Conformational and thermodynamic properties of apo A-1 of human plasma high density lipoproteins.

作者：Alan R. Tall, G. Graham Shipley, DONALD M. SMALL · 发表于：Journal of Biological Chemistry · 年份：1976 · DOI：10.1016/s0021-9258(17)33407-5 · 被引用次数：100 · 研究领域：Computational Drug Discovery Methods、Plant biochemistry and biosynthesis、Protein Structure and Dynamics

Conformational changes of apo A-1, the principal apoprotein of human plasma high density lipoprotein, have been studied by differential scanning calorimetry and ultraviolet difference spectroscopy as a function of temperature, pH, concentration of apoprotein, and urea concentration. Calorimetry shows that apo A-1 (5 to 40 mg/ml, pH 9.2) undergoes a two-state, reversible denaturation (enthalpy = 64 +/- 8.9 kcal/mole), between 43--71 degrees (midpoint temperature, Tm = 54 degrees), associated with a rise in heat capacity (deltaCvd) of 2.4 +/- 0.5 kcal/mole/degrees C. Apo A-1 (0.2 to 0.4 mg/ml, pH 9.2) develops a negative difference spectrum between 42--70 degrees, with Tm = 53 degrees. The enthalpy (deltaH = 59 +/- 5.7 kcal/mole at Tm) and heat capacity change (2.7 +/- 0.9 kcal/mole/degrees C) in the spectroscopic experiments were not significantly different from the calorimetric values. Below pH 9 and above pH 11, the calorimetric Tm and deltaH of denaturation are decreased. In the pH range of reversible denaturation (6.5 to 11.8), delatH and Tm are linearly related, showing that the heat capacity change (ddeltaH/dT) associated with denaturation is independent of Tm. In urea solutions, the calorimetric Tm and deltaH of denaturation are decreased. At 25 degrees, apo A-1 develops a negative difference spectrum between 1.4 and 3 M urea. Fifty per cent of the spectral change occurs in 2.4 M urea, which corresponds to the urea concentration obtained by extrapolation of the calorime...