Association of lipids with milk α- and β-caseins

We report the molecular interaction and the binding sites of cholesterol (CHOL), 1,2-dioleoyl-3-trimethylammonium-propane (DOTAP), dioctadecyldimethyl-ammoniumbromide (DDAB), and dioleoylphosphatidylethanolamine (DOPE) with milk alpha- and beta-caseins in aquous solution at physiological conditions. Fourier transform infrared (FTIR), fluorescence spectroscopic methods and molecular modeling were used to determine the binding sites of lipid-protein complexes and the effect of lipid interaction on the stability and conformation of alpha- and beta-caseins. Structural analysis showed that lipids bind casein via mainly hydrophobic contact with association constants of KCHOL-alpha-casein =1.0 (+/- 0.1) X 10(4) M-1KDOPE-alpha-casein = 5.0 (+/- 0.07)x 10(3)M(-1), KDDAB-alpha-casein = 2.0 (+/- 0.06) x 10(4) M-1, KDOTAP-alpha-casein = 1.5 (+/- 0.6) x 10(3) M-1, (+/- 0.3) x 10(4) M-1, KDOPE-beta-casein = 1.5 (+/- 0.06) x 10(3) M-1, KDDAB-beta-casein = 1.7 (+/- 0.3)x10(4) M-1 and KDOTAP-beta-casein = 2.1 ( 0.5) x 104 M-1. The average number of binding sites occupied by lipid molecules on protein (n) were from 0.7 to 1.1. Docking showed different binding sites for alpha- and beta-caseins toward lipid complexation with the free binding energies from -10 to -13 kcal/mol. Casein conformation was altered by lipid interaction with a reduction of a-helix and beta-sheet and an increase of random coil and turn structure suggesting a partial protein unfolding.Abbreviations: Cascasein; CHOLcholesterol; DOTAP1,2-dioleoyl-3-trimethylammonium-propane; DDABdioctadecyldimethylammonium bromide; DOPEdioleoylphosphatidylethanolamine; FTIRFourier transform infrared spectroscopy; CDcircular dichroism. (C) 2014 Elsevier B.V. All rights reserved.