Mutation H(M202)L does not lead to the formation of a heterodimer of the primary electron donor in reaction centers of Rhodobacter sphaeroides when combined with mutation I(M206)H

• Published in: Photosynthesis research 2020-12, Vol.146 (1-3), p.109-121
• Main Authors: Khristin, Anton M., Zabelin, Alexey A., Fufina, Tatiana Yu, Khatypov, Ravil A., Proskuryakov, Ivan I., Shuvalov, Vladimir A., Shkuropatov, Anatoly Ya, Vasilieva, Lyudmila G.
• Format: Article
• Language: English
• Subjects: Bacteriochlorophyll; Biochemistry; Biomedical and Life Sciences; Electron transfer; Histidine; Isoleucine; Leucine; Life Sciences; Ligands; Mutants; Mutation; Original Article; Plant Genetics and Genomics; Plant Physiology; Plant Sciences; Reaction centers; Titration
• ISSN: 0166-8595; 1573-5079
• DOI: 10.1007/s11120-020-00728-9

In photosynthetic reaction centers (RCs) of purple bacteria, conserved histidine residues [His L173 and His M202 in Rhodobacter ( Rba. ) sphaeroides ] are known to serve as fifth axial ligands to the central Mg atom of the bacteriochlorophyll (BChl) molecules (P A and P B , respectively) that constitute the homodimer (BChl/BChl) primary electron donor P. In a number of previous studies, it has been found that replacing these residues with leucine, which cannot serve as a ligand to the Mg ion of BChl, leads to the assembly of heterodimer RCs with P represented by the BChl/BPheo pair. Here, we show that a homodimer P is assembled in Rba. sphaeroides RCs if the mutation H(M202)L is combined with the mutation of isoleucine to histidine at position M206 located in the immediate vicinity of P B . The resulting mutant H(M202)L/I(M206)H RCs are characterized using pigment analysis, redox titration, and a number of spectroscopic methods. It is shown that, compared to wild-type RCs, the double mutation causes significant changes in the absorption spectrum of the P homodimer and the electronic structure of the radical cation P + , but has only minor effect on the pigment composition, the P/P + midpoint potential, and the initial electron-transfer reaction. The results are discussed in terms of the nature of the axial ligand to the Mg of P B in mutant H(M202)L/I(M206)H RCs and the possibility of His M202 participation in the previously proposed through-bond route for electron transfer from the excited state P* to the monomeric BChl B A in wild-type RCs.