Probing the primary donor environment in the histidineM200-->leucine and histidineL173-->leucine heterodimer mutants of Rhodobacter capsulatus by light-induced Fourier transform infrared difference spectroscopy

Monitoring molecular events during photo-driven ubiquinone pool reduction in PufX+ and PufX- membranes from Rhobobacter capsulatus by time-resolved FTIR difference spectroscopy

The PufX protein is found in the photosynthetic membranes of several purple bacteria and is involved in ubiquinol-ubiquinone exchange at the QB site of the reaction center. We have studied quinone pool reduction in chromatophores from PufX+ and PufX- strains of Rhodobacter capsulatus by time-resolved FTIR difference spectroscopy under and after continuous illumination. To our knowledge, it is the first time that quinone pool reduction has been directly followed in real time in Rba. capsulatus membranes. Thanks to the availability in the literature of IR marker bands for protein conformational changes, ubiquinone consumption, ubiquinol production, Q---QH2 quinhydrone complex formation, as well as for RC-bound QA- and QB- semiquinone species, it is possible to follow all the molecular events associated with light-induced quinone pool reduction. In Rba. capsulatus PufX + chromatophores, these events resemble the ones found in Rba. sphaeroides wild-type membranes. In PufX- chromatophores the situation is different. Spectra recorded during 22.7 s of illumination showed a much smaller amount of photoreduced quinol, consistent with previous observations that PufX is required for efficient QH2/Q exchange at the QB site of the RC. Q consumption and QH2 formation are rapidly associated with QA- formation, showing that the structure of the RC-LH1 complex in PufX- membranes does not provide efficient access to the QB site of the RC to a large fraction of the quinone pool, evidently because the LH1 ring increases in size to impair access to the RC. The presence of a positive band at 1560 cm-1 suggests also the transient formation, in a fraction of chromatophores or of RC-LH1 complexes, of a Q---QH2 quinhydrone complex. Experiments carried out after 2-flash and 10-flash sequences make it possible to estimate that the size of the quinone pool with access to the QB site in PufX- membranes is ≥ 5 ubiquinone molecules per RC. The results are discussed in the framework of the current knowledge of protein organization and quinone pool reduction in bacterial photosynthetic membranes.

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.

Dehydration affects the electronic structure of the primary electron donor in bacterial photosynthetic reaction centers: evidence from visible-NIR and light-induced difference FTIR spectroscopy

The photosynthetic reaction center (RC) is a membrane pigment-protein complex that catalyzes the initial charge separation reactions of photosynthesis. Following photoexcitation, the RC undergoes conformational relaxations which stabilize the charge-separated state. Dehydration of the complex inhibits its conformational dynamics, providing a useful tool to gain insights into the relaxational processes. We analyzed the effects of dehydration on the electronic structure of the primary electron donor P, as probed by visible-NIR and light-induced FTIR difference spectroscopy, in RC films equilibrated at different relative humidities r. Previous FTIR and ENDOR spectroscopic studies revealed that P, an excitonically coupled dimer of bacteriochlorophylls, can be switched between two conformations, P866 and P850, which differ in the extent of delocalization of the unpaired electron between the two bacteriochlorophyll moieties (PL and PM) of the photo-oxidized radical P(+). We found that dehydration (at r = 11%) shifts the optical Qy band of P from 866 to 850-845 nm, a large part of the effect occurring already at r = 76%. Such a dehydration weakens light-induced difference FTIR marker bands, which probe the delocalization of charge distribution within the P(+) dimer (the electronic band of P(+) at 2700 cm(-1), and the associated phase-phonon vibrational modes at around 1300, 1480, and 1550 cm(-1)). From the analysis of the P(+) keto C[double bond, length as m-dash]O bands at 1703 and 1713-15 cm(-1), we inferred that dehydration induces a stronger localization of the unpaired electron on PL(+). The observed charge redistribution is discussed in relation to the dielectric relaxation of the photoexcited RC on a long (10(2) s) time scale.

Primary charge separation in the photosystem II core from Synechocystis: a comparison of femtosecond visible/midinfrared pump-probe spectra of wild-type and two P680 mutants

