Apolipoproteins

Lipoprotein complexes Triaglycerol lipid droplets Cholesterol esters Phospholipids Apolipoproteins

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

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

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

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

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.

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

Dimer of ApoA-I Consists of two monomers arranged antiparallel

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

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

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

Apolipoprotein A-II Exchangeable apolipoprotein Associated with HDL

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

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

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

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

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)

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

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

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

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

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

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

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.