Apolipoprotein amphipathic helices, properties

The key structural features predicted for the amphipathic helix by the original model (Segrest et al., 1974) enabled three laboratories to study independently how amino acid variability determined the properties of the amphipathic helix (Kanellis et al., 1980 Fukushima et al., 1980 Sparrow et al., 1981). The strategy adapted by these investigators was based, not on the primary sequence of naturally occurring apolipoproteins, but on incorporating the periodicity of the secondary structural features of the amphipathic helix motif into the sequences of the peptide analogs. 

While lipoproteins are the products of many different genes, the major apolipoproteins share properties distinguishing them from most lipid-free and membrane-associated proteins. For example, apolipoproteins consist of a single polypeptide chain that has relatively little tertiary structure. Most apolipoproteins contain stretches of amphipathic alpha-helix, whose hydrophobic face can be turned to the lipid surface of the particle. The apolipoproteins are flexible, as is reflected in their unusually small free energy of unfolding. As these apolipoproteins expand and contract at the cell surface, different protein domains are exposed that are detectable with monoclonal antibodies. These properties reflect the role of apolipoproteins at the surface of lipoprotein particles whose size changes as they circulate. 

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