Properties and Crystal Structure of the Cereibacter sphaeroides Photosynthetic Reaction Center with Double Amino Acid Substitution I(L177)H + F(M197)H

The photosynthetic reaction center of the purple bacterium Cereibacter sphaeroides with two site-directed mutations Ile-L177-His and M197 Phe-His is of double interest. The substitution I(L177)H results in strong binding of a bacteriochlorophyll molecule with L-subunit. The second mutation F(M197)H introduces a new H-bond between the C2-acetyl carbonyl group of the bacteriochlorophyll P_B and His-M197, which is known to enhance the stability of the complex. Due to this H-bond, π -electron system of P finds itself connected to an extensive H-bonding network on the periplasmic surface of the complex. The crystal structure of the double mutant reaction center obtained with 2.6 Å resolution allows clarifying consequences of the Ile L177 - His substitution. The value of the P/P⁺ midpoint potential in the double mutant RC was found to be ~20 mV less than the sum of potentials measured in the two RCs with single mutations I(L177)H and F(M197)H. The protein environment of the BChls P_A and B_B were found to be similar to that in the RC with single substitution I(L177)H, whereas an altered pattern of the H-bonding networks was found in the vicinity of bacteriochlorophyll P_B. The data obtained are consistent with our previous assumption on a correlation between the bulk of the H-bonding network connected with the π-electron system of the primary electron donor P and the value of its oxidation potential.

In memory of Vladimir Anatolievich Shuvalov (1943-2022): an outstanding biophysicist

We present here a tribute to one of the foremost biophysicists of our time, Vladimir Anatolievich Shuvalov, who made important contributions in bioenergetics, especially on the primary steps of conversion of light energy into charge-separated states in both anoxygenic and oxygenic photosynthesis. For this, he and his research team exploited pico- and femtosecond transient absorption spectroscopy, photodichroism & circular dichroism spectroscopy, light-induced FTIR (Fourier-transform infrared) spectroscopy, and hole-burning spectroscopy. We remember him for his outstanding leadership and for being a wonderful mentor to many scientists in this area. Reminiscences by many [Suleyman Allakhverdiev (Russia); Robert Blankenship (USA); Richard Cogdell (UK); Arvi Freiberg (Estonia); Govindjee Govindjee (USA); Alexander Krasnovsky, jr, (Russia); William Parson (USA); Andrei Razjivin (Russia); Jian- Ren Shen (Japan); Sergei Shuvalov (Russia); Lyudmilla Vasilieva (Russia); and Andrei Yakovlev (Russia)] have included not only his wonderful personal character, but his outstanding scientific research.

Properties of Mutant Photosynthetic Reaction Centers of Purple Non-Sulfur Bacteria Cereibacter sphaeroides with M206 Ile→Gln Substitution

In the structure of photosynthetic reaction center (RC) of the purple bacterium Cereibacter sphaeroides the highly conserved amino acid residue Ile-M206 is located near the bacteriochlorophyll dimer P, which is the primary electron donor, and the monomeric bacteriochlorophyll B_A, which is the nearest electron acceptor. Since Ile-M206 is close to the C2-acetyl group of bacteriochlorophyll P_B, the hydroxyl group of Tyr-M210, and to the C9-keto group of bacteriochlorophyll B_A, as well as to the water molecule near the latter group, this site can be used for introducing mutations in order to study mechanisms of primary photochemical processes in the RC. Previously it was shown that the Ile→Glu substitution at the M204 position (analog of M206 in the RC of C. sphaeroides) in the RC of the closely related purple non-sulfur bacterium Rhodobacter capsulatus significantly affected kinetics of the P⁺H_A^- state formation, whereas the M204 Ile→Gln substitution led to the loss of BChl B_A molecule from the complex structure. In the present work, it is shown that the single I(M206)Q or double I(M206)Q + F(M208)A amino acid substitutions in the RC of C. sphaeroides do not change the pigment composition and do not markedly influence redox potential of the primary electron donor. However, substitution of Ile M206 by Gln affected positions and amplitudes of the absorption bands of bacteriochlorophylls, increased lifetime of the primary electron donor P* excited state from 3.1 ps to 22 ps, and decreased quantum yield of the P⁺Q_A^- state formation to 60%. These data suggest significant changes in the pigment-protein interactions in the vicinity of the primary electron donor P and the nearest electron acceptor B_A. A considerable decrease was also noticed in the resistance of the mutant RC to thermal denaturation, which was more pronounced in the RC with the double substitution I(M206)Q + F(M208)A. This was likely associated with the disruption of the dense packing of the protein near bacteriochlorophylls P_B and B_A. Possible reasons for different effects of identical mutations on the properties of two highly homologous RCs from closely related purple non-sulfur bacteria are discussed.

