In the photosynthetic reaction centre (RC) from the purple bacterium Rhodobacter sphaeroides, the primary quinone, a ubiquinone-10 (QA), has been substituted by anthraquinone. Three-dimensional crystals have been grown from the modified RC and its structure has been determined by X-ray crystallography to 2.4 Å resolution. The bindings of the head-group from ubiquinone-10 and of the anthraquinone ring are very similar. In particular, both rings are parallel to each other and the hydrogen bonds connecting the native ubiquinone-10 molecule to AlaM260 and HisM219 are conserved in the anthraquinone containing RC. The space of the phytyl tail missing in the anthraquinone exchanged RC is occupied by the alkyl chain of a detergent molecule. Other structural changes of the QA-binding site are within the limit of resolution. Our structural data bring strong credit to the very large amount of spectroscopic data previously achieved in anthraquinone-replaced RCs and which have participated in the determination of the energetics of the quinone system in bacterial RCs.
