| Abstract
| - Reaction centers with the double mutation Phe M197 to Arg and Gly M203 to Asp (FM197R/GM203D) have been crystallized from an antenna-deficient strain of Rhodobacter sphaeroides, and thestructure has been determined at 2.7 Å resolution. Unlike in reaction centers with a single FM197R mutation,the Arg M197 residue in the FM197R/GM203D reaction center adopts a position similar to that of thenative Phe residue in the wild-type reaction center. Asp M203 is packed in such a way that the γ-carboxygroup interacts with the backbone carbonyl of Arg M197. The Asp M203 residue takes up part of thevolume that is occupied in the wild-type reaction center by a water molecule. This water has been proposedto form a hydrogen bond interaction with the 9-keto carbonyl group of the active branch accessorybacteriochlorophyll, particularly when the primary donor bacteriochlorophylls are oxidized. The GM203Dmutation therefore appears to remove the possibility of this hydrogen bond interaction by exclusion ofthis water molecule, as well as altering the local dielectric environment of the 9-keto carbonyl group. Weexamine whether the observed structural changes can provide new or alternative explanations for theabsorbance and electron-transfer properties of reaction centers with the FM197R and GM203D mutations.
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