It is now quite well accepted that charge separation in PS2 reaction centers starts predominantly from the accessory chlorophyll B(A) and not from the special pair P(680). To identify spectral signatures of B(A,) and to further clarify the process of primary charge separation, we compared the femtosecond-infrared pump-probe spectra of the wild-type (WT) PS2 core complex from the cyanobacterium Synechocystis sp. PCC 6803 with those of two mutants in which the histidine residue axially coordinated to P(B) (D2-His(197)) has been changed to Ala or Gln. By analogy with the structure of purple bacterial reaction centers, the mutated histidine is proposed to be indirectly H-bonded to the C(9)=O carbonyl of the putative primary donor B(A) through a water molecule. The constructed mutations are thus expected to perturb the vibrational properties of B(A) by modifying the hydrogen bond strength, possibly by displacing the H-bonded water molecule, and to modify the electronic properties and the charge localization of the oxidized donor P(680)(+). Analysis of steady-state light-induced Fourier transform infrared difference spectra of the WT and the D2-His(197)Ala mutant indeed shows that a modification of the axially coordinating ligand to P(B) induces a charge redistribution of P(680)(+). In addition, a comparison of the time-resolved visible/midinfrared spectra of the WT and mutants has allowed us to investigate the changes in the kinetics of primary charge separation induced by the mutations and to propose a band assignment identifying the characteristic vibrations of B(A).

Characterization of a mutant strain of Rhodovulum sulfidophilum lacking the pufA and pufB genes encoding the polypeptides for the light-harvesting complex 1 (B 870)

Contradictory results on the effectiveness of energy transfer from the light harvesting complex 2 (LH2) directly to the reaction center (RC) in mutant strains lacking the core light-harvesting complex 1 (LH1) have been obtained with cells of Rhodobacter capsulatus and Rhodobacter sphaeroides. A LH1(-) mutant of Rhodovulum sulfidophilum, named rsLRI, was constructed by deletion of the pufBA genes, resulting in a kanamycin resistant photosynthetically positive clone. To restore the wild type phenotype, a complemented strain C2 was constructed by inserting in trans a DNA segment containing the pufBA genes. Light-induced FTIR difference spectra indicate that the RC in the rsLRI mutant and in the C2 complemented strains are functionally and structurally identical with those in the wild type strain, demonstrating that the assembly and the function of the RC is not impaired by the LH1 deletion. The photosynthetic growth rate of the rsLRI strain increased with decreasing light intensity. At 50 W m(-2 )no photosynthetic growth was observed. These results indicate that the light energy harvested by the LH2 complex was not or inefficiently transferred to the RC; thus most of the energy necessary for photosynthetic growth is in the LH1(-) strain directly absorbed by the RC. It is supposed that in the mutant strain, RC and LH2 cannot interact in an efficient way.

Tuning of the optical and electrochemical properties of the primary donor bacteriochlorophylls in the reaction centre from Rhodobacter sphaeroides: spectroscopy and structure

A series of mutations have been introduced at residue 168 of the L-subunit of the reaction centre from Rhodobacter sphaeroides. In the wild-type reaction centre, residue His L168 donates a strong hydrogen bond to the acetyl carbonyl group of one of the pair of bacteriochlorophylls (BChl) that constitutes the primary donor of electrons. Mutation of His L168 to Phe or Leu causes a large decrease in the mid-point redox potential of the primary electron donor, consistent with removal of this strong hydrogen bond. Mutations to Lys, Asp and Arg cause smaller decreases in redox potential, indicative of the presence of weak hydrogen bond and/or an electrostatic effect of the polar residue. A spectroscopic analysis of the mutant complexes suggests that replacement of the wild-type His residue causes a decrease in the strength of the coupling between the two primary donor bacteriochlorophylls. The X-ray crystal structure of the mutant in which His L168 has been replaced by Phe (HL168F) was determined to a resolution of 2.5 A, and the structural model of the HL168F mutant was compared with that of the wild-type complex. The mutation causes a shift in the position of the primary donor bacteriochlorophyll that is adjacent to residue L168, and also affects the conformation of the acetyl carbonyl group of this bacteriochlorophyll. This conformational change constitutes an approximately 27 degrees through-plane rotation, rather than the large into-plane rotation that has been widely discussed in the context of the HL168F mutation. The possible structural basis of the altered spectroscopic properties of the HL168F mutant reaction centre is discussed, as is the relevance of the X-ray crystal structure of the HL168F mutant to the possible structures of the remaining mutant complexes.

Heterodimeric versus homodimeric structure of the primary electron donor in Rhodobacter sphaeroides reaction centers genetically modified at position M202