Effect of Detergents and Osmolytes on Thermal Stability of Native and Mutant Rhodobacter sphaeroides Reaction Centers

Photosynthetic reaction center (RC) of the purple bacterium Rhodobacter sphaeroides is one of the most well-studied transmembrane pigment-protein complexes. It is a relatively stable protein with established conditions for its isolation from membranes, purification, and storage. However, it has been shown that some amino acid substitutions can affect stability of the RC, which results in a decrease of the RCs yield during its isolation and purification, disturbs spectral properties of the RCs during storage, and can lead to sample heterogeneity. To optimize conditions for studying mutant RCs, the effect of various detergents and osmolytes on thermal stability of the complex was examined. It was shown that trehalose and, to a lesser extent, sucrose, maltose, and hydroxyectoin at 1 M concentration slow down thermal denaturation of RCs. Sodium cholate was found to have significant stabilizing effect on the structure of native and genetically modified RCs. The use of sodium cholate as a detergent has several advantages and can be recommended for the storage and investigation of the unstable mutant membrane complexes of purple bacteria in long-term experiments.

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

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.

Effect of Leucine M196 Substitution by Histidine on Electronic Structure of the Primary Electron Donor and Electron Transfer in Reaction Centers from Rhodobacter sphaeroides

In our recent X-ray study, we demonstrated that substitution of the natural leucine residue M196 with histidine in the reaction center (RC) from Rhodobacter (Rba.) sphaeroides leads to formation of a close contact between the genetically introduced histidine and the primary electron donor P (bacteriochlorophylls (BChls) P_A and P_B dimer) creating a novel pigment-protein interaction that is not observed in native RCs. In the present work, the possible nature of this novel interaction and its effects on the electronic properties of P and the photochemical charge separation in isolated mutant RCs L(M196)H are investigated at room temperature using steady-state absorption spectroscopy, light-induced difference FTIR spectroscopy, and femtosecond transient absorption spectroscopy. The results are compared with the data obtained for the RCs from Rba. sphaeroides pseudo-wild type strain. It is shown that the L(M196)H mutation results in a decrease in intensity and broadening of the long-wavelength Q_y absorption band of P at ~865 nm. Due to the mutation, there is also weakening of the electronic coupling between BChls in the radical cation P⁺ and increase in the positive charge localization on the P_A molecule. Despite the significant perturbations of the electronic structure of P, the mutant RCs retain high electron transfer rates and quantum yield of the P⁺Q_A^- state (Q_A is the primary quinone acceptor), which is close to the one observed in the native RCs. Comparison of our results with the literature data suggests that the imidazole group of histidine M196 forms a π-hydrogen bond with the π-electron system of the P_B molecule in the P dimer. It is likely that the specific (T-shaped) spatial organization of the π-hydrogen interaction and its potential heterogeneity in relation to the bonding energy is, at least partially, the reason that this type of interaction between the protein and the pigment and quinone cofactors is not realized in the native RCs.

Properties of Rhodobacter sphaeroides Reaction Centers with the Ile→Tyr Substitution at Positions L177 and M206

Studying pigment-protein interactions in the photosynthetic reaction centers (RCs) is important for the understanding of detailed mechanisms of the photochemical process. This paper describes spectral and photochemical characteristics, pigment composition, and stability of the Rhodobacter sphaeroides RCs with the I(L177)Y and I(M206)Y amino acid substitutions. The obtained data are compared with the properties of I(L177)H, I(L177)D, and I(M206)H RCs reported previously. It is shown that the I(L177)Y and I(M206)Y mutations cause a similar shift of the Q_YP band in the absorption spectra of the mutant RCs and do not affect the distribution of the electron spin density within the photo-oxidized P⁺ dimer. The differences in the position and amplitude of the Q_YB band in the I(L177)Y and I(M206)Y RCs were determined. The results indicate the possibility of new pigment-protein interactions in the vicinity of monomeric bacteriochlorophylls in the A and B chains, which might be of interest for future research.

