Conformational Studies of Biliproteins from the Insects Pieris brassicae and Cerura vinula

Chromophore conformation and protein secondary structure of biliproteins from the butterfly, Pieris brassicae, and the moth, Cerura vinula, have been investigated by absorption, circular dichroism and fluorescence spectroscopy. The chromophore of the P. brassicae protein, biliverdin IXy, has probably a cyclic-helical structure similar to that of free bile pigments of the biliverdin type. Though achiral by structure the chromophore displays strong optical activity in the native protein-bound state, but becomes inactive after urea denaturation of the protein. A minor biliprotein from P. brassicae shows absorption, circular dichroism and fluorescence spectra identical to the main biliprotein. In the biliprotein from Cerura vinula the structure of the pigment is still unknown. It has a semi-open conformation intermediate between that of the Pieris proteins and that of the phycobiliprotein, C-phycocyanin, and it retains optical activity after urea denaturation. The band widths and the size of the Stokes shifts of the fluorescence spectra indicate a high degree of conformational flexibility of the chromophores in the two Pieris pigments, and a decreased flexibility in the one from Cerura. In the biliproteins from both insects the polypeptides are low in a-helix content compared to that of phycobiliproteins. From these and earlier data, insect and algal biliproteins seem to be related only distantly if at all, but there exist also considerable differences among insect biliproteins from different species.