Using light-induced Fourier-transform infrared (FTIR) difference spectroscopy of the photo-oxidation of the primary donor (P) in chromatophores from Rhodobacter sphaeroides, we examined a series of site-directed mutants with His M202 changed to Gly, Ser, Cys, Asn or Glu in order to assess the ability of these side chains to ligate the Mg atom of one of the two bacteriochlorophylls (BChl) constituting P. In the P+QA-/PQA FTIR difference spectra of the mutants HG(M202), HS(M202), HC(M202) and HN(M202), the presence of a specific electronic transition at approximately 2650-2750 cm-1 as well as of associated vibrational (phase-phonon) bands at approximately 1560, 1480 and 1290 cm-1 demonstrate that these mutants contain a BChl/BChl homodimer like that in native reaction centers with the charge on P+ shared between the two coupled BChl. In contrast, the absence of all of these bands in HE(M202) shows that this mutant contains a BChl/bacteriopheophytin heterodimer with the charge localized on the single BChl, as previously determined for the mutant HL(M202). Furthermore, the spectra of the heterodimers HE(M202) and HL(M202) are very similar in the 4000-1200 cm-1 IR range. Perturbations of the 10a-ester and 9-keto carbonyl modes for both the P and P+ states are observed in the homodimer mutants reflecting slight variations in the conformation and/or in position of P. These perturbations are likely to be due to a repositioning of the dimer in the new protein cavity generated by the mutation.

FTIR spectroscopy of primary donor photooxidation in Photosystem I, Heliobacillus mobilis, and Chlorobium limicola. Comparison with purple bacteria

The photooxidation of the primary electron donor in several Photosystem I-related organisms (Synechocystis sp. PCC 6803, Heliobacillus mobilis, and Chlorobium limicola f. sp. thiosulphatophilum) has been studied by light-induced FTIR difference spectroscopy at 100 K in the 4000 to 1200 cm(-1) spectral range. The data are compared to the well-characterized FTIR difference spectra of the photooxidation of the primary donor P in Rhodobacter sphaeroides (both wild type and the heterodimer mutant HL M202) in order to get information on the charge localization and the extent of coupling within the (bacterio)chlorophylls constituting the oxidized primary donors. In Rb. sphaeroides RC, four marker bands mostly related to the dimeric nature of the oxidized primary donor have been previously observed at ≈2600, 1550, 1480, and 1295 cm(-1). The high-frequency band has been shown to correspond to an electronic transition (Breton et al. (1992) Biochemistry 31: 7503-7510) while the three other marker bands have been described as phase-phonon bands (Reimers and Hush (1995) Chem Phys 197: 323-332). The absence of these bands in PS I as well as in the heterodimer HL M202 demonstrates that in P700(+) the charge is essentially localized on a single chlorophyll molecule. For both H. mobilis and C. limicola, the presence of a high-frequency band at ≈ 2050 and 2450 cm(-1), respectively, and of phase-phonon bands (at ≈ 1535 and 1300 cm(-1) in H. mobilis, at ≈ 1465 and 1280 cm(-1) in C. limicola) indicate that the positive charge in the photooxidized primary donor is shared between two coupled BChls. The structure of P840(+) in C. limicola, in terms of the resonance interactions between the two BChl a molecules constituting the oxidized primary donor, is close to that of P(+) in purple bacteria reaction centers while for H. mobilis the FTIR data are interpreted in terms of a weaker coupling of the two bacteriochlorophylls.

Resonance Raman characterization of H(M200)L mutant reaction centers from Rhodobacter capsulatus. Effects of heterodimer formation on the structural and electronic properties of the cofactors

Resonance Raman (RR) spectra are reported for photosynthetic reactions centers (RCs) from the H(M200)L mutant of Rhodobacter capsulatus. In this mutant, the histidine residue which ligates the M-side bacteriochlorophyll (BCh) of the special pair primary donor (P) of wild-type RCs is replaced by a noncoordinating leucine. This results in the formation of a heterodimer primary donor (D) in which a bacteriopheophytin (BPh) replaces the M-side BCh. The RR data for the H(M200)L mutant were acquired at a large number of excitation wavelengths which span the B, Qx, and Qy absorption bands of the various bacteriochlorin cofactors in the RC. For comparison, spectra were also acquired for wild-type RCs at the same excitation wavelengths. The RR data obtained for the mutant indicate that heterodimer formation induces a variety of changes in the structural and electronic properties of the cofactors in the RC. These perturbations extend beyond the primary donor and include one of the two accessory BChs. Collectively, the RR studies indicate the following: (1) The structure of the single BCh cofactor in D [DL(BCh)] is different from that of either of the two BChs in P. However, DL(BCh) is more similar to PL than to PM. The PM cofactor is conformationally more distorted than either PL or DL(BCh). (2) The structure of the BPh cofactor in D [DM(BPh)] is similar to that of the other two BPhs in the RC. However, the frequency of the C9-keto carbonyl mode of DM(BPh) is anomalously low (1678 cm-1), as is also the case for PM. The vibrational characteristics of the C9-keto carbonyl vibrations of DM(BPh)/PM versus DL(BCh)/PL are consistent the notion that dielectric effects govern the frequency of the mode and that the effective dielectric constant is different on the L- versus M-sides of the primary donor. (3) Heterodimer formation perturbs the structural and electronic properties of one of the two accessory BChs (most likely BChL) in the RC. These perturbations are manifested as upshifts in the ring skeletal-mode frequencies and a blue-shift in the Qx absorption band (from 600 to 580 nm). The fact that heterodimer formation perturbs one of the accessory BChs suggests that global structural rearrangements occur in the protein matrix when the ligand to a cofactor in the primary donor is removed. (4) For both the H(M200)L mutant and wild-type RCs, oxidation of the primary donor significantly affects the RR cross section of the carotenoid.(ABSTRACT TRUNCATED AT 400 WORDS)