The L(M196)H mutation in Rhodobacter sphaeroides reaction center results in new electrostatic interactions

New histidine residue was introduced in M196 position in the reaction center of Rhodobacter sphaeroides in order to alter polarity of the BChl dimer's protein environment and to study how it affects properties and structure of the primary electron donor P. It was shown that in the absorption spectrum of the mutant RC the 6 nm red shift of the Q Y P band was observed together with considerable decrease of its amplitude. The mid-point potential of P/P (+) in the mutant RC was increased by +65 (±15) mV as compared to the E m P/P (+) value in the wild-type RC suggesting that the mutation resulted in new pigment-protein interactions. Crystal structure of RC L(M196)H determined at 2.4 Å resolution implies that BChl Р В and introduced histidine-M196 organize new electrostatic contact that may be specified either as π-π staking or as hydrogen-π interaction. Besides, the structure of the mutants RC shows that His-M196 apparently became involved in hydrogen bond network existing in BChl Р В vicinity that may favor stability of the mutant RC.

Expression, purification, crystallization and preliminary X-ray structure analysis of wild-type and L(M196)H-mutant Rhodobacter sphaeroides reaction centres

The electron and proton transport mediated by protein-bound cofactors in photosynthesis have been investigated by various methods in order to determine the energetics, the dynamics and the pathway of this process. In purple bacteria, primary photosynthetic charge separation and the build-up of a proton gradient across the periplasmic membrane are catalyzed by the photosynthetic reaction centre (RC). Here, the purification, crystallization and preliminary X-ray analysis of wild-type and L(M196)H-mutant RCs of Rhodobacter sphaeroides are presented, enabling study of the influence of the protein environment of the primary electron donor on the spectral properties and photochemical activity of the RC.

The site-directed mutation I(L177)H in Rhodobacter sphaeroides reaction center affects coordination of P(A) and B(B) bacteriochlorophylls

To explore the influence of the I(L177)H single mutation on the properties of the nearest bacteriochlorophylls (BChls), three reaction centers (RCs) bearing double mutations were constructed in the photosynthetic purple bacterium Rhodobacter sphaeroides, and their properties and pigment content were compared with those of the correspondent single mutant RCs. Each pair of the mutations comprised the amino acid substitution I(L177)H and another mutation altering histidine ligand of BChl P(A) or BChl B(B). Contrary to expectations, the double mutation I(L177)H+H(L173)L does not bring about a heterodimer RC but causes a 46nm blue shift of the long-wavelength P absorbance band. The histidine L177 or a water molecule were suggested as putative ligands for P(A) in the RC I(L177)H+H(L173)L although this would imply a reorientation of the His backbone and additional rearrangements in the primary donor environment or even a repositioning of the BChl dimer. The crystal structure of the mutant I(L177)H reaction center determined to a resolution of 2.9Å shows changes at the interface region between the BChl P(A) and the monomeric BChl B(B). Spectral and pigment analysis provided evidence for β-coordination of the BChl B(B) in the double mutant RC I(L177)H+H(M182)L and for its hexacoordination in the mutant reaction center I(L177)H. Computer modeling suggests involvement of two water molecules in the β-coordination of the BChl B(B). Possible structural consequences of the L177 mutation affecting the coordination of the two BChls P(A) and B(B) are discussed. This article is part of a Special Issue entitled: Photosynthesis Research for Sustainability: from Natural to Artificial.

Primary charge separation within P870* in wild type and heterodimer mutants in femtosecond time domain

Primary charge separation dynamics in the reaction center (RC) of purple bacterium Rhodobacter sphaeroides and its P870 heterodimer mutants have been studied using femtosecond time-resolved spectroscopy with 20 and 40fs excitation at 870nm at 293K. Absorbance increase in the 1060-1130nm region that is presumably attributed to P(A)(δ+) cation radical molecule as a part of mixed state with a charge transfer character P*(P(A)(δ+)P(B)(δ-)) was found. This state appears at 120-180fs time delay in the wild type RC and even faster in H(L173)L and H(M202)L heterodimer mutants and precedes electron transfer (ET) to B(A) bacteriochlorophyll with absorption band at 1020nm in WT. The formation of the P(A)(δ+)B(A)(δ-) state is a result of the electron transfer from P*(P(A)(δ+)P(B)(δ-)) to the primary electron acceptor B(A) (still mixed with P*) with the apparent time delay of ~1.1ps. Next step of ET is accompanied by the 3-ps appearance of bacteriopheophytin a(-) (H(A)(-)) band at 960nm. The study of the wave packet formation upon 20-fs illumination has shown that the vibration energy of the wave packet promotes reversible overcoming of an energy barrier between two potential energy surfaces P* and P*(P(A)(δ+)B(A)(δ-)) at ~500fs. For longer excitation pulses (40fs) this promotion is absent and tunneling through an energy barrier takes about 3ps. This article is part of a Special Issue entitled: Photosynthesis Research for Sustainability: from Natural to Artificial.

