Properties of Rhodobacter sphaeroides Photosynthetic Reaction Center with Double Amino Acid Substitution I(L177)H+H(M182)L

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T. Yu. Fufina, L. G. Vasilieva^*, R. A. Khatypov, and V. A. Shuvalov

Institute of Basic Biological Problems, Russian Academy of Sciences, ul. Institutskaya 2, 142290 Pushchino, Moscow Region, Russia; fax: (496) 733-0532; E-mail: vsyulya@mail.ru

^* To whom correspondence should be addressed.

Received July 27, 2010; Revision received October 6, 2010

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Histidine M182 in the reaction center (RC) of Rhodobacter sphaeroides serves as the fifth ligand of the bacteriochlorophyll (BChl) B_B Mg atom. When this His is substituted by an amino acid that is not able to coordinate Mg, bacteriopheophytin appears in the B_B 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 B_B 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 B_B 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 B_B 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.

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KEY WORDS: bacterial photosynthesis, Rhodobacter sphaeroides, photosynthetic reaction center, site-directed mutagenesis, bacteriochlorophyll, Mg coordination

DOI: 10.1134/S0006297911040079

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