Iron-sulfur cluster assembly: characterization of IscA and evidence for a specific and functional complex with ferredoxin

The synthesis of iron-sulfur clusters in Escherichia coli is believed to require a complex protein machinery encoded by the isc (iron-sulfur cluster) operon. The product of one member of this operon, IscA, has been overexpressed, purified, and characterized. It can assemble an air-sensitive [2Fe-2S] cluster as shown by UV-visible and resonance Raman spectroscopy. The metal form but not the apoform of IscA binds ferredoxin, another member of the isc operon, selectively, allowing transfer of iron and sulfide from IscA to ferredoxin and formation of the [2Fe-2S] holoferredoxin. These results thus suggest that IscA is involved in ferredoxin cluster assembly and activation. This is an important function because a functional ferredoxin is required for maturation of other cellular Fe-S proteins.

Primary donor structure and interactions in bacterial reaction centers from near-infrared Fourier transform resonance Raman spectroscopy

Preresonance Raman and resonance Raman spectra of the primary donor (P) from reaction centers of the Rhodobacter (Rb.) sphaeroides R26 carotenoidless strain in the P and P+ states, respectively, were obtained at room temperature with 1064-nm excitation and a Fourier transform spectrometer. These spectra clearly indicate that the chromophore modes are observable over those of the protein with no signs of interference below 1800 cm-1. The chromophore modes are dominated by those of the bacteriochlorophylls (BChl a), and it is estimated that, in the P state, ca. 65% of the Raman intensity of the BChl a modes arises from the primary donor. This permits the direct observation of a vibrational spectrum of the primary donor at preresonance with the excitonic 865-nm band. The Raman spectrum of oxidized reaction centers in the presence of ferricyanide clearly exhibits bands arising from a BChl a+ species. The magnitude of the frequency shift of a keto carbonyl of neutral P from 1691 to 1717 cm-1 upon P+ formation strongly suggests that one BChl molecule in P+ carries nearly the full +1 charge. Our results indicate that the unpaired electron in P.+ does not share a molecular orbital common to the two components of the dimer on the time scale of the resonance Raman effect (ca. 10(-13) s).

Site-specific mutagenesis of the reaction centre from Rhodobacter sphaeroides studied by Fourier transform Raman spectroscopy: mutations at tyrosine M210 do not affect the electronic structure of the primary donor

The effects of mutation of residue tyrosine M210 on the primary donor bacteriochlorophylls have been investigated by near infrared FT-Raman spectroscopy in reaction centres purified from an antenna-deficient strain of Rhodobacter sphaeroides. We find that mutation at the M210 position does not significantly perturb the distribution of the unpaired electron over the pair of bacteriochlorophyll molecules which constitute the primary donor radical cation. We conclude, therefore, that the effects of mutation of tyrosine M210 on the rate and asymmetry of primary electron transfer in reaction centres cannot be ascribed to a change in the electronic structure of the primary donor.

Changes in primary donor hydrogen-bonding interactions in mutant reaction centers from Rhodobacter sphaeroides: identification of the vibrational frequencies of all the conjugated carbonyl groups

Specific changes in the hydrogen-bonding states of the primary donor, P, in reaction centers from Rhodobacter sphaeroides bearing mutations near P were determined using near-infrared excited Fourier transform (FT) Raman spectroscopy. This technique, using 1064-nm excitation, provides the preresonantly enhanced vibrational spectrum of P in its reduced state selectively over the contributions of the other reaction center chromophores and protein and yields structural information concerning P and its hydrogen-bonding interactions. The mutations studied were as follows: Leu M160-->His, Leu L131-->His, the D9 double mutant (Leu M160-->His + Leu L131-->His), Phe M197-->His, and His L168-->Phe. These mutations were designed to introduce new, or to break existing, hydrogen bonds to the C9 and C2 carbonyl groups of P. On the basis of previous assignments [Mattioli, T. A., Hoffmann, A., Robert, B., Schrader, B., & Lutz, M. (1991) Biochemistry 30, 4648-4654], the FT Raman spectra of these mutants show the predicted changes in hydrogen bond interactions of P carbonyl groups with the protein. The results of this study have permitted us to unambiguously identify the C2 and C9 carbonyl vibrators of P in Rb. sphaeroides. The genetically introduced hydrogen bond interactions are discussed in terms of other physicochemical properties of P including the redox potential and electronic asymmetry in the P+ state. It is discussed that changes in protein hydrogen bonding to the conjugated carbonyl groups of P alone are not the sole factor that contributes to the sizeable modifications of the P/P+ redox midpoint potentials, and that the chemical nature of the hydrogen bond donor plays a significant role in this modification.

