SPECTROSCOPIC STUDY OF THE MECHANISM OF INTERACTION OF CHOLESTEROL WITH APOLIPOPROTEINS AS SIMULATED BY A SYNTHETIC FRAGMENT OF APOLIPOPROTEIN-A-I

To investigate the mechanism of cholesterol binding to apolipoproteins , the fragment of human apoprotein A-I containing amino acid residues 144-164 was synthesized. The interaction of this fragment (native and with modified functional groups) with cholesterol in alcohol-water mix tures was studied by fluorescence, circular dichroism, and NMR NOE spe ctroscopy. The fragment was found to form complexes with one and two c holesterol molecules, the association constant being as high as simila r to 10(8) M(-1). The interaction is due to hydrogen bonds formed by t he cholesterol OH-group and hydrophobic interaction of nonpolar groups of cholesterol and the peptide. This interaction requires a correspon ding conformation of the peptide, involving niches which are formed in the peptide molecule by salt bridge formation between carboxylate gro ups of side chains of Asp or Glu and guanidinium groups of Arg residue s of the peptide. At pH 6.3, the OH-group of cholesterol was found to include an H-bonded system involving a carboxylate group of Asp, one n eutral imidazole ring, and one protonated imidazole ring. At pH < 5, w hen all imidazole groups of His residues are protonated, the H-bonded system for binding the cholesterol OH-group includes Arg and Asp side chains.