Molecular organisation of the chlorophyll a/c light-harvesting complex of Pleurochloris meiringensis (Xanthophyceae). Pigment binding and secondary structure of the protein

Abstract

In our previous study by means of circular dichroism (CD) spectroscopy we have shown that the pigment organisation in the chlorophyll a/c light harvesting complex (Chl a/c LHC) isolated from Pleurochloris meiringensis significantly differs from the architecture of the main Chl a/b light harvesting antenna complexes of higher plants (Büchel and Garab. J. Photochem. Photobiol., B: Biol., 37 (1997) 118–124). In this work we measured the CD spectra in the far-UV, between 190 nm and 240 nm. and investigated the variations of the secondary structure of the protein upon treatments which affect the binding of the long wavelength Chl a. We found that low concentrations ( < 5%) of acetone, which had no noticeable effect on the (−)679 nm CD band, drastically reduced the α-helical content of the protein. In contrast, amounts of digitonin, which completely abolished this intense, non-conservative CD band of Chl a, induced only minor changes in the secondary structure of the protein complex. These data suggest that the binding site of the long-wavelength absorbing Chl a is found in a position deeply buried in the protein, and it is most likely co-ordinated by two helices.