Effects of hydrogen bonds on the redox potential and electronic structure of the bacterial primary electron donor

The primary donor, P, of photosynthetic bacterial reaction centers (RCs) is a dimer of excitonically interacting bacteriochlorophyll (BChl) molecules. The two constituents are named PL and PM to designate their close association with the L- and M-subunits, respectively, of the RC protein. A series of site-directed mutants of RCs from Rhodobacter sphaeroides has been constructed in order to model the effects of hydrogen bonding on the redox midpoint potential and electronic structure of P. The leucine residue at position M160 was genetically replaced with eight other amino acid residues capable of donating a hydrogen bond to the C9 keto carbonyl group of the PM BChl a molecule of P. Fourier transform (FT) (pre)resonance Raman spectroscopy with 1064 nm excitation was used to (i) determine the formation and strengths of hydrogen bonds on this latter keto carbonyl group in the reduced, neutral state (PO), and (ii) determine the degree of localization of the positive charge on one of the two constituent BChl molecules of P in its oxidized, radical cation state (P*+). A correlation was observed between the strength of the hydrogen bond and the increase in PO/P*+ redox midpoint potential. This correlation is less pronounced than that observed for another series of RC mutants where hydrogen bonds to the four pi-conjugated carbonyl groups of P were broken or formed uniquely involving histidinyl residues [Mattioli, T. A., Lin, X., Allen, J. P. and Williams, J. C. (1995) Biochemistry 34, 6142-6152], indicating that histidinyl residues are more effective in raising the PO/P*+ redox midpoint potential via hydrogen bond formation than are other hydrogen bond-forming residues. In addition, an increase in positive charge localization is correlated with the strength of the hydrogen bond and with the PO/P*+ redox midpoint potential. This latter correlation was analyzed using an asymmetric bacteriochlorophyll dimer model based on Hückel-type molecular orbitals in order to obtain estimates of certain energetic parameters of the primary donor. Based on this model, the correlation is extrapolated to the case of complete localization of the positive charge on PL and gives a predicted value for the P/P+ redox midpoint potential similar to that experimentally determined for the Rb. sphaeroides HL(M202) heterodimer. The model yields parameters for the highest occupied molecular orbital energies of the two BChl a constituents of P which are typical for the oxidation potential of isolated BChl a in vitro, suggesting that the protein, as compared to many solvents, does not impart atypical redox properties to the BChl a constituents of P.

Correlation between multiple hydrogen bonding and alteration of the oxidation potential of the bacteriochlorophyll dimer of reaction centers from Rhodobacter sphaeroides

The electronic absorption and vibrational Raman spectra of mutant reaction centers from Rhodobacter sphaeroides bearing multiple site-specific mutations near the primary electron donor (P), a bacteriochlorophyll dimer, are reported. These mutations bear double and triple combinations of single-point mutations that alter the H-bonding interactions between histidine residues and the C2- and C9-conjugated carbonyl groups of the primary donor [Mattioli, T.A., Williams, J.C., Allen, J.P., & Robert, B. (1994) Biochemistry 33, 1636-1643] and change the donor redox midpoint potential from 410 to 765 mV compared to 505 mV for wild type [Lin, X., Murchison, H.A., Nagarajan, V., Parson, W.W., Williams, J.C., & Allen, J.P. (1994) Proc. Natl. Acad. Sci. U.S.A. 91, 10265-10269]. Near-infrared Fourier transform Raman spectroscopy was used to determine the changes in H-bonding interactions of the primary donor in these multiple mutants. The Fourier transform Raman spectra of the mutants exhibit the predicted changes in hydrogen bond interactions of the P carbonyl groups with the protein, and they are consistent with the designed mutations. Moreover, the Raman data verify that the H-bonds formed or broken in the multiple mutants are similar in strength to those observed in the corresponding single mutants. A correlation was observed between the change in P/P.+ redox midpoint potential and the total change in H-bonding interaction energy (from -207 to 364 meV relative to wild type) as gauged by the estimated enthalpy of each H-bond formed or broken on the four conjugated carbonyls of the primary donor. Only minor changes were observed in the optical spectra of the mutant reaction centers, indicating that the addition of H-bonds from histidines has little effect in destabilizing the first electronic excited state of the dimer relative to the ground state. However a blue shift in the dimer absorption band at ca. 890 nm at 20 K was associated with the removal of the H-bond to the C2 acetyl carbonyl group via His L168. A red shift of the oxidized dimer band at ca. 1250 nm was associated with the formation of each H-bond to the C9 keto carbonyl groups.

