Surface-core relationships in human low density lipoprotein as studied by infrared spectroscopy.

The secondary structure of human apolipoprotein B at 37 degrees C is estimated to be 24% alpha-helix, 23% beta-sheet, 6% beta-turns, 24% unordered structure, and 24% "beta-strands," characterized by a band around 1618 cm-1, and consistent with extended string-like chains in contact with the lipid moiety not forming beta-sheets. When cooled to a temperature below the cholesteryl ester transition at 30 degrees C, the ordering of the low density lipoprotein core results in reversible changes in the protein conformation, decreasing the apparent amount of alpha-helix, beta-strand, and unordered structure below 30 degrees C and increasing beta-sheet and beta-turns. Lowering the ionic strength affects the core-associated transitions, shifting their temperature from 30 to 20 degrees C, and modifying protein conformation below the transition. An additional thermal event is observed at 75 degrees C, leading to irreversible protein denaturation. In the broad temperature range between the 30 and 75 degrees 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.