A difference Fourier transform infrared spectroscopic study of chlorophyll oxidation in hydroxylamine-treated photosystem II

In oxygenic photosynthesis, photosystem II is the chlorophyll-containing reaction center that carries out the light-induced transfer of electrons from water to plastoquinone. Fourier transform infrared spectroscopy can be used to obtain information about the structural changes that accompany electron transfer in photosystem II. The vibrational difference spectrum associated with the reduction of photosystem II acceptor quinones is of interest. Previously, a high concentration of the photosystem II donor, hydroxylamine, has been used to obtain a spectrum attributed to QA- -QA (Berthomieu, C., Nabedryk, E., Mantele, W. and Breton, J. FEBS Lett. (1990) 269, 363). Here, we use electron paramagnetic resonance, Fourier transform infrared spectroscopy, and 15N isotopic labeling to show that the difference infrared spectrum, obtained under these conditions, also exhibits a contribution from the oxidation of chlorophyll.

Crystallographic analyses of site-directed mutants of the photosynthetic reaction center from Rhodobacter sphaeroides

Seven site-directed mutants of the bacterial photosynthetic reaction center (RC) from the 2.4.1 and WS 231 wild-type strains of Rhodobacter sphaeroides have been crystallized and their X-ray diffraction analyzed to resolutions between 3.0 and 4.0 A. The mutations can be divided into four distinct categories: (1) mutations altering cofactor composition that affect electron transfer and quantum yield, His M202-->Leu (M202HL), His L173-->Leu (L173HL), and Leu M214-->His (M214LH); (2) a mutation in the proposed pathway of electron transfer altering electron-transfer kinetics, Tyr M210-->Phe (M210YF); (3) a mutation around the non-heme iron resulting in an iron-less reaction center, His M219-->Cys (M219HC); and (4) mutations around the secondary electron acceptor, a ubiquinone, affecting proton transfer and quinone turnover, Glu L212-->Gln (L212EQ) and Asp L213-->Asn (L213DN). Residues L173 and M202 are within bonding distance of the respective magnesiums of the two bacteriochlorophylls of the BChl special pair, while M214 is close to the bacteriopheophytin on the active A branch of the RC. The L173HL and M202HL crystal structures show that the respective bacteriochlorophylls are replaced with bacteriopheophytins (i.e., loss of magnesium) without significant structural perturbations to the surrounding main-chain or side-chain atoms. In the M214LH mutant, the bacteriopheophytin has been replaced by a bacteriochlorophyll, and the side chain of His M214 is within ligand distance of the magnesium. The M210YF, L212EQ, and L213DN mutants show no significant tertiary structure changes near the mutation sites. The M219HC diffraction data indicate that the overall tertiary structure of the reaction center is maintained in the absence of the non-heme iron.

Picosecond infrared studies of the dynamics of the photosynthetic reaction center

The changes in the vibrational transitions of the protein and redox cofactors of the photosynthetic reaction center were examined by picosecond infrared spectroscopy. The spectra in the vibrational mid-infrared region (1800-1550 cm-1) of hydrated and partially dehydrated reaction centers were investigated from 50 ps to 4 ns after photoinitiation of the electron transfer. Features in the infrared difference spectra were identified with both protein and redox cofactor vibrational modes and correlated with electron transfer events whose kinetics were measured in the infrared and visible regions. The observed protein response is confined to a few amide I transitions (1644 cm-1, 1661 cm-1, 1665 cm-1) and carboxylic residues (1727 cm-1). About 85% of the observed signal corresponded to alterations in the cofactor-associated ester and keto carbonyls. The amide I and carboxylic transitions appeared prior to 50 ps, suggesting that the primary electron transfer event is coupled with a specific piece of the protein backbone and to glutamic or aspartic residues nearby the special pair. Infrared absorption changes accompanying bacteriochlorophyll-dimer cation formation dominated the signal at all times investigated. Infrared spectral changes observed in hydrated and partially dehydrated reaction centers were distinctly different; a band at 1665 cm-1 with a spectral width of 6 cm-1 in the hydrated protein, corresponding to a protein amide I bleach, was not present in the dehydrated film. These differences are discussed in terms of the markedly different electron transfer kinetics observed in the presence of water.