Effects of hydrogen bonding to a bacteriochlorophyll-bacteriopheophytin dimer in reaction centers from Rhodobacter sphaeroides

The properties of the primary electron donor in reaction centers from Rhodobacter sphaeroides have been investigated in mutants containing a bacteriochlorophyll (BChl)--bacteriopheophytin (BPhe) dimer with and without hydrogen bonds to the conjugated carbonyl groups. The heterodimer mutation His M202 to Leu was combined with each of the following mutations: His L168 to Phe, which should remove an existing hydrogen bond to the BChl molecule; Leu L131 to His, which should add a hydrogen bond to the BChl molecule; and Leu M160 to His and Phe M197 to His, each of which should add a hydrogen bond to the BPhe molecule [Rautter, J., Lendzian, F., Schulz, C., Fetsch, A., Kuhn M., Lin, X., Williams, J. C., Allen J. P., & Lubitz, W. (1995) Biochemistry 34, 8130-8143]. Pigment extractions and Fourier transform Raman spectra confirm that all of the mutants contain a heterodimer. The bands in the resonance Raman spectra arising from the BPhe molecule, which is selectively enhanced, exhibit the shifts expected for the addition of a hydrogen bond to the 9-keto and 2-acetyl carbonyl groups. The oxidation--reduction midpoint potential of the donor is increased by approximately 85 mV by the addition of a hydrogen bond to the BChl molecule but is only increased by approximately 15 mV by the addition of a hydrogen bond to the BPhe molecule. An increase in the rate of charge recombination from the primary quinone is correlated with an increase in the midpoint potential. The yield of electron transfer to the primary quinone is 5-fold reduced for the mutants with a hydrogen bond to the BPhe molecule. Room- and low-temperature optical absorption spectra show small differences from the features that are typical for the heterodimer, except that a large increase in absorption is observed around 860-900 nm for the donor Qy band in the mutant that adds a hydrogen bond to the BChl molecule. The changes in the optical spectra and the yield of electron transfer are consistent with a model in which the addition of a hydrogen bond to the BChl molecule increases the energy of an internal charge transfer state while the addition to the BPhe molecule stabilizes this state. The results show that the properties of the heterodimer are different depending on which side is hydrogen-bonded and suggest that the hydrogen bonds alter the energy of the internal charge transfer state in a well-defined manner.

Structure, spectroscopic, and redox properties of Rhodobacter sphaeroides reaction centers bearing point mutations near the primary electron donor

Single mutations of three amino acid residues in the vicinity of the primary electron donor, P, in the reaction center (RC) from Rhodobacter (Rb.) sphaeroides were constructed and characterized in order to study the effects of hydrogen-bonding on the physical properties of P. The mutations, Phe M197-->Tyr, Met L248-->Thr, and Ser L244-->Gly, represent single amino acid changes near P designed to introduce residues found in Rhodopseudomonas (Rps.) viridis and to, thus, probe the effects of nonconserved residues. The mutations were designed to change the nonconserved H-bonding interactions of P in Rb. sphaeroides, at the level of a C2 acetyl, a C9 keto, and a C10 ester carbonyl of P, respectively, to those present in Rps. viridis. The Fourier transform (pre)resonance Raman (FTRR) spectra of P, in its reduced and oxidized states, from reaction centers of these mutants were studied to determine modifications of H-bond interactions of the pi-conjugated C2 acetyl and C9 keto carbonyl groups and the C10 carbomethoxy ester carbonyl groups of P. The vibrational spectra of reduced P in the Met L248-->Thr and Ser L244-->Gly mutants reveal no evidence for changes in the H-bonding pattern of P; this suggests that for Rb. sphaeroides wild type, Ser L244 is not H-bonded to the C10 ester carbonyl of PL. The vibrational spectrum of reduced P from the Phe M197-->Tyr mutant compared to that of wild type can unambiguously be interpreted in terms of the formation of a new H-bond with an acetyl carbonyl of P, specifically PM. Correlating with the new H-bond, the Phe M197-->Tyr mutant exhibits an electronic absorption spectrum where the P absorption band is significantly perturbed. Intact cell and chromatophore photobleaching spectra of the same mutant indicate that the P absorption band has red-shifted by ca. 10 nm; no such behavior is observed for the other mutants. As well, the P-->BPheL electron transfer rate does not seem to strongly depend on the H-bonding of the C2 acetyl carbonyl of PM to a tyrosine residue. The EPR zero-field splitting parameters, E and D, of the primary donor triplet are only slightly modified in the mutant reaction centers, on the order of 1%.(ABSTRACT TRUNCATED AT 400 WORDS)

Conformation of bacteriochlorophyll molecules in photosynthetic proteins from purple bacteria

Fourier transform near-infrared resonance Raman spectroscopy can be used to obtain information on the bacteriochlorophyll a (BChl a) molecules responsible for the redmost absorption band in photosynthetic complexes from purple bacteria. This technique is able to distinguish distortions of the bacteriochlorin macrocycle as small as 0.02 A, and a systematic analysis of those vibrational modes sensitive to BChl a macrocycle conformational changes was recently published [Näveke et al. (1997) J. Raman Spectrosc. 28, 599-604]. The conformation of the two BChl a molecules constituting the primary electron donor in bacterial reaction centers, and of the 850 and 880 nm-absorbing BChl a molecules in the light-harvesting LH2 and LH1 proteins, has been investigated using this technique. From this study it can be concluded that both BChl a molecules of the primary electron donor in the photochemical reaction center are in a conformation close to the relaxed conformation observed for pentacoordinate BChl a in diethyl ether. In contrast, the BChl a molecules responsible for the long-wavelength absorption transition in both LH1 and LH2 antenna complexes are considerably distorted, and furthermore there are noticeable differences between the conformations of the BChl molecules bound to the alpha- and beta-apoproteins. The molecular conformations of the pigments are very similar in all the antenna complexes investigated.

