Abstract
A comparison of the protein structure in chromatophores, quantasomes and reaction centres of Rhodopseudomonas viridis is made by investigating ultraviolet circular dichroism and polarized infrared spectra of the intact chromatophore membrane, the isolated quantasome and the reaction centre reconstituted into phosphatidyl choline vesicles. A quantasome (photoreceptor unit) is made of a reaction centre surrounded by a ring of antenna complexes. Conformational analysis of the circular dichroism data indicates that intact chromatophores and quantasomes contain more α-helical structure (57%) than reaction centres (47%). Infrared dichroism data show that α-helical segments are on the average closer to the membrane normal in quantasomes ( ф α = 28° ) than in reaction centres ( ф α = 38°C ). This suggests a higher content of α-helices and a better orientation of transmembrane α-helical rods in the antennae surrounding the reaction centre. Our data are discussed in view of the results previously obtained by infrared dichroism of reaction centre films of Rps. sphaeroides (Nabedryk, E., Tiede, D.M., Dutton, P.L. and Breton, J. (1984) in Advances in Photosynthesis Research (Sybesma, C., ed.), Vol. II, pp. 177–180, Martinus Nijhoff/Dr. W. Junk Publishers, Dordrecht, The Netherlands) and by X-rays of reaction centre crystals of Rps. viridis (Deisenhofer, J., Epp, O., Miki, K., Huber, R. and Michel, H. (1984) J. Mol. Biol. 180, 385–398). Our results also imply that the secondary structure and orientation of the protein is comparable in bacteriochlorophyll a- and bacteriochlorophyll b-containing reaction centres. Furthermore, our data on the orientation of the α-helices in the reaction centre of Rps. viridis imply that the C-2 symmetry axis observed in the model derived from X-ray crystallography is oriented in vivo along the normal to the membrane plane.
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