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Circular dichroism of c phycoerythrin a conformational analysis


, : Circular dichroism of c phycoerythrin a conformational analysis. Zeitschrift fuer Naturforschung Section C Journal of Biosciences 35(5-6): 367-375

An extensive study of the chiroptical properties of [Pseudanabaena W1173] C-phycoerythrin [C-PE] and the .alpha.- and .beta.-subunits in the spectral region from 700-200 nm is presented. Based on the vis[visual spectrum]-circular dichroism [CD] inherently chiral conformations are proposed for the covalently linked chromophores. By mean residue ellipticities and the experimental CD spectra in the region of the N .fwdarw. .pi.* peptide transition the .alpha.-helix contents of the apoproteins of the .alpha.- and .beta.-subunits are estimated to amount to 60% and 40%, respectively. The CD spectrum of native C-phycoerythrin is congruent with a linear superposition of the .alpha.- and .beta.-subspectra, in the whole spectral region studied. Since .alpha.- and .beta.-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.degree.-40.degree. C the thermally induced changes of the chromophores in native C-phycoerythrin are not associated with changes of the secondary structure of the apoprotein. Unfolding occurs at 60.degree.-70.degree. C, but slowly leads to irreversible denaturation. Protein unfolding starts at 3 M urea. The random coil secondary structure of the apoproteins is reached at 8 M urea. At this concentration the absorbance and the optical activity of the chromophores 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 chromophore indicating that a multistep process is operative during unfolding. The CD results on denaturation are supplemented by absorption and emission spectroscopy.

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