Resonance Raman characterization of Rhodobacter sphaeroides reaction centers bearing site-directed mutations at tyrosine M210

Resonance Raman (RR) spectroscopy and low-temperature absorption spectroscopy have been used to investigate the structural changes in the reaction centers (RCs) of Rhodobacter sphaeroides induced by site-specific mutations on the tyrosine (Y) M210 residue. RCs in which Y M210 has been genetically replaced with phenylalanine (F) or leucine (L) exhibit a 5-fold decrease in their primary electron-transfer kinetics (Finkele et al., 1990). The general similarity of RR spectra of the wild-type RCs as compared to those of the two mutant RCs indicates that no significant global structural changes occur upon mutation at the level of any of the six bacteriochlorin pigments. In the RR spectra of the two mutant RCs there is a conspicuous absence of contributions from the BPheM prosthetic group, which is interpreted in terms of a change in the resonance enhancement conditions of this chromophore. Low-temperature adsorption spectroscopy reveals marked shifts in the Qx absorption band of BPheM. This shift is interpreted as arising from a destabilization of the protein in the vicinity of BPheM and accounts for the change in resonance condition for this chromophore in its RR contributions. As well, there is a 3-nm red shift of the Qy absorption band of the BChls from 803 to 806 nm for the mutant RCs. Difference RR spectra yielding structural information concerning, selectively, the primary donor (P) indicate that the structure of the P binding pocket is conserved for these mutant RCs. The tyrosine M210 is not observed to be engaged in a hydrogen bond with either of the acetyl or keto carbonyls of P.

Time-resolved and steady-state spectroscopic analysis of membrane-bound reaction centers from Rhodobacter sphaeroides: comparisons with detergent-solubilized complexes

The spectroscopic analysis of the antenna-deficient Rhodobacter sphaeroides strain RCO1 has been extended to an investigation of the kinetics and spectroscopy of primary charge separation. Global analysis of time-resolved difference spectra demonstrated that the rate of charge separation in membrane-bound reaction centers is slightly slower than in detergent-solubilized reaction centers from the same strain. A kinetic analysis of the decay of the primary donor excited state at single wavelengths was carried out using a high repetition rate laser system, with the reaction centers being maintained in the open state using a combination of phenazine methosulfate and horse heart cytochrome c. The kinetics of primary charge separation in both membrane-bound and solubilized reaction centers were found to be non-monoexponential, with two exponential decay components required for a satisfactory description of the decay of the primary donor excited state. The overall rate of charge separation in membrane-bound reaction centers was slowed if the primary acceptor quinone was reduced using sodium ascorbate. This slowing was caused, in part, by an increase in the relative amplitude of the slower of the two exponential components. The acceleration in the rate of charge separation observed on removal of the reaction center from the membrane did not appear to be caused by a significant change in the electrochemical properties of the primary donor. The influence of the environment of the reaction center on primary charge separation is discussed together with the origins of the non-monoexponential decay of the primary donor excited state.

Calcium binding to the photosystem II subunit CP29

We have identified a Ca(2+)-binding site of the 29-kDa chlorophyll a/b-binding protein CP29, a light harvesting protein of photosystem II most likely involved in photoregulation. (45)Ca(2+) binding studies and dot blot analyses of CP29 demonstrate that CP29 is a Ca(2+)-binding protein. The primary sequence of CP29 does not exhibit an obvious Ca(2+)-binding site therefore we have used Yb(3+) replacement to analyze this site. Near-infrared Yb(3+) vibronic side band fluorescence spectroscopy (Roselli, C., Boussac, A., and Mattioli, T. A. (1994) Proc. Natl. Acad. Sci. U. S. A. 91, 12897-12901) of Yb(3+)-reconstituted CP29 indicated a single population of Yb(3+)-binding sites rich in carboxylic acids, characteristic of Ca(2+)-binding sites. A structural model of CP29 presents two purported extra-membranar loops which are relatively rich in carboxylic acids, one on the stromae side and one on the lumenal side. The loop on the lumenal side is adjacent to glutamic acid 166 in helix C of CP29, which is known to be the binding site for dicyclohexylcarbodiimide (Pesaresi, P., Sandonà, D., Giuffra, E. , and Bassi, R. (1997) FEBS Lett. 402, 151-156). Dicyclohexylcarbodiimide binding prevented Ca(2+) binding, therefore we propose that the Ca(2+) in CP29 is bound in the domain including the lumenal loop between helices B and C.