Coherent phenomena of charge separation in reaction centers of LL131H and LL131H/LM160H/FM197H mutants of Rhodobacter sphaeroides

Primary stage of charge separation and transfer of charges was studied in reaction centers (RCs) of point mutants LL131H and LL131H/LM160H/FM197H of the purple bacterium Rhodobacter sphaeroides by differential absorption spectroscopy with temporal resolution of 18 fsec at 90 K. Difference absorption spectra measured at 0-4 psec delays after excitation of dimer P at 870 nm with 30 fsec step were obtained in the spectral range of 935-1060 nm. It was found that a decay of P* due to charge separation is considerably slower in the mutant RCs in comparison with native RCs of Rba. sphaeroides. Coherent oscillations were found in the kinetics of stimulated emission of the P* state at 940 nm. Fourier analysis of the oscillations revealed a set of characteristic bands in the frequency range of 20-500 cm(-1). The most intense band has the frequency of ~130 cm(-1) in RCs of mutant LL131H and in native RCs and the frequency of ~100 cm(-1) in RCs of the triple mutant. It was found that an absorption band of bacteriochlorophyll anion B(A)(-) which is registered in the difference absorption spectra of native RCs at 1020 nm is absent in the analogous spectra of the mutants. The results are analyzed in terms of the participation of the B(A) molecule in the primary electron transfer in the presence of a nuclear wave packet moving along the inharmonic surface of P* potential energy.

Primary electron transfer in reaction centers of YM210L and YM210L/HL168L mutants of Rhodobacter sphaeroides

The role of tyrosine M210 in charge separation and stabilization of separated charges was studied by analyzing of the femtosecond oscillations in the kinetics of decay of stimulated emission from P* and of a population of the primary charge separated state P(+)B(A)(-) in YM210L and YM210L/HL168L mutant reaction centers (RCs) of Rhodobacter sphaeroides in comparison with those in native Rba. sphaeroides RCs. In the mutant RCs, TyrM210 was replaced by Leu. The HL168L mutation placed the redox potential of the P(+)/P pair 123 mV below that of native RCs, thus creating a theoretical possibility of P(+)B(A)(-) stabilization. Kinetics of P* decay at 940 nm of both mutants show a significant slowing of the primary charge separation reaction in comparison with native RCs. Distinct damped oscillations in these kinetics with main frequency bands in the range of 90-150 cm(-1) reflect mostly nuclear motions inside the dimer P. Formation of a very small absorption band of B(A)(-) at 1020 nm is registered in RCs of both mutants. The formation of the B(A)(-) band is accompanied by damped oscillations with main frequencies from ~10 to ~150 cm(-1). Only a partial stabilization of the P(+)B(A)(-) state is seen in the YM210L/HL168L mutant in the form of a small non-oscillating background of the 1020-nm kinetics. A similar charge stabilization is absent in the YM210L mutant. A model of oscillatory reorientation of the OH-group of TyrM210 in the electric fields of P(+) and B(A)(-) is proposed to explain rapid stabilization of the P(+)B(A)(-) state in native RCs. Small oscillatory components at ~330-380 cm(-1) in the 1020-nm kinetics of native RCs are assumed to reflect this reorientation. We conclude that the absence of TyrM210 probably cannot be compensated by lowering of the P(+)B(A)(-) free energy that is expected for the double YM210L/HL168L mutant. An oscillatory motion of the HOH55 water molecule under the influence of P(+) and B(A)(-) is assumed to be another potential contributor to the mechanism of P(+)B(A)(-) stabilization.

