Circular Dichroism of C-Phycoerythrin: A Conformational Analysis

An extensive study o f the chiroptical properties o f C-phycoerythrin and the aand ^-subunits in the spectral region from 700 200 nm is presented. Based on the VIS-circular dichroism inherently chiral conformations are proposed for the co­ valently linked chromophores. By means o f mean residue ellipticities and the experimental circular dichroism spectra in the region o f the n -* n* peptide transition the a-helix contents o f the apoproteins o f the acand ßsubunits are estimated to amount to 60% and 40%, respectively. The circular dichroism spectrum o f native C-phycoerythrin is congruent with a linear superposition o f the aand /?-subspectra, in the whole spectral region studied. Since aand /?-subunits are associated in native C-phycoerythrin as revealed by sedimentation analysis the interactions between the subunits in the native chromoprotein are not accompanied by substantial conformational changes. In the temperature range 0 ° 4 0 °C the thermally induced changes o f the chromophores in native C-phycoerythrin are not associated with changes o f the secondary structure o f the apoprotein. Unfolding occurs at 60 0 7 0 °C but slowly leads to irreversible denaturation. Protein unfolding starts at 3 M urea. The random coil secondary structure o f the apoproteins is reached at 8 M urea. At this concentration the absorbance and the optical activity o f the chromo­ phores are reduced by a factor 3 and 10, respectively. The conformational changes in the peptide with increasing denaturant concentration are not synchronous with those induced in the Chromo­ phore indicating that a multistep process is operative during unfolding. The CD results on dena­ turation are supplemented by absorption and emission spectroscopy.