HELIX HELIX INTERACTIONS IN RECONSTITUTED HIGH-DENSITY-LIPOPROTEINS

In this work we calculated the ionic interactions between adjacent amphipathic helices of apo A-I and apo A-IV. The calculation of the electrostatic potential around the helices helps identify the charged residues susceptible to form salt bridges between adjacent helices. An estimation of the stability of the different pairs of helices is derived from the calculation of the energy of interaction between contiguous helices at a water/lipid interface after energy minimization. The most stable energetic conformation corresponds to the 17-residue helices oriented anti-parallel and separated by a stretch of 5 residues in an extended beta-strand conformation, as calculated through the 'stereoalphabet' calculation procedure. In a pair of helices, the hydrophobic faces are directed towards the lipid core of the discoidal phospholipid-apolipoprotein complex and the hydrophobic lipid-protein interactions are major determinants for the stability of the complex. Interactions between polar residues located on the opposite face of the helix and water molecules can also contribute to the overall energy of the system. Finally, salt bridge formation between residues of opposite charge along the edge of the helical segments contribute to the cooperativity of the phospholipid-apolipoprotein complex formation. The mode of assembly of the amphipathic helical repeats of the apolipoproteins around the edge of a discoidal complex is therefore determined both by the hydrophobic character of the residues and by the charge complementarity along the edge of the helices which increases the structural stability and determines the relative orientation of the helices. Although these helices are too hydrophilic to span the phospholipid bilayer, their arrangement around the edge of the phospholipid bilayer is determined by the principles described for transmembrane helices. Our data suggest that ionic interactions can contribute to the stabilization of the protein structure in reconstituted HDL and to the cooperative interactions between the helices of apo A-I and A-IV and the lipids.