Small-angle neutron scattering study of the association between porcine pancreatic colipase and taurodeoxycholate micelles☆☆☆

Abstract Small-angle neutron scattering studies have shown the association of porcine colipase with bile salts micelles to be a lateral one. The molecular structure parameters of the individual components were determined first. A radius of gyration of 13.9 A is found for colipase, which implies a non-spherical shape for this molecule. The size of taurodeoxycholate micelles is controlled by the ionic strength of the solution. In 0.15 m -NaCl their volume is comparable to that of colipase; they are elongated with an axial ratio of about 2. At higher ionic strengths the elongation of the micelles increases. In 0.15 m -NaCl the complex is found to be an association of one colipase molecule with a volume of detergent corresponding to that of one free micelle. The contrast variation study of the radius of gyration shows that in the complex the centre of masses of the protein and of the detergent are well-separated: a distance between 29 and 45 A has been estimated. The value of the radius of gyration of the complex at high contrast, and the agreement between the contrast variation analysis and a straightforward application of the parallel axes theorem indicate that the complex is formed by the juxtaposition of the protein and a preformed micelle, which has approximately the same size and shape as a free micelle. There is only one localized surface contact between the protein and the micelle, which implies that colipase possesses a relatively well-defined binding site.