Circular dichroism study of horse colipase interaction with bile salt

CD studies have been carried out on horse pancreatic colipases which contain a tryptophan in place of the phenylalanine found at position 52 in the pig protein. From the CD spectra of native proteins in the far ultraviolet region, it was estimated that the secondary structure consists of equal amounts of β-structure and aperiodic arrangements. Colipases contain no α-helical structure. The CD spectra of horse and pig colipases display a positive band at 226 nm the pH dependence of which is characteristic of aromatic chromophores. The near ultraviolet region of the CD spectra of horse and pig colipases contains well-resolved bands at 283–284 nm and 277–278 nm, which corresponds to the tyrosines, with one band at 266–268 nm and one shoulder at 261–263 nm reflecting the contribution of the phenylalanine residues. Comparative analysis of the contribution of sidechains in the aromatic region of the spectra is consistent with a lower solvent accessibility of the lone tryptophan of horse colipase B as compared to isocolipase A. Study of the near ultraviolet CD spectrum of colipase B in the presence of micellar concentration of sodium taurodeoxycholate reveals that the tryptophan (Trp₅₂) moves towards a more hydrophobic environment upon the formation of the colipase-bile salt complex. These results support the conclusion that this residue belongs to the previously identified hydrophobic domain of the colipase molecule which interacts with organized lipids (lipid binding site). The conservative substitution of this residue for a phenylalanine in the pig protein suggests that the aromatic structure might be of critical importance for the binding of colipase to the lipid-water interface.