Structure and binding site of the primary electron acceptor in the reaction center of Chlorobium

In isolated, chlorosome-free reaction centers from Chlorobium limicola f thiosulphatophilum, a chlorin pigment exhibits a Qy absorption band at 672 nm (Feiler, U., Nitschke, W., & Michel, H. (1992) Biochemistry 31, 2608-2614). To characterize the chemical nature of this chlorin pigment and its interactions within the reaction-center protein, selective enhancement of its Raman scattering was achieved by resonant excitation within its Soret band. This is the first time that structural studies of this pigment were performed on the native reaction-center protein. The obtained resonance Raman spectra were consistent with a single population of a chlorophyll a(-like) pigment, possessing a vinyl group on ring I, but not with bacteriochlorophyll c or bacteriophaeophytin c. The stretching frequencies of the C9-keto carbonyl of this pigment indicates that it is H-bonded to the reaction-center protein. The strength of this H-bond is very close to those of the keto carbonyls of the primary electron acceptors in purple bacterial reaction centers and D1/D2 particles. Since in membranes of Chlorobiaceae a transient bleaching at 670 nm is due to the primary acceptor in the reaction center (Nuijs, A. M., Vasmel, H., Joppe, H. L. P., Duysens, L. N. M., & Amesz, J. (1985a) Biochim. Biophys. Acta 907, 24-34), we thus conclude that the primary acceptor in Chlorobium reaction centers is the characterized chlorophyll a(-like) pigment.(ABSTRACT TRUNCATED AT 250 WORDS)

Functional Implications of the Propionate 7−Arginine 220 Interaction in the FixLH Oxygen Sensor from Bradyrhizobium japonicum

BjFixL from Bradyrhizobium japonicum is a heme-based oxygen sensor implicated in the signaling cascade that enables the bacterium to adapt to fluctuating oxygen levels. Signal transduction is initiated by the binding of O(2) to the heme domain of BjFixL, resulting in protein conformational changes that are transmitted to a histidine kinase domain. We report structural changes of the heme and its binding pocket in the Fe(II) deoxy and Fe(III) met states of the wild-type BjFixLH oxygen sensor domain and four mutants of the highly conserved residue arginine 220. UV-visible, electron paramagnetic resonance, and resonance Raman spectroscopies all showed that the heme iron of the R220H mutant is unexpectedly six-coordinated at physiological pH in the Fe(III) state but undergoes pH- and redox-dependent coordination changes. This behavior is unprecedented for FixL proteins, but is reminiscent of another oxygen sensor from E. coli, EcDos. All mutants in their deoxy states are five-coordinated Fe(II), although we report rupture of the residue 220-propionate 7 interaction and structural modifications of the heme conformation as well as propionate geometry and flexibility. In this work, we conclude that part of the structural reorganization usually attributed to O(2) binding in the wild-type protein is in fact due to rupture of the Arg220-P7 interaction. Moreover, we correlate the structural modifications of the deoxy Fe(II) states with k(on) values and conclude that the Arg220-P7 interaction is responsible for the lower O(2) and CO k(on) values reported for the wild-type protein.

Symmetric structural features and binding site of the primary electron donor in the reaction center of Chlorobium

The protein binding interactions of the constituent bacteriochlorophyll a molecules of the primary electron donor, P840, in isolated reaction centers from Chlorobium limicola f thiosulphatophilum and the electronic symmetry of the radical cation P840+. were determined using near-infrared Fourier transform (FT) Raman spectroscopy excited at 1064 nm. The FT Raman vibrational spectrum of P840 indicates that it is constituted of a single population of BChl a molecules which are spectrally indistinguishable. The BChl a molecules of P840 are pentacoordinated with only one axial ligand on the central Mg atom, and the pi-conjugated C2 acetyl and C9 keto carbonyls are free of hydrogen-bonding interactions. The FT Raman spectrum of P840+. exhibits a 1707 cm-1 band attributable to a BChl a C9 keto carbonyl group vibrational frequency that has upshifted 16 cm-1 upon oxidation of P840; this upshift is exactly one-half of that expected for the one-electron oxidation of monomeric BChl a in vitro. The 16 cm-1 upshift, thus, indicates that the resulting +1 charge is equally shared between two BChl a molecules. This situation is markedly different from that of the oxidized primary donor of the purple bacterial reaction center of Rhodobacter sphaeroides, (i) which exhibits a 1717 cm-1 band that has upshifted 26 cm-1, indicating an asymmetric distribution of the resulting +1 charge over the two constituent BChl a molecules, and (ii) whose H-bonding pattern with respect to the pi-conjugated carbonyl groups is asymmetric.(ABSTRACT TRUNCATED AT 250 WORDS)

Fanconi's anemia: clinical and radiographic oral manifestations

BACKGROUND

Fanconi's anemia (FA) is a rare autosomal recessive disorder characterized by progressive bone marrow failure, congenital abnormalities, and predisposition to malignancies. There are 11 genetic subtypes characterized by complementation groups - FA- A, B, C, D1, D2, E, F, G, I, J, and L.

OBJECTIVE

To evaluate and describe clinical, oral and radiographic manifestations of patients with FA.

METHODS

A quantitative analysis of clinical manifestations, oral lesions and panoramic radiographs was performed in 33 patients.

RESULTS

Clinical manifestations included melanin skin pigmentation, skin vascular and ocular anomalies. Melanin pigmentation on oral mucosa, traumatic lesions, gingival bleeding, dental biofilm and gingival alterations were the main oral manifestations that were found. Oral and clinical manifestations were not dependent on patient's sex. No significant statistical difference between females and males was detected. Dental anomalies were not remarkable either at clinical or at radiographic examinations. Although several dental anomalies were observed in patients with FA, the correlation between this disease was not established from this study. Panoramic radiographs showed agenesis, taurodontism, radicular anomalies such as dilaceration, tapering, and foreshortening.

CONCLUSION

This study suggests that gingival alterations are associated with defective oral hygiene but not with hematologic conditions. It also helps elucidate oral manifestations of FA. These patients are living longer and need special dental care.

Influence of Asn/His L166 on the hydrogen-bonding pattern and redox potential of the primary donor of purple bacterial reaction centers

The primary electron donor (P) of the photosynthetic reaction center (RC) from the purple bacterium Rhodobacter (Rb.) sphaeroides is constituted of two bacteriochlorophyll molecules in excitonic interaction. The C2 acetyl carbonyl group of one of the two bacteriochlorophyll molecules (PL), the one more closely associated with the L polypeptide subunit, is engaged in a hydrogen bond with histidine L168, while the other pi-conjugated carbonyl groups of P are free from such hydrogen-bonding interactions. The three-dimensional X-ray crystal structures of the RC from several strains of Rb. sphaeroides reveal that asparagine L166 probably interacts indirectly with P through His L168. Such an interaction is expected to modulate the hydrogen bond between P and His L168, a residue which is highly conserved in purple bacteria. RC mutants of Rb. sphaeroides where asparagine L166 was genetically replaced by leucine [NL(L166)], histidine [NH(L166)], and aspartate [ND(L166)] were studied using Fourier transform (FT) Raman spectroscopy. All of these mutations resulted in an increase in the strength of the hydrogen bond between His L168 and the acetyl carbonyl group of P(L), as observed in the FT Raman spectrum, by the 2-4 cm(-1) decrease in vibrational frequency of the 1620 cm(-1) band which has been assigned to this specific acetyl carbonyl group [Mattioli, T. A., Lin, X., Allen, J. P., & Williams, J. C. (1995) Biochemistry 34, 6142-6152]. At pH 8, the NH(L166) mutation showed the greatest change in the P0/P.+ redox midpoint potential (515 mV), increasing it by ca. 30 mV compared to that of wild type (485 mV). A similar increase in P0/P.+ redox midpoint potential for NH(L166) compared to that of wild type is also observed at pH 5, 6, and 9.5. The p0/P.+ midpoint potential of the NL(L166) mutant was comparable to that of wild type at all pH values. In contrast, for the ND(L166) mutant, the midpoint potential shows a markedly different pH dependency, being 25 mV higher than wild type at pH 5 but 20 mV lower than wild type at pH 9.5. The hydrogen bond interactions of the primary electron donor from Rhodospirillum (Rsp.) centenum were determined from the FT Raman vibrational spectrum which exhibits a 1616 cm(-1) band similar to what is seen in the NH(L166) and ND(L166) Rb. sphaeroides mutants. Comparison of the sequence of the L subunit determined for the Rsp. centenum RC with that of other species indicates that positions L166 and L168 are occupied by His residues. The stronger hydrogen bond between the conserved His L168 and the acetyl carbonyl group of P(L), observed in the primary donor of Rsp. centenum and of several bacterial species which are known to possess a histidine residue at the analogous L166 position, is proposed to be due to interactions between these two histidine residues.

Structure and protein binding interactions of the primary donor of the Chloroflexus aurantiacus reaction center

