Electrostatic interactions in an integral membrane protein

The effects of charge-charge interactions on the midpoint reduction potential (E(m)()) of the primary electron donor (P) in the photosynthetic reaction center of Rhodobacter sphaeroides were investigated by introducing mutations of ionizable amino acids at selected sites. The mutations were designed to alter the electrostatic environment of P, a bacteriochlorophyll dimer, without greatly affecting its structure or molecular orbitals. Two arginine residues at homologous positions in the L and M subunits [residues (L135) and (M164)], Asp (L155), Tyr (L164), and Cys (L247) were changed independently. Arginine (L135) was replaced by Lys, Leu, Gln, or Glu; Arg (M164), by Leu or Glu; Asp (L155), by Asn; Tyr (L164), by Phe; and Cys (L247), by Lys or Asp. The R(L135)E/C(L247)K double mutant also was made. The shift in the E(m)() of P/P(+) was measured in each mutant and was compared with the effect predicted by electrostatics calculations using several different computational approaches. A simple distance-dependent dielectric screening factor reproduced the effects remarkably well. By contrast, microscopic methods that considered the reaction field in the protein and solvent but did not include explicit counterions overestimated the changes in the E(m)() considerably. Including counterions for the charged residues reduced the calculated effects of the mutations in molecular dynamics calculations. The results show that electrostatic interactions of P with ionizable amino acid residues are strongly screened, and suggest that counterions make major contributions to this screening. The screening also could reflect penetration of water or other relaxations not taken into account because of incomplete sampling of configurational space.