoA-I
Orients the protein to provide necessary contacts with enzyme
ApoA-I interacts with LCAT through positively charged residues on the side chains of the ApoA-I helices

21. ApoA-II displaces ApoA-I
ApoA-II can completely displace ApoA-I from HDL
Two molecules of ApoA-II displace one of ApoA-I
Flexibility of ApoA-II allows displacement onto any size HDL which ApoA-I is bound to
ApoA-II forms a more stable complex with HDL
Head to tail interactions
C terminal helix more hydrophobic than any helix from ApoA-I
ApoA-II does not activate LCAT

22. atherosclerosis
HDL and ApoA-I negatively correlated with atherosclerosis
ApoA-II positively correlated with atherosclerosis
Deposits of fat and cholesterol building up in lining of arteries
Atherosclerosis->cardiovascular heart disease

23. How HDL prevents Atherosclerosis
HDL transports cholesterol from peripheral tissues to liver for catabolism
LCAT converts cholesterol into cholesterol esters
ApoA-I needs to be bound to HDL to activate LCAT

24. Conclusion ApoA-I HDL Activate LCAT Helps prevent atherosclerosis
ApoA-II
Does not activate
LCAT
Leads to higher
Levels of
Atherosclerosis
HDL

25. References
The medical biochemistry page
Molecular biochemsitry, Joyce J. Diwan
Bolanos-Garcia et al., 2003.  On the structure and function of apolipoproteins: more than a family of lipid-binding proteins. Progress in Biophysics and Molecular Biology. 83:47-68.
Borhani et al., Crystal structure of truncated human apolipoprotein A-I suggests a lipid-bound conformation. Proc. Natl. Acad. Sci. 94:
Kumar et al., Structures of Apolipoprotein A-II and a lipid-surrogate complex provide insights into Apolipoprotien-lipid interactions. Biochemistry. 41:
Mahley et al. 1984. Plasma lipoproteins: apolipoprotein structure and function. Journal of lipid research. 25: