Mg coordination by amino acid side chains is not required for assembly and function of the special pair in bacterial photosynthetic reaction centers

A bound iron porphyrin is redox active in hybrid bacterial reaction centers modified to possess a four-helix bundle domain

In this paper we report the design of hybrid reaction centers with a novel redox-active cofactor. Reaction centers perform the primary photochemistry of photosynthesis, namely the light-induced transfer of an electron from the bacteriochlorophyll dimer to a series of electron acceptors. Hybrid complexes were created by the fusion of an artificial four-helix bundle to the M-subunit of the reaction center. Despite the large modification, optical spectra show that the purified hybrid reaction centers assemble as active complexes that retain the characteristic cofactor absorption peaks and are capable of light-induced charge separation. The four-helix bundle could bind iron-protoporphyrin in either a reduced and oxidized state. After binding iron-protoporphyrin to the hybrid reaction centers, light excitation results in a new derivative signal with a maximum at 402 nm and minimum at 429 nm. This signal increases in amplitude with longer light durations and persists in the dark. No signal is observed when iron-protoporphyrin is added to reaction centers without the four-helix bundle domain or when a redox-inactive zinc-protoporphyrin is bound. The results are consistent with the signal arising from a new redox reaction, electron transfer from the iron-protoporphyrin to the oxidized bacteriochlorophyll dimer. These outcomes demonstrate the feasibility of binding porphyrins to the hybrid reaction centers to gain new light-driven functions.

In Situ, Protein-Mediated Generation of a Photochemically Active Chlorophyll Analogue in a Mutant Bacterial Photosynthetic Reaction Center

All possible natural amino acids have been substituted for the native LeuL185 positioned near the B-side bacteriopheophytin (HB) in the bacterial reaction center (RC) from Rhodobacter sphaeroides. Additional mutations that enhance electron transfer to the normally inactive B-side cofactors are present. Approximately half of the isolated RCs with Glu at L185 contain a magnesium chlorin (CB) in place of HB. The chlorin is not the common BChl a oxidation product 3-desvinyl-3-acetyl chlorophyll a with a C-C bond in ring D and a C═C bond in ring B but has properties consistent with reversal of these bond orders, giving 17,18-didehydro BChl a. In such RCs, charge-separated state P+CB- forms in ∼5% yield. The other half of the GluL185-containing RCs have a bacteriochlorophyll a (BChl a) denoted βB in place of HB. Residues His, Asp, Asn, and Gln at L185 yield RCs with ≥85% βB in the HB site, while most other amino acids result in RCs that retain HB (≥95%). To the best of our knowledge, neither bacterial RCs that harbor five BChl a molecules and one chlorophyll analogue nor those with six BChl a molecules have been reported previously. The finding that altering the local environment within a cofactor binding site of a transmembrane complex leads to in situ generation of a photoactive chlorin with an unusual ring oxidation pattern suggests new strategies for amino acid control over pigment type at specific sites in photosynthetic proteins.

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.

Consequences of saturation mutagenesis of the protein ligand to the B-side monomeric bacteriochlorophyll in reaction centers from Rhodobacter capsulatus

In bacterial reaction centers (RCs), photon-induced initial charge separation uses an A-side bacteriochlorophyll (BChl, B_A) and bacteriopheophytin (BPh, H_A), while the near-mirror image B-side B_B and H_B cofactors are inactive. Two new sets of Rhodobacter capsulatus RC mutants were designed, both bearing substitution of all amino acids for the native histidine M180 (M-polypeptide residue 180) ligand to the core Mg ion of B_B. Residues are identified that largely result in retention of a BChl in the B_B site (Asp, Ser, Pro, Gln, Asn, Gly, Cys, Lys, and Thr), ones that largely harbor the Mg-free BPh in the B_B site (Leu and Ile), and ones for which isolated RCs are comprised of a substantial mixture of these two RC types (Ala, Glu, Val, Met and, in one set, Arg). No protein was isolated when M180 is Trp, Tyr, Phe, or (in one set) Arg. These findings are corroborated by ground state spectra, pigment extractions, ultrafast transient absorption studies, and the yields of B-side transmembrane charge separation. The changes in coordination chemistries did not reveal an RC with sufficiently precise poising of the redox properties of the B_B-site cofactor to result in a high yield of B-side electron transfer to H_B. Insights are gleaned into the amino acid properties that support BChl in the B_B site and into the widely observed multi-exponential decay of the excited state of the primary electron donor. The results also have direct implications for tuning free energies of the charge-separated intermediates in RCs and mimetic systems.

Primary and Higher Order Structure of the Reaction Center from the Purple Phototrophic Bacterium Blastochloris viridis: A Test for Native Mass Spectrometry

The reaction center (RC) from the phototrophic bacterium Blastochloris viridis was the first integral membrane protein complex to have its structure determined by X-ray crystallography and has been studied extensively since then. It is composed of four protein subunits, H, M, L, and C, as well as cofactors, including bacteriopheophytin (BPh), bacteriochlorophyll (BCh), menaquinone, ubiquinone, heme, carotenoid, and Fe. In this study, we utilized mass spectrometry-based proteomics to study this protein complex via bottom-up sequencing, intact protein mass analysis, and native MS ligand-binding analysis. Its primary structure shows a series of mutations, including an unusual alteration and extension on the C-terminus of the M-subunit. In terms of quaternary structure, proteins such as this containing many cofactors serve to test the ability to introduce native-state protein assemblies into the gas phase because the cofactors will not be retained if the quaternary structure is seriously perturbed. Furthermore, this specific RC, under native MS, exhibits a strong ability not only to bind the special pair but also to preserve the two peripheral BCh's.

Native Mass Spectrometry Characterizes the Photosynthetic Reaction Center Complex from the Purple Bacterium Rhodobacter sphaeroides

Native mass spectrometry (MS) is an emerging approach to study protein complexes in their near-native states and to elucidate their stoichiometry and topology. Here, we report a native MS study of the membrane-embedded reaction center (RC) protein complex from the purple photosynthetic bacterium Rhodobacter sphaeroides. The membrane-embedded RC protein complex is stabilized by detergent micelles in aqueous solution, directly introduced into a mass spectrometer by nano-electrospray (nESI), and freed of detergents and dissociated in the gas phase by collisional activation. As the collision energy is increased, the chlorophyll pigments are gradually released from the RC complex, suggesting that native MS introduces a near-native structure that continues to bind pigments. Two bacteriochlorophyll a pigments remain tightly bound to the RC protein at the highest collision energy. The order of pigment release and their resistance to release by gas-phase activation indicates the strength of pigment interaction in the RC complex. This investigation sets the stage for future native MS studies of membrane-embedded photosynthetic pigment-protein and related complexes.Graphical Abstract.

Evaluation of a biohybrid photoelectrochemical cell employing the purple bacterial reaction centre as a biosensor for herbicides

The Rhodobacter sphaeroides reaction centre is a relatively robust and tractable membrane protein that has potential for exploitation in technological applications, including biohybrid devices for photovoltaics and biosensing. This report assessed the usefulness of the photocurrent generated by this reaction centre adhered to a small working electrode as the basis for a biosensor for classes of herbicides used extensively for the control of weeds in major agricultural crops. Photocurrent generation was inhibited in a concentration-dependent manner by the triazides atrazine and terbutryn, but not by nitrile or phenylurea herbicides. Measurements of the effects of these herbicides on the kinetics of charge recombination in photo-oxidised reaction centres in solution showed the same selectivity of response. Titrations of reaction centre photocurrents yielded half maximal inhibitory concentrations of 208 nM and 2.1 µM for terbutryn and atrazine, respectively, with limits of detection estimated at around 8 nM and 50 nM, respectively. Photocurrent attenuation provided a direct measure of herbicide concentration, with no need for model-dependent kinetic analysis of the signal used for detection or the use of prohibitively complex instrumentation, and prospects for the use of protein engineering to develop the sensitivity and selectivity of herbicide binding by the Rba. sphaeroides reaction centre are discussed.

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The Rhodobacter sphaeroides reaction centre was used as a biosensor for herbicides.

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Herbicide concentration was assessed through the attenuation of a photocurrent.

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The biosensor showed selectivity for triazine herbicides.

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The limit of detection of the biosensor was in the low nanomolar range.

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Photocurrent attenuation is a simple and direct basis for a herbicide biosensor.

Influence of protein interactions on oxidation/reduction midpoint potentials of cofactors in natural and de novo metalloproteins

As discussed throughout this special issue, oxidation and reduction reactions play critical roles in the function of many organisms. In photosynthetic organisms, the conversion of light energy drives oxidation and reduction reactions through the transfer of electrons and protons in order to create energy-rich compounds. These reactions occur in proteins such as cytochrome c, a heme-containing water-soluble protein, the bacteriochlorophyll-containing reaction center, and photosystem II where water is oxidized at the manganese cluster. A critical measure describing the ability of cofactors in proteins to participate in such reactions is the oxidation/reduction midpoint potential. In this review, the basic concepts of oxidation/reduction reactions are reviewed with a summary of the experimental approaches used to measure the midpoint potential of metal cofactors. For cofactors in proteins, the midpoint potential not only depends upon the specific chemical characteristics of cofactors but also upon interactions with the surrounding protein, such as the nature of the coordinating ligands and protein environment. These interactions can be tailored to optimize an oxidation/reduction reaction carried out by the protein. As examples, the midpoint potentials of hemes in cytochromes, bacteriochlorophylls in reaction centers, and the manganese cluster of photosystem II are discussed with an emphasis on the influence that protein interactions have on these potentials. This article is part of a Special Issue entitled: Metals in Bioenergetics and Biomimetics Systems.

Influence of the axial ligand on the cationic properties of the chlorophyll pair in photosystem II from Thermosynechococcus vulcanus

Influence of the axial ligand of PD1 chlorophyll (D1-His-198) on the Em of monomer chlorophylls PD1 and PD2, and the PD1•+/PD2•+ charge ratio was investigated by theoretical calculations using the PSII crystal structure of Thermosynechococcus vulcanus analyzed at 1.9-Å resolution. It was found that the Em(PD1)/Em(PD2) values and PD1•+/PD2•+ ratio remained unchanged upon D1-H198Q mutation. However, Em(PD1) was increased in the D1-H198A mutant, resulting in a more even distribution of the positive charge over PD1/PD2. Introduction of a water molecule as an axial ligand resulted in equal Em values and PD1•+/PD2•+ ratios between the mutant and wild-type, thus confirming the presence of the water ligand in the mutant.

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.

Purification of His6-tagged Photosystem I from Chlamydomonas reinhardtii

We have developed a rapid method for isolation of the Photosystem I (PS1) complex from Chlamydomonas reinhardtii using epitope tagging. Six histidine residues were genetically added to the N-terminus of the PsaA core subunit of PS1. The His(6)-tagged PS1 could be purified with a yield of 80-90% from detergent-solubilized thylakoid membranes within 3 h in a single step using a Ni-nitrilotriacetic acid (Ni-NTA) column. Immunoblots and low-temperature fluorescence analysis indicated that the His(6)-tagged PS1 preparation was highly pure and extremely low in uncoupled pigments. Moreover, the introduced tag appeared to have no adverse effect upon PS1 structure/function, as judged by photochemical assays and EPR spectroscopy of isolated particles, as well as photosynthetic growth tests of the tagged strain.

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).

Charge Delocalization in the Special-Pair Radical Cation of Mutant Reaction Centers of Rhodobacter sphaeroides from Stark Spectra and Nonadiabatic Spectral Simulations

Stark and absorption spectra for the hole-transfer band of the bacteriochlorophyll special pair in the wild-type and L131LH, M160LH, and L131LH/M160LH mutants of the bacterial reaction center of Rhodobacter sphaeroides are presented, along with extensive analyses based on nonadiabatic spectral simulations. Dramatic changes in the Stark spectra are induced by the mutations, changes that are readily interpreted in terms of the redox-energy asymmetry and degree of charge localization in the special-pair radical cation. The effect of mutagenesis on key properties such as the electronic coupling within the special pair and the reorganization energy associated with intervalence hole transfer are determined for the first time. Results for the L131LH and M160LH/L131LH mutants indicate that these species can be considered as influencing the special pair primarily through modulation of the redox asymmetry, as is usually conceptualized, but M160LH is shown to develop a wide range of effects that can be interpreted in terms of significant mutation-induced structural changes in and around the special pair. The nonadiabatic spectra simulations are performed using both a simple two-state 1-mode and an extensive four-state 70-mode model, which includes the descriptions of additional electronic states and explicitly treats the major vibrational modes involved. Excellent agreement between the two simulation approaches is obtained. The simple model is shown to reproduce key features of the Stark effect of the main intervalence transition, while the extensive model quantitatively reproduces most features of the observed spectra for both the electronic and the phase-phonon regions, thus giving a more comprehensive description of the effect of the mutations on the properties of the special-pair radical cation. These results for a series of closely related mixed-valence complexes show that the Stark spectra provide a sensitive indicator for the properties of the mixed-valence complexes and should serve as an instructive example on the application of nonadiabatic simulations to the study of mixed-valence complexes in general as well as other chemical systems akin to the photosynthetic special pair.

Investigation of Rhodobacter capsulatus PufX interactions in the core complex of the photosynthetic apparatus

The photosynthetic apparatus of purple bacteria in the genus Rhodobacter includes a core complex consisting of the reaction centre (RC), light-harvesting complex 1 (LH1), and the PufX protein. PufX modulates LH1 structure and facilitates photosynthetic quinone/quinol exchange. We deleted RC/LH1 genes in pufX+ and pufX++ (merodiploid) strains of Rhodobacter capsulatus, which reduced PufX levels regardless of pufX gene copy number and location. Photosynthetic growth of RC-only strains and independent assembly kinetics of the RC and LH1 were unaffected by pufX merodiploidy, but the absorption spectra of strains expressing the RC plus either LH1 alpha or beta indicated that PufX may influence bacteriochlorophyll binding environments. Significant self-association of the PufX transmembrane segment was detected in a hybrid protein expression system, consistent with a role of PufX in core complex dimerization, as proposed for other Rhodobacter species. Our results indicate that in R. capsulatus PufX has the potential to be a central, homodimeric core complex component, and its cellular level is increased by interactions with the RC and LH1.

Low-frequency resonance Raman studies of the H(M202)G cavity mutant of bacterial photosynthetic reaction centers

Low-frequency (90-435 cm(-1)) NIR-excitation (875-900 nm) resonance Raman (RR) studies are reported for the H(M202)G cavity mutant of bacterial photosynthetic reaction centers (RCs) from Rb. sphaeroides that was first described by Goldsmith et al. [(1996) Biochemistry 35: 2421-2428]. In this mutant, the His residue that axially ligates the Mg ion of the M-side bacteriochlorophyll (BChl) of the special pair primary donor (P) is replaced by a non-ligating Gly residue. Regardless, the Mg ion of P(M) in the H(M202)G RCs remains pentacoordinates and is presumably ligated by a water molecule, although this axial ligand has not been definitively identified. The low-frequency RR studies of the H(M202)G RCs are accompanied by studies of RCs exchanged with D(2)O and incubated with imidazole (Im). The RR studies of the cavity mutant RCs reveal the following: (1) The structure of P(M) in the H(M202)G RCs is different from that of the wild-type, consistent with an altered BChl core. (2) A water ligand for P(M) in the H(M202)G RCs is generally consistent with the low-frequency RR spectra. The Mg-OH(2) stretching vibration is tentatively assigned to a band at 318 cm(-1), a frequency higher than that of the Mg-His stretch of the native pigment ( approximately approximately 235 cm(-1)). (3) The BChl core structure of P(M) in the cavity mutant is rendered similar (but not identical) to that of the wild-type when the adventitious water axial ligand is replaced by Im. (4) Exchange with D(2)O results in more global structural changes, likely involving the protein, which in turn affect the structure of the BChls in P. (5) Assignment of the low-frequency vibrational spectrum of P is generally more complex than originally suggested.

Asymmetric electron transfer in cyanobacterial Photosystem I: charge separation and secondary electron transfer dynamics of mutations near the primary electron acceptor A0

Point mutations were introduced near the primary electron acceptor sites assigned to A0 in both the PsaA and PsaB branches of Photosystem I in the cyanobacterium Synechocystis sp. PCC 6803. The residues Met688PsaA and Met668PsaB, which provide the axial ligands to the Mg2+ of the eC-A3 and eC-B3 chlorophylls, were changed to leucine and asparagine (chlorophyll notation follows Jordan et al., 2001). The removal of the ligand is expected to alter the midpoint potential of the A0/A0- redox pair and result in a change in the intrinsic charge separation rate and secondary electron transfer kinetics from A0- to A1. The dynamics of primary charge separation and secondary electron transfer were studied at 690 nm and 390 nm in these mutants by ultrafast optical pump-probe spectroscopy. The data reveal that mutations in the PsaB branch do not alter electron transfer dynamics, whereas mutations in the PsaA branch have a distinct effect on electron transfer, slowing down both the primary charge separation and the secondary electron transfer step (the latter by a factor of 3-10). These results suggest that electron transfer in cyanobacterial Photosystem I is asymmetric and occurs primarily along the PsaA branch of cofactors.

Structure, dynamics, and energetics of the primary photochemistry of photosystem II of oxygenic photosynthesis

Recent progress in two-dimensional and three-dimensional electron and X-ray crystallography of Photosystem II (PSII) core complexes has led to major advances in the structural definition of this integral membrane protein complex. Despite the overall structural and kinetic similarity of the PSII reaction centers to their purple non-sulfur photosynthetic bacterial homologues, the different cofactors and subtle differences in their spatial arrangement result in significant differences in the energetics and mechanism of primary charge separation. In this review we discuss some of the recent spectroscopic, structural, and mutagenic work on the primary and secondary electron transfer reactions in PSII, stressing what is experimentally novel, what new insights have appeared, and where questions of interpretation remain.

Design, synthesis and properties of synthetic chlorophyll proteins

A chemoselective method is described for coupling chlorophyll derivatives with an aldehyde group to synthetic peptides or proteins modified with an aminoxyacetyl group at the epsilon-amino group of a lysine residue. Three template-assembled antiparallel four-helix bundles were synthesized for the ligation of one or two chlorophylls. This was achieved by coupling unprotected peptides to cysteine residues of a cyclic decapeptide by thioether formation. The amphiphilic helices were designed to form a hydrophobic pocket for the chlorophyll derivatives. Chlorophyll derivatives Zn-methyl-pheophorbide b and Zn-methyl-pyropheophorbide d were used. The aldehyde group of these chlorophyll derivatives was ligated to the modified lysine group to form an oxime bond. The peptide-chlorophyll conjugates were characterized by electrospray mass spectrometry, analytical HPLC, and UV/visible spectroscopy. Two four-helix bundle chlorophyll conjugates were further characterized by size-exclusion chromatography, circular dichroism, and resonance Raman spectroscopy.

Enantiomeric discrimination of Ru-substrates by cytochrome P450cam

Molecules with photosensitizers attached to substrates (Wilker et al., Angew. Chem. Int. Ed. 38 (1999) 90-92) or cofactors (Hamachi et al., J. Am. Chem. Soc. 121 (1999) 5500-5506) can rapidly deliver redox equivalents to buried active sites. The structure of cytochrome P450cam (P450) co-crystallized with a prototypal sensitizer-substrate, [Ru-C9-Ad]Cl2, has been determined (Dmochowski et al., Proc. Natl. Acad. Sci. USA 96 (1999) 12987-12990); and, in separate UV-vis absorption and time-resolved luminescence experiments, the binding of the lambda and delta enantiomers of Ru-C9-Ad to P450 has been measured. The results, KD(delta/lambda) approximately 2, indicate that the bipyridyl ligands of the lambda isomer interact more favorably with hydrophobic residues at the entrance to the substrate channel. We conclude that enantiospecific interactions may be exploited in the design of enzyme-metallosubstrate conjugates.

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.

Relationship between altered structure and photochemistry in mutant reaction centers in which bacteriochlorophyll replaces the photoactive bacteriopheophytin

Qy-excitation resonance Raman (RR) spectra are reported for two mutant reaction centers (RCs) from Rhodobacter capsulatus in which the photoactive bacteriopheophytin (BPhL) is replaced by a bacteriochlorophyll (BChl) molecule, designated beta. The pigment change in both mutants is induced via introduction of a histidine residue near the photoactive cofactor. In one mutant, L(M212)H, the histidine is positioned over the core of the cofactor and serves as an axial ligand to the Mg+2 ion. In the other mutant, F(L121)H/F(L97)V, the histidine is positioned over ring V of the cofactor, which is nominally too distant to permit bonding to the Mg+2 ion. The salient observations are as follows: (1) The beta cofactor in F(L121)H/F(L97)V RCs is a five-coordinate BChl molecule. However, there is no evidence for the formation of a Mg-His bond. This bond is either much weaker than in the L(M212)H RCs or completely absent, the latter implying coordination by an alternative ligand. The different axial ligation for beta in the F(L121)H/F(L97)V versus L(M212)H RCs in turn leads to different conformations of the BChl macrocycles. (2) The C9-keto group of beta in F(L121)H/F(L97)V RCs is free of hydrogen bonding interactions, unlike the L(M212)H RCs in which the C9-keto of beta is hydrogen bonded to Glu L104. The interactions between other peripheral substituents of beta and the protein are also different in the F(L121)H/F(L97)V RCs versus L(M212)H RCs. Accordingly, the position and orientation of beta in the protein is different in the two beta-containing RCs. Nonetheless, previous studies have shown that the primary electron transfer reactions are very similar in the two mutants but differ in significant respects compared to wild-type RCs. Collectively, these observations indicate that changes in the conformation of a photoactive tetrapyrrole macrocycle or its interactions with the protein do not necessarily lead to significantly perturbed photochemistry and do not underlie the altered primary events in beta-type RCs.

Photosynthetic reaction centers

The reaction center is the key component for the primary events in the photochemical conversion of light into chemical energy. After excitation by light, a charge separation that spans the cell membrane is formed in the reaction center in a few hundred picoseconds with a quantum yield of essentially one. A conserved pattern in the cofactors and core proteins of reaction centers from different organisms can be defined based on comparisons of the three dimensional structure of two types of reaction centers. Different functional aspects of the reaction center are discussed, including the properties of the bacteriochlorophyll or chlorophyll dimer that constitutes the primary electron donor, the pathway of electron transfer, and the different functional roles of the electron acceptors. The implication of these results on the evolution of the reaction center is presented.

Interactions of chlorophyll a with synthesized peptide in aqueous solution

The interactions between chlorophyll a and synthesized peptides have been studied using optical spectroscopy. Three 30-residue peptides are designed and synthesized: an amphiphilic peptide without histidine (L), an amphiphilic peptide with histidine (L/H) and a hydrophilic peptide (K/E). These peptide properties thereby allow us to examine the effect of the peptide hydrophobicity and/or histidine residue on pigment-peptide interactions. On mixing with peptides, chlorophyll a has a main absorption band in the Qy region with the maximum at 672 nm. For all three peptides, fluorescence patterns show that at a low concentration of the peptide (0.05 mM) in aqueous solution, the energy is transferred among various forms of the pigment. Only peptide L/H at high concentration (0.5 mM) in solution retains the Qy band of chlorophyll a at 672 nm, and the emission is that typically seen for the monomeric form of the pigment. The aggregation of chlorophyll a is suppressed most strongly in the presence of the peptides L/H. The results suggest that chlorophyll a is ligated to a histidine residue, located in the hydrophobic region of the peptides L/H, and in surrounded or shielded by the peptide alpha-helixes.

Chemical complementation identifies a proton acceptor for redox-active tyrosine D in photosystem II

Through the use of site-directed mutagenesis and chemical rescue, we have identified the proton acceptor for redox-active tyrosine D in photosystem II (PSII). Effects of chemical rescue on the tyrosyl radical were monitored by EPR spectroscopy. We also have acquired the Fourier-transform infrared (FT-IR) spectrum associated with the oxidation of tyrosine D and concomitant protonation of the acceptor. Mutant and isotopically labeled PSII samples are used to assign vibrational lines in the 3,600-3,100 cm-1 region to N-H modes of His-189 in the D2 polypeptide. When His-189 in D2 is changed to a leucine (HL189D2) in PSII, dramatic alterations of both EPR and FT-IR spectra are observed. When imidazole is introduced into HL189D2 samples, results from both EPR and FT-IR spectroscopy argue that imidazole is functionally reconstituted into an accessible pocket and that imidazole acts as a chemical mimic for His-189. Small perturbations of EPR and FT-IR spectra are consistent with access to this pocket in wild-type PSII, as well. Structures of the analogous site in bacterial reaction centers suggest that an accessible pocket, large enough to contain imidazole, is bordered by tyrosine D and His-189 in the D2 polypeptide. These data provide evidence that His-189 in the D2 polypeptide of PSII acts as a proton acceptor for redox-active tyrosine D and that proton transfer to the imidazole ring facilitates the efficient oxidation/reduction of tyrosine D.

Crystal structure of the bacteriochlorophyll a protein from Chlorobium tepidum

The bacteriochlorophyll (BChl) a protein from Chlorobium tepidum, which participates in energy transfer in green photosynthetic bacteria, has been crystallized using the sitting drop method of vapor diffusion. X-ray diffraction data collected from these crystals indicate that the crystals belong to the cubic space group P4132 with cell dimensions of a=b=c=169.5 A. A native X-ray diffraction data set has been collected to a resolution of 2.2 A. The initial solution was determined by using the molecular replacement method using the structure of the previously solved BChl a protein from Prosthecochloris aestuarii. A unique rotation and translation solution was obtained for two monomers in the asymmetric unit giving a pseudo-body centered packing. After rebuilding and refinement the model yields an R factor of 19.0%, a free R-factor of 28.3%, and good geometry with root-mean-square deviations of 0.013 A and 2.1 degrees for the bond lengths and angles, respectively. The structure of the BChl a protein from C. tepidum consists of three identical subunits related by a 3-fold axis of crystallographic symmetry. In each subunit the polypeptide backbone forms large beta-sheets and encloses a central core of seven BChl a molecules. The distances between neighboring bacteriochlorin systems within a subunit range between 4 A to 11 A and that between two bacteriochlorins from different subunits is more than 20 A. The overall structure is comparable with that of P. aestuarii but significant differences are observed for the individual bacteriochlorophyll structures. The surface of the trimer has a hydrophobic region that is modeled as the complex being a peripheral membrane protein partially embedded in the membrane. A general model is presented for the membrane organization with two of the bacteriochlorophyll structures in the membrane and transferring energy to the reaction center complex. In this model these two bacteriochlorophyll structures serve a similar role to the cofactors of integral membrane light-harvesting complexes although the protein structure surrounding the cofactors is significantly different for the BChl a protein compared with the integral membrane complexes.