Mutation H(M202)L does not lead to the formation of a heterodimer of the primary electron donor in reaction centers of R
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ORIGINAL ARTICLE
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 Anton M. Khristin1 · Alexey A. Zabelin1 · Tatiana Yu. Fufina1 · Ravil A. Khatypov1 · Ivan I. Proskuryakov1 · Vladimir A. Shuvalov1 · Anatoly Ya. Shkuropatov1 · Lyudmila G. Vasilieva1 Received: 29 November 2019 / Accepted: 17 February 2020 © Springer Nature B.V. 2020
Abstract 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 (PA and PB, 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 BA in wild-type RCs. Keywords Homodimer primary electron donor · Axial ligand · Reaction center · Electron transfer · Rhodobacter sphaeroides Abbreviations RC Reaction center Wt Wild type BChl Bacteriochlorophyll BPheo Bacteriopheophytin P Dimer of BChls in the RC PA and PB BChls constituting P BA and BB Monomeric BChls in the active and inactive cofactors branch, respectively HA and HB BPheos in the active and inactive cofactors branch, respectively QA Primary quinone acceptor * Lyudmila G. Vasilieva [email protected] 1
Institute of Basic Biological Problems of the Russian Academy of Sciences, Pushchino Scientific Center for Biological Research of the Russian Academy of Sciences, Pushchino, 142290 Moscow, Russian Federation
QB Secondary quinone acceptor Rba. Rhodobacter FTIR Fourier transform infrared EPR Electron paramagnetic resonance
Introduction In plant cells, algae, and phototrophic microorganisms, the harvested light energy is converted into the energy of chemical bonds as a result of photosynthesis. A ke
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