Comparative Study of Spectral and Functional Properties of Reaction Centers of Wild Type and Double Mutant H(L173)L/I(L177)H of the Purple Bacterium Cereibacter sphaeroides

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Abstract

Previously, we found that in the reaction center (RC) of the purple bacterium Cereibacter sphaeroides, the formation of a heterodimeric primary electron donor (P), caused by the substitution of His-L173 by Leu, was compensated by a second mutation Ile-L177 – His. Significant changes in the spectral properties, pigment composition and redox potential of P, observed in the H(L173)L RC, are restored to the corresponding characteristics of the native RC in the RC H(L173)L/I(L177)H, with the difference that the energy of the long-wavelength QY optical transition of P increases significantly (by ~75 meV). In this work, using light-induced FTIR difference spectroscopy, it was shown that in the RC with double mutation, the homodimeric structure of P is preserved with partially altered electronic properties: the electronic coupling in the radical-cation of the P+ dimer is weakened and the localization of the positive charge on one of its halves increases. The results of the study of the triple mutant H(L173)L/I(L177)H/F(M197)H are consistent with the assumption that the observed changes in the P+ electronic structure, as well as considerable blue shift of QY P absorption band in the RC H(L173)L/ I(L177)H, are associated with a modification of the spatial position and/or geometry of P. Using femtosecond absorption difference spectroscopy, it was shown that the mutant H(L173)L/I(L177)H RC retains a sequence of reactions P* → P+BA− → P+HA− → P+QA− with electron transfer rates and the quantum yield of the final state P+QA− close to those observed in the wild-type RC (P* is the singlet-excited state of P; BA, HA, and QA are molecules of bacteriochlorophyll, bacteriopheophytin, and ubiquinone in the active A-branch of cofactors, respectively). The obtained results, together with previously published data for the RC with a symmetrical double mutation H(M202)L/I(M206)H, demonstrate that by introducing additional point amino acid substitutions, the photochemical activity of the isolated RC from C. sphaeroides can be maintained at a high level even in the absence of important structural elements – axial histidine ligands to the primary electron donor P.

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T. Yu. Fufina

Pushchino Scientific Center for Biological Research of the Russian Academy of Sciences

Author for correspondence.
Email: vsyulya@mail.ru
Russian Federation, 142290, Pushchino, Moscow Region

A. A. Zabelin

Pushchino Scientific Center for Biological Research of the Russian Academy of Sciences

Email: vsyulya@mail.ru
Russian Federation, 142290, Pushchino, Moscow Region

R. A. Khatypov

Pushchino Scientific Center for Biological Research of the Russian Academy of Sciences

Email: vsyulya@mail.ru
Russian Federation, 142290, Pushchino, Moscow Region

A. M. Khristin

Pushchino Scientific Center for Biological Research of the Russian Academy of Sciences

Email: vsyulya@mail.ru
Russian Federation, 142290, Pushchino, Moscow Region

A. Ya. Shkuropatov

Pushchino Scientific Center for Biological Research of the Russian Academy of Sciences

Email: vsyulya@mail.ru
Russian Federation, 142290, Pushchino, Moscow Region

L. G. Vasilieva

Pushchino Scientific Center for Biological Research of the Russian Academy of Sciences

Email: vsyulya@mail.ru
Russian Federation, 142290, Pushchino, Moscow Region

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Supplementary files

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2. Fig. 1. Structure of the reaction centre of C. sphaeroides (PDB ID: 3v3y) (a). L, L-subunit; M, M-subunit; H, H-subunit; P, dimer of BChl molecules; BA and BB, monomeric BChl molecules; HA and HB, monomeric Bfeo molecules; QA and QB, ubiquinones; car, carotenoid. The yellow arrow shows the electron transfer pathway in RC. Protein environment of bacteriochlorophylls in the structure of wild-type RCs (b). The location of symmetrical residues His-L173 and -M202, Ile-M206 and -L177, His-L168 and Phe-M197 is shown. In the structure of monomeric BHL BB, the 2-acetyl and 9-keto groups of the macrocycle are marked by numbers. The dotted lines show coordination and H-bonds between BChl molecules and the protein

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3. Fig. 2. Electronic absorption spectra of wild-type C. sphaeroides RC (1) and H(L173)L/I(L177)H(2) and H(L173)L/I(L177)H/F(M197)H(3) mutants measured at room temperature (a) and 100 K (b). The spectra are normalized in the absorption band QY H at 760 nm

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4. Fig. 3. Thermally dependent change in the amplitude of the QY B band in the wild-type C. sphaeroides RC (■), RC H(L173)L/I(L177)H(▲) and RC H(L173)L/I(L177)H/F(M197)H(●) at 48 °C

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5. Fig. 4. Photoinduced differential (light-minus-dark) IR Fourier spectra of P+Q-/PQ wild-type C. sphaeroides RC (1), mutant RC H(L173)L/I(L177)H(2) and RC H(L173)L/I(L177)H/F(M197)H(3), measured at room temperature. a is the spectral range of 4500-1100 cm -1. The spectra are normalized by the amplitude of the differential signal at ~1750/~1740 cm -1. The region at ~3700-3070 cm -1, saturated due to strong absorption of the sample and water, is excluded from the figure. b is the low–frequency spectral range of 1800-1100 cm−1 from panel (a), shown on an enlarged scale

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6. Fig. 5. DADS for wild-type C. sphaeroides RC (a, b) and double mutant RC H(L173)L/I(L177)H (c, d) in the visible (500-720 nm) and near-infrared (750-1050 nm) spectral regions. The inserts on panels (b) and (d) show spectral features in the range of 980-1050 nm on the ΔA scale, increased by 2.5 times

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7. Fig. 6. Target analysis of spectral-temporal data for wild-type C. sphaeroides RC and double mutant RC H(L173)L/I(L177)H. a is the kinetic model used in the analysis. b and c are concentration profiles of states for wild–type RC and H(L173)L/I(L177)H, respectively. The time scale is represented on a logarithmic scale

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8. Fig. 7. SADS (species associated difference spectra) for wild-type C. sphaeroides RC (black) and double mutant RC H(L173)L/I(L177)H (gray), obtained as a result of target analysis using the model shown in Fig. 6, a. a – are shown lifetimes of states; b – states are indicated; n.z. – undamped

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