Apolipoproteins

1. Apolipoproteins

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

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

17. ApoA-II-BOG 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-I ApoA-II Does not activate LCAT Leads to higher Levels of Atherosclerosis HDL

25. References • The medical biochemistry page http://www.indstate.edu/thcme/mwking/lipoproteins.html • Molecular biochemsitry, Joyce J. Diwan http://www.rpi.edu/dept/bcbp/molbiochem/MBWeb/mb2/part1/lipoprot.htm • 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., 1997. Crystal structure of truncated human apolipoprotein A-I suggests a lipid-bound conformation. Proc. Natl. Acad. Sci. 94:12291-12296. • Kumar et al., 2002. Structures of Apolipoprotein A-II and a lipid-surrogate complex provide insights into Apolipoprotien-lipid interactions. Biochemistry. 41:11681-11691. • Mahley et al. 1984. Plasma lipoproteins: apolipoprotein structure and function. Journal of lipid research. 25:1277-1294.