Soret resonance, QX resonance, and QY near-infrared Fourier transform (FT) (pre)resonance Raman spectroscopies were used to determine pigment-protein interactions of specific bacteriochlorin molecules in the reaction center from Chloroflexus aurantiacus. FT Raman spectroscopy, using 1064 nm excitation, was used to selectively obtain preresonance and resonance vibrational Raman spectra of the primary donor (P) of reaction centers (RCs) from Chloroflexus aurantiacus in the Po and P.+ states, respectively. The FT Raman spectrum of RCs in their neutral P (Po) state exhibits bands at 1605, 1632, 1648, and 1696 cm-1 which are attributable to P in its resting neutral state. Specifically, the latter three Raman bands can be assigned to the conjugated C2 acetyl and C9 keto carbonyl groups of the bacteriochlorophyll (BChl) molecules constituting P. The observation of at least three such bands is indicative of a non-monomeric nature of P, consistent with the proposal that it is a dimer of BChl molecules. The 1632 cm-1 band is consistent only with a hydrogen bonded BChl acetyl carbonyl, while the 1648 cm-1 band is assigned to a non-hydrogen bonded acetyl carbonyl. The 1696 cm-1 band is consistent only with a non-hydrogen bonded keto carbonyl group; from the unusually high intensity of this latter band compared to the others, we propose that the 1696 cm-1 band contains contributions from two keto carbonyl groups, both free of hydrogen bonds. From published protein sequence alignments of the L and M subunits of Rhodobacter (Rb.) sphaeroides and Chloroflexus aurantiacus we assign the 1632 cm-1 band as arising from the C2 acetyl carbonyl of the analogous PM constituent of P, which is hydrogen bonded to tyrosine M187 in the Chloroflexus RC, and propose a pigment-protein structural model for the primary donor of Chloroflexus aurantiacus. The FT Raman spectrum of RCs in the P degrees+ state indicates that one component of the 1696 cm-1 band has upshifted 21 cm-1 to 1717 cm-1. Compared to Rb. sphaeroides which showed a 26 cm-1 upshift for the corresponding band, the 21 cm-1 upshift indicates that the + charge is more delocalized over the P.+ species of Chloroflexus; we estimate that ca. 65% of the + charge is localized on one of the two BChl molecules of the Chloroflexus primary donor as compared to ca. 80% for Rb. sphaeroides. The consequences of the proposed structure of the Chloroflexus primary donor in terms of its Po/P.+ redox midpoint potential are discussed.

Hydrogen-bond interactions of the primary donor of the photosynthetic purple sulfur bacterium Chromatium tepidum

We have used near-infrared Fourier transform (pre)resonance Raman spectroscopy to determine the protein interactions with the bacteriochlorophyll (BChl) dimer constituting the primary electron donor, P, in the reaction center (RC) from the thermophilic purple sulfur bacterium Chromatium tepidum. In addition, we report the alignment of partial sequences of the L and M protein subunits of C. tepidum RCs in the vicinity of the primary donor with those of Rhodobacter sphaeroides and Rhodopseudomonas viridis. Taken together, these results enable us to propose the hydrogen-bonding pattern and the H-bond donors to the conjugated carbonyl groups of P. Selective excitation (1064-nm laser radiation) of the FT (pre)-resonance Raman spectra of P in its neutral (P degree) and oxidized (P degree +) states were obtained via their electronic absorption bands at 876 and 1240 nm, respectively. The P degree spectrum exhibits vibrational frequencies at 1608, 1616, 1633, and 1697 cm-1 which bleach upon P oxidation. The P degree + spectrum exhibits new bands at 1600, 1639, and 1719 cm-1. The 1608-cm-1 band, which downshifts to 1600 cm-1 upon oxidation, is assigned to a CaCm methine bridge stretching mode of the P dimer, indicating that each BChl molecule possesses a single axial ligand (His L181 and His M201, from the sequence alignment). The 1616- and 1633-cm-1 bands correspond to two H-bonded pi-conjugated acetyl carbonyl groups of each BChl molecule. with different H-bond strengths: the 1616-cm-1 band is assigned to the PL C2 acetyl group which is H-bonded to a histidine residue (His L176), while the 1633-cm-1 band is assigned to the PM C2 acetyl carbonyl, H-bonded to a tyrosine residue (Tyr M196). Both PL and PM C9 keto carbonyls are free from interactions and vibrate at the same frequency (1697 cm-1). Thus, the H-bond pattern of the primary donor of C. tepidum differs from that of Rb. sphaeroides in the extra H-bond to the PM C2 acetyl carbonyl group; that of PL is H-bonded to a histidine residue in both primary donors (His L168 in Rb. sphaeroides and His L176 in C. tepidum). The P degree/P degree + redox midpoint potentials were measured to be +497 and +526 mV for isolated C. tepidum RCs with and without the associated tetraheme cytochrome c subunit, respectively, and +502 mV for intracytoplasmic membranes. The positive charge localization was estimated to be 69% in favor of PL, indicating a more delocalized situation over the primary donor of C. tepidum than that of Rb. sphaeroides (estimated to be 80% on PL). These differences in physicochemical properties are discussed with respect to the proposed structural model for the microenvironment of the primary donor of C. tepidum.

