Abstract
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.
Highlights
From the Wepartamento de Bioquimica, Uniuersidad del Pais Vasco, Apartado 644, E-4BOBO Bilbao, Spain and the IlRuder Boshouic Institute, Bijenicha 54, 41001 Zagreb, Croatia
In the broad temperature range between the 30 and 75°C transitions, apolipoprotein B is stable toward both temperature and ionic strength changes
Human serum low density lipoprotein (LDL)1 is a major carrier of serum cholesterol in humans
Summary
From the Wepartamento de Bioquimica, Uniuersidad del Pais Vasco, Apartado 644, E-4BOBO Bilbao, Spain and the IlRuder Boshouic Institute, Bijenicha 54, 41001 Zagreb, Croatia. 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 a-helix, p-strand, and unordered structure below 30°C and increasing p-sheet and p-turns. Human serum low density lipoprotein (LDL) is a major carrier of serum cholesterol in humans It is described as a spherical particle containing a hydrophobic core of cholesteryl esters and triglycerides surrounded by an amphipathic monolayer of phospholipid and cholesterol in which a single molecule of apoB is located [1,2,3]. ApoB is one of the largest proteins known, containing 4536 amino acid residues, and is extremely insoluble in aqueous media. 1 The abbreviations used are: LDL, low density lipoprotein; apoB, apolipoprotein B-I00; IR, infrared spectroscopy Tel.: 34-4-464-7700 (ext. 2407); Fax: 34-4-464-8500. 1 The abbreviations used are: LDL, low density lipoprotein; apoB, apolipoprotein B-I00; IR, infrared spectroscopy
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