Reversible folding reactions of human apolipoprotein A-I: pressure and guanidinium chloride effects.
Apolipoprotein A-I, the major structural polypeptide of human high-density lipoproteins, activates lecithin: cholesterol acyltransferase, the cholesterol ester-forming enzyme in plasma. Apolipoprotein A-I, like several other apolipoproteins, exhibits structural adaptability, which is manifest in a low free energy of stabilization and facile changes in secondary structure. We have investigated the dual effects of guanidinium chloride (GdmCl) and pressure perturbation at low GdmCl concentrations on apolipoproteins A-I conformational states, using fluorescence detection. Pressure alone (up to 3 kilobar) is insufficient to fully denature apolipoprotein A-I, and results in formation of metastable state(s). However, in conjunction with low concentrations of GdmCl the calculated volume change upon pressure denaturation increases from approx. -50 ml/mol to -90 ml/mol. The free energy of denaturation by pressure perturbation ranges from 1.4 to 1.8 kcal/mol, but the conformational states induced by pressure and GdmCl perturbation are most likely different. The physico-chemical properties of native and pressure-denatured conformational states can be, readily and reversibly, measured by fluorescence techniques. Biological activity of apolipoprotein A-I in the form of lecithin: cholesterol acyltransferase activation, is also reversible upon pressure perturbation. Samples of apolipoprotein A-I exposed to 2 kbar for an hour activated lecithin: cholesterol acyltransferase equally well as controls. To delineate more precisely the conformational states of apolipoprotein A-I under pressure, time-dependent anisotropy decay measurements, capable of resolving rotational heterogeneity, will be required.
D016632:Apolipoprotein A-I; D001054:Apolipoproteins A / Q000494:pharmacology*; D055598:Chemical Phenomena; D002621:Chemistry; D004789:Enzyme Activation / Q000187:drug effects; D005454:Fluorescence Polarization; D019791:Guanidine; D006146:Guanidines; D006801:Humans; D007862:Phosphatidylcholine-Sterol O-Acyltransferase / Q000378:metabolism*; D011312:Pressure; D011487:Protein Conformation; D011489:Protein Denaturation; D013329:Structure-Activity Relationship; D013816:Thermodynamics
D016632:Apolipoprotein A-I; D001054:Apolipoproteins A; D006146:Guanidines; D007862:Phosphatidylcholine-Sterol O-Acyltransferase; D019791:Guanidine