PARENT SESSION
Posters P5A Type II reaction centres : Structure. Abstracts (289-312)

Structural description of reaction center mutants that restore proton transfer to the l212ala-l213ala double mutant. Raj Pokkuluri^*,, Deborah Hanson¹, Phil Laible¹, Stephan Ginell¹, George Johnson¹, Marianne Schiffer¹, ¹ Biosciences Division, Argonne, IL, U.S.A

ABSTRACT- When acidic residues L212Glu and L213Asp near the secondary quinone, Q_B, of the photosynthetic reaction center (RC) are mutated to Ala, proton transfer to Q_B is interrupted. We have determined the structures of two RCs derived from photocompetent revertants of the photosynthetically incompetent double mutant L212Ala-L213Ala (AA): 1) the structure of AA+M5Asn to Asp revertant RC where an acidic residue was added, and 2) that of the AA+M233Arg to Leu revertant RC where a basic residue was removed. Although the second-site compensatory mutation sites are 15 Angstroms away from Q_B, these substitutions restore proton transfer by altering the electrostatic field near Q_B by changing side chain and main chain orientations and water positions. Our previously determined structure of the AA mutant RC was used in rigid body refinement for the determination of the structures of the revertant RCs. Preliminary fits of the structures to the difference electron density maps show that in the AA+M5Asp revertant RC, the location of M5Asp was changed leading to the reorientation of H194Gln; further, there are many small changes that propogate the influence of the compensatory mutation to Q_B. In the AA+M233Leu revertant RC, side chains of residues M236Glu, H118Arg, H122Glu and H177Arg shifted between 3 and 5 Angstroms. Rearrangement of the H177Arg side chain makes it more distant from H173Glu and H170Asp than in the AA structure. We have predicted (Biochemistry 34:8390, 1995) that charged residues will reorient as a result of the compensatory mutations in an 'electrostatic dominoes' effect that moves the effective negative charge close to Q_B to functionally compensate for the roles of the removed acidic residues L212Glu and L213Asp in proton delivery to the Q_B anion. This work is supported by NIH grant 1 R01 GM063849

KEY WORDS: electrostatic dominoes, proton transfer, RC mutant Structure, side chain rearrangements