title: Fluorescence quenching studies of apolipoprotein A-I in solution and in lipid-protein complexes: protein dynamics.
abstract: Fluorescence lifetime and intensity quenching studies of human plasma apolipoprotein A-I (apo A-I) in aqueous solution and in recombinant lipoprotein complexes with dimyristoylphosphatidylcholine (DMPC) indicate differences in conformational dynamics. In aqueous solution, the bimolecular quenching constants (k*) for lipid-free apo A-I fluorescence quenching by oxygen and acrylamide are 2.4 X 10(9) and 0.38 X 10(9) M-1 s-1, respectively. These values are independent of the oligomeric form of the protein. There is no correlation between the relatively small k* for apo A-I, which reflects rapid, low-amplitude protein fluctuations, and the labile conformational changes of apo A-I folding reactions, like denaturation, which occur on a slower time scale. In recombinant DMPC/apo A-I complexes (100:1 molar ratio) the protein increases in amphiphilic alpha-helical structure as it blankets the lipid matrix. The apparent k* for oxygen quenching of apo A-I fluorescence in the complex is large and increases in a temperature-dependent manner. We have introduced a two-compartment model, which discriminates the source of quencher molecules as aqueous or lipid, to describe oxygen quenching of DMPC/apo A-I fluorescence. The magnitude and temperature dependence of the apparent k* predominantly reflect the partitioning of oxygen between the two phases rather than being a probe of the lipid physical state. Calculations of the helical hydrophobic moment in apo A-I indicate that tryptophan residues 8 and 72 occur at the lipid-protein interface of amphiphilic alpha-helices, whereas the other two tryptophan residues (50, 108) lie on the nonpolar faces of amphiphilic helices.(ABSTRACT TRUNCATED AT 250 WORDS)
mesh_terms: D000595:Amino Acid Sequence; D016632:Apolipoprotein A-I; D001054:Apolipoproteins A / Q000097:blood*; D004134:Dimyristoylphosphatidylcholine; D006801:Humans; D007700:Kinetics; D008075:Lipoproteins, HDL / Q000097:blood*; D008433:Mathematics; D011487:Protein Conformation; D012996:Solutions; D013050:Spectrometry, Fluorescence; D013816:Thermodynamics
chemical_list: D016632:Apolipoprotein A-I; D001054:Apolipoproteins A; D008075:Lipoproteins, HDL; D012996:Solutions; D004134:Dimyristoylphosphatidylcholine