Efficient exchange of the primary quinone acceptor QA in isolated reaction centers of Rhodopseudomonas viridis

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The National Academy of Sciences of the USA

RESUMO

A key step in the conversion of solar energy into chemical energy by photosynthetic reaction centers (RCs) occurs at the level of the two quinones, QA and QB, where electron transfer couples to proton transfer. A great deal of our understanding of the mechanisms of these coupled reactions relies on the seminal work of Okamura et al. [Okamura, M. Y., Isaacson, R. A., & Feher, G. (1975) Proc. Natl. Acad. Sci. USA 88, 3491–3495], who were able to extract with detergents the firmly bound ubiquinone QA from the RC of Rhodobacter sphaeroides and reconstitute the site with extraneous quinones. Up to now a comparable protocol was lacking for the RC of Rhodopseudomonas viridis despite the fact that its QA site, which contains 2-methyl-3-nonaprenyl-1,4-naphthoquinone (menaquinone-9), has provided the best x-ray structure available. Fourier transform infrared difference spectroscopy, together with the use of isotopically labeled quinones, can probe the interaction of QA with the RC protein. We establish that a simple incubation procedure of isolated RCs of Rp. viridis with an excess of extraneous quinone allows the menaquinone-9 in the QA site to be almost quantitatively replaced either by vitamin K1, a close analogue of menaquinone-9, or by ubiquinone. To our knowledge, this is the first report of quinone exchange in bacterial photosynthesis. The Fourier transform infrared data on the quinone and semiquinone vibrations show a close similarity in the bonding interactions of vitamin K1 with the protein at the QA site of Rp. viridis and Rb. sphaeroides, whereas for ubiquinone these interactions are significantly different. The results are interpreted in terms of slightly inequivalent quinone–protein interactions by comparison with the crystallographic data available for the QA site of the two RCs.

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