THE STRUCTURAL ROLE OF THE CAROTENOID IN THE BACTERIAL LIGHT-HARVESTING PROTEIN-2 (LH2) OF RHODOBACTER-CAPSULATUS - A FOURIER-TRANSFORM RAMAN-SPECTROSCOPY AND CIRCULAR-DICHROISM STUDY

In previous work (Zurdo J, Fernandez-Cabrera C and Ramirez JM (1993) B iochem J 290: 531-537), it had been shown that selective extraction of the carotenoid from the light-harvesting protein 2 (LH2) of Rhodobact er capsulatus induced the dissociation of 800-nm absorbing bacteriochl orophyll (Bchl), a 10-nm red shift of 854-nm Bchl, and a decrease of t he stability of the protein in detergent solution. In the present stud y, the Fourier transform Raman and near-infrared circular dichroism sp ectra of native and carotenoid-depleted LH2 membrane preparations were compared. It was found that while the coupled carbonyls of 854-nm Bch l remained specifically H-bonded to the peptides after carotenoid extr action, the optical activity of the near-infrared electronic transitio n was significantly altered. Given the excitonic origin of such optica l activity, our data suggest that carotenoid extraction elicits a rear rengement of the chromophore cluster and of the associated polypeptide subunits. This implies a significant role of the carotenoid in mainta ining the native quaternary structure of the protein, which would be c onsistent with the observed dissociation of 800-nm Bchl and the loss o f solubilized LH2 stability that result from carotenoid removal. There is no evidence for a similar role of the carotenoid in the LH1 protei n.