Direct vibrational structure of protein metal-binding sites from near-infrared Yb3+ vibronic side band spectroscopy

Near-infrared Yb3+ vibronic side band (VSB) spectroscopy is used to obtain structural information of metal binding sites in metalloproteins. This technique provides a selective "IR-like" vibrational spectrum of those ligands chelated to the Yb3+ ion. VSB spectra of various model complexes of Yb3+ representing different ligand types were studied to provide references for the VSB spectra of Yb(3+)-reconstituted metalloproteins. Ca2+ in the calcium-binding protein parvalbumin and Fe3+ in the iron-transporting protein transferrin were replaced with Yb3+. The fluorescence of Yb3+ reconstituted into these two proteins exhibits weak VSBs whose energy shifts, with respect to the main 2F5/2-->2F7/2 Yb3+ electronic transition, represent the vibrational frequencies of the Yb3+ ligands. The chemical nature of the ligands of the Yb3+ in these proteins, as deduced by the observed VSB frequencies, is entirely in agreement with their known crystal structures. For transferrin, replacement of the 12CO3(2-) metal counterion with 13CO3(2-) yielded the expected isotopic shift for the VSBs corresponding to the carbonate vibrational modes. This technique demonstrates enormous potential in elucidating the localized structure of metal binding sites in proteins.

Molecular emission and electronic structure of associated chlorophyll a

We report the wavelength-selective Raman and fluorescence spectra of monomeric and aggregate species of chlorophyll a in solution. The excitation profile for the molecular emission of the various chlorophyll complexes permit the construction of part of the electronic energy level diagram. For a T-shaped dimeric chlorophyll a, pseudo-exciton splittings of 667 cm−1 in the Soret and 397 cm−1 in the Qy bands are obtained.

Temperature-dependent behavior of bacteriochlorophyll and bacteriopheophytin in the photosynthetic reaction center from Rhodobacter sphaeroides

We have reexamined the temperature dependence of resonance Raman (RR) spectra of the bacteriochlorin cofactors bound to reaction centers from Rhodobacter sphaeroides. Three types of resonant excitations were performed, namely, Soret band, bacteriopheophytin Qx-band, and near-infrared, Qy-band (pre)resonances. Sample temperature was varied from 300 to 10 K. In both Soret-resonant and Qy-preresonant Raman spectra, the ca. 1610-cm(-1) band corresponding to a bacteriochlorophyll CaCm methine bridge stretching mode is observed to increase in frequency by 4-6 cm(-1) as temperature is decreased from 300 to 15 K. This upshift is interpreted as arising from a change in conformation of the bacteriochlorophyll macrocycles. It may be nonspecific to the protein-bound cofactors, since a similar 4-cm(-1) upshift was observed in the same temperature range for BChl a in solution. Qx-resonant Raman spectra of either of the two bacteriopheophytin (BPhe) cofactors were obtained selectively using excitations at 537 and 546 nm. No significant frequency shift was observed for the CaCm stretching mode of BPheL between 200 and 15 K. We conclude, at variance with a previous report, that the macrocycle of the BPheL primary electron acceptor does not undergo any significant conformational change in the 200-15 K temperature range. Qy-preresonant excitation of RCs at 1064 nm provided selective Raman information on the primary electron donor (P primary). The stretching frequencies of the two conjugated keto and acetyl carbonyl groups of the M-branch primary donor BChl cofactor (P(M)) did not significantly change between 300 and 10 K. In contrast the keto carbonyl stretching frequency of cofactor P(L) was observed to upshift by 5 cm(-1), while its acetyl carbonyl frequency downshifted by 2 cm(-1). The latter shift indicated that the strong H-bond between the acetyl group of P(L) and His L168 may have slightly strengthened at 10 K. Excitation at 1064 nm of chemically oxidized RCs selectively provided RR spectra of the primary donor in its radical P.+ state. These spectra can be interpreted as a decrease of the localization of the positive charge on P(L) from 78% to 63% when the temperature decreased from 300 to 10 K resulting in a more electronically symmetric dimer. Possible origins of the temperature dependence of the positive charge delocalization in P.+ are discussed.

Near-infrared Yb(3+) vibronic sideband spectroscopy: application to Ca(2+)-binding proteins

We have used near-infrared (NIR) vibronic fluorescence spectroscopy to study the vibrational structure of ligands associated with model complexes of the lanthanide Yb(3+). This technique exploits the similar binding properties of the lanthanide Yb(3+) to probe Ca(2+)-binding sites in proteins. The (NIR) fluorescence of complexed Yb(3+) exhibits, in addition to main 0-0 (2F5/2----2F7/2) electronic transition of Yb(3+), weak vibronic sidebands which provide infrared-like, local vibrational spectra of the chelates (inner sphere ligands) of Yb(3+). A similar approach has been used for the lanthanide Gd(3+) (MacGregor, R.B., Jr (1989) Arch. Biochem. Biophys. 274, 312-316) which fluoresces in the UV and which is usually complicated by amino-acid residues fluorescing in the same spectral region. In this same spectral region, other complications in studying photosynthetic membranes occur in the form of the excitation wavelength being actinic, promoting photodegradation of the membranes, as well as the reabsorption of Gd(3+) fluorescence. NIR excitation and fluorescence detection of Yb(3+) avoid these problems when studying photosynthetic membranes. A preliminary study has been conducted here on rat muscle parvalbumin.