Femtosecond phase of charge separation in reaction centers of Chloroflexus aurantiacus

Difference absorption spectroscopy with temporal resolution of approximately 20 fsec was used to study the primary phase of charge separation in isolated reaction centers (RCs) of Chloroflexus aurantiacus at 90 K. An ensemble of difference (light-minus-dark) absorption spectra in the 730-795 nm region measured at -0.1 to 4 psec delays relative to the excitation pulse was analyzed. Comparison with analogous data for RCs of HM182L mutant of Rhodobacter sphaeroides having the same pigment composition identified the 785 nm absorption band as the band of bacteriopheophytin Phi(B) in the B-branch. By study the bleaching of this absorption band due to formation of Phi(B)(-), it was found that a coherent electron transfer from P* to the B-branch occurs with a very small delay of 10-20 fsec after excitation of dimer bacteriochlorophyll P. Only at 120 fsec delay electron transfer from P* to the A-branch occurs with the formation of bacteriochlorophyll anion B(A)(-) absorption band at 1028 nm and the appearance of P* stimulated emission at 940 nm, as also occurs in native RCs of Rb. sphaeroides. It is concluded that a nuclear wave packet motion on the potential energy surface of P* after a 20-fsec light pulse excitation leads to the coherent formation of the P(+)Phi(B)(-) and P(+)B(A)(-) states.

Primary light-energy conversion in tetrameric chlorophyll structure of photosystem II and bacterial reaction centers: I. A review

The purpose of the review is to show that the tetrameric (bacterio)chlorophyll ((B)Chl) structures in reaction centers of photosystem II (PSII) of green plants and in bacterial reaction centers (BRCs) are similar and play a key role in the primary charge separation. The Stark effect measurements on PSII reaction centers have revealed an increased dipole moment for the transition at approximately 730 nm (Frese et al., Biochemistry 42:9205-9213, 2003). It was found (Heber and Shuvalov, Photosynth Res 84:84-91, 2005) that two fluorescent bands at 685 and 720 nm are observed in different organisms. These two forms are registered in the action spectrum of Q(A) photoreduction. Similar results were obtained in core complexes of PSII at low temperature (Hughes et al., Biochim Biophys Acta 1757: 841-851, 2006). In all cases the far-red absorption and emission can be interpreted as indication of the state with charge transfer character in which the chlorophyll monomer plays a role of an electron donor. The role of bacteriochlorophyll monomers (B(A) and B(B)) in BRCs can be revealed by different mutations of axial ligand for Mg central atoms. RCs with substitution of histidine L153 by tyrosine or leucine and of histidine M182 by leucine (double mutant) are not stable in isolated state. They were studied in antennaless membrane by different kinds of spectroscopy including one with femtosecond time resolution. It was found that the single mutation (L153HY) was accompanied by disappearance of B(A) molecule absorption near 802 nm and by 14-fold decrease of photochemical activity measured with ms time resolution. The lifetime of P(870)* increased up to approximately 200 ps in agreement with very low rate of the electron transfer to A-branch. In the double mutant L153HY + M182HL, the B(A) appears to be lost and B(B) is replaced by bacteriopheophytin Phi(B) with the absence of any absorption near 800 nm. Femtosecond measurements have revealed the electron transfer to B-branch with a time constant of approximately 2 ps. These results are discussed in terms of obligatory role of B(A) and Phi(B) molecules located near P for efficient electron transfer from P*.

Substitution of Isoleucine L177 by Histidine Affects the Pigment Composition and Properties of the Reaction Center of the Purple Bacterium Rhodobacter sphaeroides

Using site-directed mutagenesis, we obtained the mutant of the purple bacterium Rhodobacter sphaeroides with Ile to His substitution at position 177 in the L-subunit of the photosynthetic reaction center (RC). The mutant strain forms stable and photochemically active RC complexes. Relative to the wild type RCs, the spectral and photochemical properties of the mutant RC differ significantly in the absorption regions corresponding to the primary donor P and the monomer bacteriochlorophyll (BChl) absorption. It is shown that the RC I(L177)H contains only three BChl molecules compared to four BChl molecules in the wild type RC. Considering the fact that the properties of both isolated and membrane-associated mutant RCs are similar, we conclude that the loss of a BChl molecule from the mutant RC is caused by the introduced mutation but not by the protein purification procedure. The new mutant missing one BChl molecule but still able to perform light-induced reactions forming the charge-separated state P+QA- appears to be an interesting object to study the mechanisms of the first steps of the primary electron transfer in photosynthesis.