* To whom correspondence should be addressed.
Received July 27, 2010; Revision received October 6, 2010
Histidine M182 in the reaction center (RC) of Rhodobacter sphaeroides serves as the fifth ligand of the bacteriochlorophyll (BChl) BB Mg atom. When this His is substituted by an amino acid that is not able to coordinate Mg, bacteriopheophytin appears in the BB binding site instead of BChl (Katilius, E., et al. (1999) J. Phys. Chem. B, 103, 7386-7389). We have shown that in the presence of the additional mutation I(L177)H the coordination of the BChl BB Mg atom in the double mutant I(L177)H+H(M182)L RC still remains. Changes in the double mutant RC absorption spectrum attributed to BChl absorption suggest that BChl BB Mg atom axial ligation might be realized not from the usual α-side of the BChl macrocycle, but from the opposite, β-side. Weaker coordination of BChl BB Mg atom compared to the other mutant RC BChl molecules suggests that not an amino acid residue but a water molecule might be a possible ligand. The results are discussed in the light of the structural changes that occurred in the RC upon Ile/His substitution in the L177 position.
KEY WORDS: bacterial photosynthesis, Rhodobacter sphaeroides, photosynthetic reaction center, site-directed mutagenesis, bacteriochlorophyll, Mg coordination