Structural consequences of the replacement of glycine M203 with aspartic acid in the reaction center from Rhodobacter sphaeroides

Reaction centers with the double mutation Phe M197 to Arg and Gly M203 to A sp (FM197R/ GM203D) have been crystallized from an antenna-deficient strain of Rhodobacter sphaeroides, and the structure has been determined at 2.7 A ngstrom resolution. Unlike in reaction centers with a single FM197R mutatio n, the Arg M197 residue in the FM197R/GM203D reaction center adopts a posit ion similar to that of the native Phe residue in the wild-type reaction cen ter. Asp M203 is packed in such a way that the gamma-carboxy group interact s with the backbone carbonyl of Arg M197. The Asp M203 residue takes up par t of the volume that is occupied in the wild-type reaction center by a wate r molecule. This water has been proposed to form a hydrogen bond interactio n with the 9-keto carbonyl group of the active branch accessory bacteriochl orophyll, particularly when the primary donor bacteriochlorophylls are oxid ized. The GM203D mutation therefore appears to remove the possibility of th is hydrogen bond interaction by exclusion of this water molecule, as well a s altering the local dielectric environment of the 9-keto carbonyl group. W e examine whether the observed structural changes can provide new or altern ative explanations for the absorbance and electron-transfer properties of r eaction centers with the FM197R and GM203D mutations.