Surface-Core Relationships in Human Low Density Lipoprotein as Studied by Infrared Spectroscopy

The secondary structure of human apolipoprotein B at 37°C is estimated to be 24% α-helix, 23% β-sheet, 6% β-turns, 24% unordered structure, and 24% “β-strands,” characterized by a band around 1618 cm , and consistent with extended string-like chains in contact with the lipid moiety not forming β-sheets. When cooled to a temperature below the cholesteryl ester transition at 30°C, the ordering of the low density lipoprotein core results in reversible changes in the protein conformation, decreasing the apparent amount of α-helix, β-strand, and unordered structure below 30°C and increasing β-sheet and β-turns. Lowering the ionic strength affects the core-associated transitions, shifting their temperature from 30 to 20°C, and modifying protein conformation below the transition. An additional thermal event is observed at 75°C, leading to irreversible protein denaturation. In the broad temperature range between the 30 and 75°C transitions, apolipoprotein B is stable toward both temperature and ionic strength changes. After thermal denaturation, the protein retains a certain degree of ordered structure.