X-ray Structure Analyses of Photosynthetic Reaction Center Variants from Rhodobacter sphaeroides: Structural Changes Induced by Point Mutations at Position L209 Modulate Electron and Proton Transfer

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Abstract
The structures of the reaction center variants Pro L209 → Tyr, Pro L209 → Phe, and Pro L209 → Glu from the photosynthetic purple bacterium Rhodobacter sphaeroides have been determined by X-ray crystallography to 2.6−2.8 Å resolution. These variants were constructed to interrupt a chain of tightly bound water molecules that was assumed to facilitate proton transfer from the cytoplasm to the secondary quinone QB [Baciou, L., and Michel, H. (1995) Biochemistry34, 7967−7972]. However, the amino acid exchanges Pro L209 → Tyr and Pro L209 → Phe do not interrupt the water chain. Both aromatic side chains are oriented away from this water chain and interact with three surrounding polar side chains (Asp L213, Thr L226, and Glu H173) which are displaced by up to 2.6 Å. The conformational changes induced by the bulky aromatic rings of Tyr L209 and Phe L209 lead to unexpected displacements of QB compared to the wild-type protein. In the structure of the Pro L209 → Tyr variant, QB is shifted by ∼4 Å and is now located at a position similar to that reported for the wild-type reaction center after illumination [Stowell, M. H. B., et al. (1997) Science276, 812−816]. In the Pro L209 → Phe variant, the electron density map reveals an intermediate QB position between the binding sites of the wild-type protein in the dark and the Pro L209 → Tyr protein. In the Pro L209 → Glu reaction center, the carboxylic side chain of Glu L209 is located within the water chain, and the binding site of QB remains unchanged compared to the wild-type structure.